NestPullup.cs source code in C# .NET

Source code for the .NET framework in C#

                        

Code:

/ Net / Net / 3.5.50727.3053 / DEVDIV / depot / DevDiv / releases / Orcas / SP / ndp / fx / src / DataEntity / System / Data / Query / PlanCompiler / NestPullup.cs / 4 / NestPullup.cs

                            //---------------------------------------------------------------------- 
// 
//      Copyright (c) Microsoft Corporation.  All rights reserved.
// 
// 
// @owner  [....], [....]
//--------------------------------------------------------------------- 
 
using System;
//using System.Diagnostics; // Please use PlanCompiler.Assert instead of Debug.Assert in this class... 
using System.Collections.Generic;
using System.Globalization;

using System.Data.Common; 
using md = System.Data.Metadata.Edm;
using System.Data.Query.InternalTrees; 
using System.Data.Query.PlanCompiler; 

namespace System.Data.Query.PlanCompiler { 

    /// 
    /// This class "pulls" up nest operations to the root of the tree
    ///  
    /// 
    /// The goal of this module is to eliminate nest operations from the query - more 
    /// specifically, the nest operations are pulled up to the root of the query instead. 
    ///
    internal class NestPullup : BasicOpVisitorOfNode { 

        #region private state

        private PlanCompiler m_compilerState; 

        ///  
        /// map from a collection var to the node where it's defined; the node should be 
        /// the node that should be used as the replacement for the var if it is referred
        /// to in an UnnestOp (through a VarRef)  Note that we expect this to contain the 
        /// PhysicalProjectOp of the node, so we can use the VarList when mapping vars to
        /// the copy; (We'll remove the PhysicalProjectOp when we copy it...)
        /// 
        private Dictionary m_definingNodeMap = new Dictionary(); 

        ///  
        /// map from var to the var we're supposed to replace it with 
        /// 
        private VarRemapper m_varRemapper; 

        /// 
        /// Map from VarRef vars to what they're referencing; used to enable the defining
        /// node map to contain only the definitions, not all the references to it. 
        /// 
        private Dictionary m_varRefMap = new Dictionary(); 
 
        #endregion
 
        #region constructor

        private NestPullup(PlanCompiler compilerState) {
            m_compilerState = compilerState; 
            m_varRemapper = new VarRemapper(compilerState.Command);
        } 
 
        #endregion
 
        #region Process Driver

        internal static void Process(PlanCompiler compilerState) {
            NestPullup np = new NestPullup(compilerState); 
            np.Process();
        } 
 
        /// 
        /// The driver routine. Does all the hard work of processing 
        /// 
        private void Process() {
            PlanCompiler.Assert(Command.Root.Op.OpType == OpType.PhysicalProject, "root node is not physicalProject?");
            Command.Root = VisitNode(Command.Root); 
        }
 
        #endregion 

        #region private methods 

        #region VisitorHelpers

        ///  
        /// the iqt we're processing
        ///  
        private Command Command { get { return m_compilerState.Command; } } 

        ///  
        /// is the node a NestOp node?
        /// 
        /// 
        ///  
        private static bool IsNestOpNode(Node n) {
            PlanCompiler.Assert(n.Op.OpType != OpType.SingleStreamNest, "illegal singleStreamNest?"); 
            return (n.Op.OpType == OpType.SingleStreamNest || n.Op.OpType == OpType.MultiStreamNest); 
        }
 
        /// 
        /// Not Supported common processing
        ///
        /// For all those cases where we don't intend to support 
        /// a nest operation as a child, we have this routine to
        /// do the work. 
        ///  
        /// 
        ///  
        /// 
        private Node NestingNotSupported(Op op, Node n) {
            // First, visit my children
            VisitChildren(n); 
            m_varRemapper.RemapNode(n);
 
            // Make sure we don't have a child that is a nest op. 
            foreach (Node chi in n.Children) {
                if (IsNestOpNode(chi)) { 
                    throw EntityUtil.NestingNotSupported(op, chi.Op);
                }
            }
            return n; 
        }
 
        ///  
        /// Follow the VarRef chain to the defining var
        ///  
        /// 
        /// 
        private Var ResolveVarReference(Var refVar) {
            Var x = refVar; 
            while (m_varRefMap.TryGetValue(x, out x)) {
                refVar = x; 
            } 
            return refVar;
        } 

        /// 
        /// Update the replacement Var map with the vars from the pulled-up
        /// operation; the shape is supposed to be identical, so we should not 
        /// have more vars on either side, and the order is guaranteed to be
        /// the same. 
        ///  
        /// 
        ///  
        private void UpdateReplacementVarMap(IEnumerable fromVars, IEnumerable toVars) {
            IEnumerator toVarEnumerator = toVars.GetEnumerator();

            foreach (Var v in fromVars) { 
                if (!toVarEnumerator.MoveNext()) {
                    throw EntityUtil.InternalError(EntityUtil.InternalErrorCode.ColumnCountMismatch, 2); 
                } 
                m_varRemapper.AddMapping(v, toVarEnumerator.Current);
            } 

            if (toVarEnumerator.MoveNext()) {
                throw EntityUtil.InternalError(EntityUtil.InternalErrorCode.ColumnCountMismatch, 3);
            } 
        }
 
        #region remapping helpers 

        ///  
        /// Replace a list of sortkeys *IN-PLACE* with the corresponding "mapped" Vars
        /// 
        /// sortkeys
        /// the mapping info for Vars 
        private static void RemapSortKeys(List sortKeys, Dictionary varMap) {
            if (sortKeys != null) { 
                foreach (InternalTrees.SortKey sortKey in sortKeys) { 
                    Var replacementVar;
                    if (varMap.TryGetValue(sortKey.Var, out replacementVar)) { 
                        sortKey.Var = replacementVar;
                    }
                }
            } 
        }
 
        ///  
        /// Produce a "mapped" sequence of the input Var sequence - based on the supplied
        /// map 
        /// 
        /// input var sequence
        /// var->var map
        /// the mapped var sequence 
        private IEnumerable RemapVars(IEnumerable vars, Dictionary varMap) {
            foreach (Var v in vars) { 
                Var mappedVar; 
                if (varMap.TryGetValue(v, out mappedVar)) {
                    yield return mappedVar; 
                }
                else {
                    yield return v;
                } 
            }
        } 
 
        /// 
        /// Produce a "mapped" varList 
        /// 
        /// 
        /// 
        ///  
        private VarList RemapVarList(VarList varList, Dictionary varMap) {
            VarList newVarList = Command.CreateVarList(RemapVars(varList, varMap)); 
            return newVarList; 
        }
 
        /// 
        /// Produce a "mapped" varVec
        /// 
        ///  
        /// 
        ///  
        private VarVec RemapVarVec(VarVec varVec, Dictionary varMap) { 
            VarVec newVarVec = Command.CreateVarVec(RemapVars(varVec, varMap));
            return newVarVec; 
        }

        #endregion
 
        #endregion
 
        #region AncillaryOp Visitors 

        ///  
        /// VarDefOp
        ///
        /// Essentially, maintains m_varRefMap, adding an entry for each VarDef that has a
        /// VarRef on it. 
        /// 
        ///  
        ///  
        /// 
        public override Node Visit(VarDefOp op, Node n) { 
            VisitChildren(n);

            // perform any "remapping"
            m_varRemapper.RemapNode(n); 

            if (n.Child0.Op.OpType == OpType.VarRef) { 
                m_varRefMap.Add(op.Var, ((VarRefOp)n.Child0.Op).Var); 
            }
            return n; 
        }

        /// 
        /// VarRefOp 
        /// 
        ///  
        /// When we remove the UnnestOp, we are left with references to it's column vars that 
        /// need to be fixed up; we do this by creating a var replacement map when we remove the
        /// UnnestOp and whenever we find a reference to a var in the map, we replace it with a 
        /// reference to the replacement var instead;
        /// 
        /// 
        ///  
        /// 
        public override Node Visit(VarRefOp op, Node n) { 
            // First, visit my children (do I have children?) 
            VisitChildren(n);
            // perform any "remapping" 
            m_varRemapper.RemapNode(n);
            return n;
        }
 
        #endregion
 
        #region ScalarOp Visitors 
        /// 
        /// We don't yet support nest pullups over Case 
        /// 
        /// 
        /// 
        ///  
        public override Node Visit(CaseOp op, Node n) {
            // Make sure we don't have a child that is a nest op. 
            foreach (Node chi in n.Children) { 
                if (chi.Op.OpType == OpType.Collect) {
                    throw EntityUtil.NestingNotSupported(op, chi.Op); 
                }
                else if (chi.Op.OpType == OpType.VarRef) {
                    Var refVar = ResolveVarReference(((VarRefOp)chi.Op).Var);
                    if (m_definingNodeMap.ContainsKey(refVar)) { 
                        throw EntityUtil.NestingNotSupported(op, chi.Op);
                    } 
                } 
            }
 
            return VisitDefault(n);
        }
        #endregion
 
        #region RelOp Visitors
 
        ///  
        /// Default RelOp processing:
        /// 
        /// We really don't want to allow any NestOps through; just fail if we don't have
        /// something coded.
        /// 
        ///  
        /// 
        ///  
        protected override Node VisitRelOpDefault(RelOp op, Node n) { 
            return NestingNotSupported(op, n);
        } 

        /// 
        /// ApplyOp/JoinOp common processing
        ///  
        /// 
        /// If one of the inputs to any JoinOp/ApplyOp is a NestOp, then the NestOp 
        /// can be pulled above the join/apply if every input to the join/apply has 
        /// a key(s). The keys of the NestOp are augmented with the keys of the
        /// other join inputs: 
        ///
        ///     JoinOp/ApplyOp(NestOp(X, ...), Y) => NestOp(JoinOp/ApplyOp(X, Y), ...)
        ///
        /// In addition, if the NestOp is on a 'nullable' side of a join (i.e. right side of 
        /// LeftOuterJoin/OuterApply or either side of FullOuterJoin), the driving node
        /// of that NestOp (X) is capped with a project with a null sentinel and 
        /// the dependant collection nodes (the rest of the NestOp children) 
        /// are filtered based on that sentinel:
        /// 
        ///  LOJ/OA/FOJ (X, NestOp(Y, Z1, Z2, ..ZN))  =>  NestOp( LOJ/OA/FOJ (X, PROJECT (Y, v = 1)), FILTER(Z1, v!=null), FILTER(Z2, v!=null), ... FILTER(ZN, v!=null))
        ///         FOJ (NestOp(Y, Z1, Z2, ..ZN), X)  =>  NestOp( LOJ/OA/FOJ (PROJECT (Y, v = 1), X), FILTER(Z1, v!=null), FILTER(Z2, v!=null), ... FILTER(ZN, v!=null))
        ///
        ///  Also, FILTER(Zi, v != null) may be transformed to push the filter below any NestOps. 
        ///
        /// Requires: Every input to the join/apply must have a key. 
        ///  
        /// 
        ///  
        /// 
        private Node ApplyOpJoinOp(Op op, Node n) {
            // First, visit my children
            VisitChildren(n); 

            // Now determine if any of the input nodes are a nestOp. 
            int countOfNestInputs = 0; 

            foreach (Node chi in n.Children) { 
                NestBaseOp nestOp = chi.Op as NestBaseOp;
                if (null != nestOp) {
                    countOfNestInputs++;
 
                    if (OpType.SingleStreamNest == chi.Op.OpType) {
                        // There should not be a SingleStreamNest in the tree, because we made a decision 
                        // that in essence means the only way to get a SingleStreamNest is to have a 
                        // PhysicalProject over something with an underlying NestOp.   Having
                        // 
                        //      Project(Collect(PhysicalProject(...)))
                        //
                        // isn�t good enough, because that will get converted to a MultiStreamNest, with
                        // the SingleStreamNest as the input to the MultiStreamNest. 
                        throw EntityUtil.InternalError(EntityUtil.InternalErrorCode.JoinOverSingleStreamNest);
                    } 
                } 
            }
 
            // If none of the inputs are a nest, then we don't really need to do anything.
            if (0 == countOfNestInputs) {
                return n;
            } 

            // We can only pull the nest over a Join/Apply if it has keys, so 
            // we can order things; if it doesn't have keys, we throw a NotSupported 
            // exception.
            foreach (Node chi in n.Children) { 
                if (op.OpType != OpType.MultiStreamNest && chi.Op.IsRelOp) {
                    KeyVec keys = Command.PullupKeys(chi);

                    if (null == keys || keys.NoKeys) { 
                        throw EntityUtil.KeysRequiredForJoinOverNest(op);
                    } 
                } 
            }
 
            // Alright, we're OK to pull the nestOp over the joinOp/applyOp.
            //
            // That means:
            // 
            // (1) build a new list of children for the nestOp and for the joinOp/applyOp
            // (2) build the new list of collectionInfos for the new nestOp. 
            List newNestChildren = new List(); 
            List newJoinApplyChildren = new List();
            List newCollectionInfoList = new List(); 

            foreach (Node chi in n.Children) {
                if (chi.Op.OpType == OpType.MultiStreamNest) {
                    newCollectionInfoList.AddRange(((MultiStreamNestOp)chi.Op).CollectionInfo); 

                    // SQLBUDT #615513: If the nest op is on a 'nullable' side of join 
                    // (i.e. right side of LeftOuterJoin/OuterApply or either side of FullOuterJoin) 
                    // the driving node of that nest operation needs to be capped with a project with
                    // a null sentinel and the dependant collection nodes need to be filtered based on that sentinel. 
                    //
                    //  LOJ/OA/FOJ (X, MSN(Y, Z1, Z2, ..ZN))  =>  MSN( LOJ/OA/FOJ (X, PROJECT (Y, v = 1)), FILTER(Z1, v!=null), FILTER(Z2, v!=null), ... FILTER(ZN, v!=null))
                    //         FOJ (MSN(Y, Z1, Z2, ..ZN), X)  =>  MSN( LOJ/OA/FOJ (PROJECT (Y, v = 1), X), FILTER(Z1, v!=null), FILTER(Z2, v!=null), ... FILTER(ZN, v!=null))
                    // 
                    //  Note: we transform FILTER(Zi, v != null) to push the filter below any MSNs.
                    if ((op.OpType == OpType.FullOuterJoin) 
                        || ((op.OpType == OpType.LeftOuterJoin || op.OpType == OpType.OuterApply) && n.Child1.Op.OpType == OpType.MultiStreamNest)) { 
                        Var sentinelVar = null;
                        newJoinApplyChildren.Add(AugmentNodeWithInternalConstant(chi.Child0, 1, out sentinelVar)); 
                        for (int i = 1; i < chi.Children.Count; i++) {
                            // Push the filter bellow all MultiStremNest-s, because this part of the tree has
                            // already been visited and it is expected that the MultiStreamNests have bubbled up
                            // above the filters. 
                            Node newFilterChild = chi.Children[i];
                            Node newFilterParent = null; 
                            while (newFilterChild.Op.OpType == OpType.MultiStreamNest) { 
                                newFilterParent = newFilterChild;
                                newFilterChild = newFilterChild.Child0; 
                            }

                            Node newFilterNode = CapWithIsNotNullFilter(newFilterChild, sentinelVar);
                            Node newNestChild; // the original child or the filter, depending on whether the filter got pushed 

                            if (newFilterParent != null) { 
                                newFilterParent.Child0 = newFilterNode; 
                                newNestChild = chi.Children[i];
                            } 
                            else {
                                newNestChild = newFilterNode;
                            }
                            newNestChildren.Add(newNestChild); 
                        }
                    } 
                    else { 
                        newJoinApplyChildren.Add(chi.Child0);
                        for (int i = 1; i < chi.Children.Count; i++) { 
                            newNestChildren.Add(chi.Children[i]);
                        }
                    }
                } 
                else {
                    newJoinApplyChildren.Add(chi); 
                } 
            }
 
            // (3) create the new Join/Apply node using the existing op and the
            //     new list of children from (1).
            Node newJoinApplyNode = Command.CreateNode(op, newJoinApplyChildren);
 
            // (4) insert the apply op as the driving node of the nestOp (put it
            //     at the beginning of the new nestOps' children. 
            newNestChildren.Insert(0, newJoinApplyNode); 

            // (5) build an updated list of output vars based upon the new join/apply 
            //     node, and ensure all the collection vars from the nestOp(s) are
            //     included.
            ExtendedNodeInfo xni = newJoinApplyNode.GetExtendedNodeInfo(Command);
            VarVec newOutputVars = Command.CreateVarVec(xni.Definitions); 

            foreach (CollectionInfo ci in newCollectionInfoList) { 
                newOutputVars.Set(ci.CollectionVar); 
            }
 
            // (6) create the new nestop
            NestBaseOp newNestOp = Command.CreateMultiStreamNestOp(new List(), newOutputVars, newCollectionInfoList);
            Node newNode = Command.CreateNode(newNestOp, newNestChildren);
            return newNode; 
        }
 
        ///  
        /// Input =>  Filter(input, Ref(var) is not null)
        ///  
        /// 
        /// 
        /// 
        private Node CapWithIsNotNullFilter(Node input, Var var) { 
            Node varRefNode = Command.CreateNode(Command.CreateVarRefOp(var));
            Node predicateNode = Command.CreateNode( 
                                            Command.CreateConditionalOp(OpType.Not), 
                                            Command.CreateNode(
                                                    Command.CreateConditionalOp(OpType.IsNull), 
                                                    varRefNode));


            Node filterNode = Command.CreateNode(Command.CreateFilterOp(), input, predicateNode); 
            return filterNode;
        } 
 
        /// 
        /// ApplyOp common processing 
        /// 
        /// 
        /// 
        ///  
        protected override Node VisitApplyOp(ApplyBaseOp op, Node n) {
            return ApplyOpJoinOp(op, n); 
        } 

        ///  
        /// DistinctOp
        /// 
        /// 
        /// The input to a DistinctOp cannot be a NestOp � that would imply that 
        /// we support distinctness over collections - which we don�t.
        ///  
        ///  
        /// 
        ///  
        public override Node Visit(DistinctOp op, Node n) {
            return NestingNotSupported(op, n);
        }
 
        /// 
        /// FilterOp 
        ///  
        /// 
        /// If the input to the FilterOp is a NestOp, and if the filter predicate 
        /// does not reference any of the collection Vars of the nestOp, then the
        /// FilterOp can be simply pushed below the NestOp:
        ///
        ///     Filter(Nest(X, ...), pred) => Nest(Filter(X, pred), ...) 
        ///
        /// Note: even if the filter predicate originally referenced one of the 
        /// collection vars, as part of our bottom up traversal, the appropriate 
        /// Var was replaced by a copy of the source of the collection. So, this
        /// transformation should always be legal. 
        /// 
        /// 
        /// 
        ///  
        public override Node Visit(FilterOp op, Node n) {
            // First, visit my children 
            VisitChildren(n); 

            // see if the child is a nestOp 
            NestBaseOp nestOp = n.Child0.Op as NestBaseOp;

            if (null != nestOp) {
#if DEBUG 
                // check to see if the predicate references any of the collection
                // expressions. If it doesn't, then we can push the filter down, but 
                // even if it does it's probably OK. 
                NodeInfo predicateNodeInfo = Command.GetNodeInfo(n.Child1);
                foreach (CollectionInfo ci in nestOp.CollectionInfo) { 
                    PlanCompiler.Assert(!predicateNodeInfo.ExternalReferences.IsSet(ci.CollectionVar), "predicate references collection?");
                }
#endif //DEBUG
 
                // simply pull up the nest child above ourself.
                Node nestOpNode = n.Child0; 
                Node nestOpInputNode = nestOpNode.Child0; 
                n.Child0 = nestOpInputNode;
                nestOpNode.Child0 = n; 

                // recompute node info - no need to perform anything for the predicate
                Command.RecomputeNodeInfo(n);
                Command.RecomputeNodeInfo(nestOpNode); 
                return nestOpNode;
            } 
 
            return n;
        } 

        /// 
        /// GroupByOp
        ///  
        /// 
        /// At this point in the process, there really isn't a way we should actually 
        /// have a NestOp as an input to the GroupByOp, and we currently aren't allowing 
        /// you to specify a collection as an aggregation Var or key, so if we find a
        /// NestOp anywhere on the inputs, it's a NotSupported situation. 
        /// 
        /// 
        /// 
        ///  
        public override Node Visit(GroupByOp op, Node n) {
            return NestingNotSupported(op, n); 
        } 

        ///  
        /// JoinOp common processing
        /// 
        /// 
        ///  
        /// 
        protected override Node VisitJoinOp(JoinBaseOp op, Node n) { 
            return ApplyOpJoinOp(op, n); 
        }
 
        /// 
        /// ProjectOp
        /// 
        ///  
        /// There are three cases to handle in ProjectOp:
        /// 
        ///  (1) The input is not a NestOp; but the ProjectOp locally defines some Vars 
        ///      as collections:
        /// 
        ///         ProjectOp(X,{a,CollectOp(PhysicalProjectOp(Y)),b,...}) ==> MsnOp(ProjectOp'(X,{a,b,...}),Y)
        ///         ProjectOp(X,{a,VarRef(ref-to-collect-var-Y),b,...})    ==> MsnOp(ProjectOp'(X,{a,b,...}),copy-of-Y)
        ///
        ///     Where: 
        ///         ProjectOp' is ProjectOp less any vars that were collection vars, plus
        ///                    any additional Vars needed by the collection. 
        /// 
        ///  (2) The input is a NestOp, but the ProjectOp does not local define some Vars
        ///      as collections: 
        ///
        ///         ProjectOp(MsnOp(X,Y,...)) => MsnOp'(ProjectOp'(X),Y,...)
        ///
        ///      Where: 
        ///         ProjectOp' is ProjectOp plus any additional Vars needed by NestOp
        ///                    (see NestOp.Outputs � except the collection vars) 
        /// 
        ///         MsnOp'     should be MsnOp. Additionally, its Outputs should be enhanced
        ///                    to include any Vars produced by the ProjectOp 
        ///
        ///  (3) The combination of both (1) and (2) -- both the vars define a collection,
        ///      and the input is also a nestOp.  we handle this by first processing Case1,
        ///      then processing Case2. 
        /// 
        ///  
        ///  
        /// 
        public override Node Visit(ProjectOp op, Node n) { 
#if DEBUG
            string input = Dump.ToXml(Command, n);
#endif //DEBUG
 
            // First, visit my children
            VisitChildren(n); 
            m_varRemapper.RemapNode(n); 

            // First, convert any nestOp inputs; 
            Node newNode = ProjectOpCase1(n);

            // Then, if we have a NestOp as an input (and we didn't
            // produce a NestOp when handling Case1) pull it over our 
            // ProjectOp.
            if (newNode.Op.OpType == OpType.Project && IsNestOpNode(newNode.Child0)) { 
                newNode = ProjectOpCase2(newNode); 
            }
 
            // Finally we fold any nested NestOps into one.
            newNode = MergeNestedNestOps(newNode);

#if DEBUG 
            int size = input.Length;// GC.KeepAlive makes FxCop Grumpy.
            string output = Dump.ToXml(Command, newNode); 
#endif //DEBUG 
            return newNode;
        } 

        /// 
        /// Fold nested MultiStreamNestOps into one:
        /// 
        ///     MSN(MSN(X,Y),Z) ==> MSN(X,Y,Z)
        /// 
        /// NOTE: It would be incorrect to merge NestOps from the non-driving node 
        ///       into one nest op, because that would change the intent.  Instead,
        ///       we let those go through the tree and wait until we get to the top 
        ///       level PhysicalProject, when we'll use the ConvertToSingleStreamNest
        ///       process to handle them.
        ///
        /// NOTE: We should never have three levels of nestOps, because we should 
        ///       have folded the lower two together when we constructed one of them.
        /// 
        /// We also remove unreferenced collections, that is, if any collection is 
        /// not referred to by the top level-NestOp, we can safely remove it from
        /// the merged NestOp we produce. 
        /// 
        /// 
        /// 
        private Node MergeNestedNestOps(Node nestNode) { 

            // First, determine if there is anything we can actually do.  If we 
            // aren't given a NestOp or if it's driving node isn't a NestOp we 
            // can just ignore this.
            if (!IsNestOpNode(nestNode) || !IsNestOpNode(nestNode.Child0)) { 
                return nestNode;
            }

#if DEBUG 
            string input = Dump.ToXml(Command, nestNode);
#endif //DEBUG 
            NestBaseOp nestOp = (NestBaseOp)nestNode.Op; 
            Node nestedNestNode = nestNode.Child0;
            NestBaseOp nestedNestOp = (NestBaseOp)nestedNestNode.Op; 

            // Get the collection Vars from the top level NestOp
            VarVec nestOpCollectionOutputs = Command.CreateVarVec();
            foreach (CollectionInfo ci in nestOp.CollectionInfo) { 
                nestOpCollectionOutputs.Set(ci.CollectionVar);
            } 
 
            // Now construct a new list of inputs, collections; and output vars.
            List newNestInputs = new List(); 
            List newCollectionInfo = new List();
            VarVec newOutputVars = Command.CreateVarVec(nestOp.Outputs);

            // Add the new DrivingNode; 
            newNestInputs.Add(nestedNestNode.Child0);
 
            // Now add each of the nested nodes collections, but only when they're 
            // referenced by the top level nestOp's outputs.
            for (int i = 1; i < nestedNestNode.Children.Count; i++) { 
                CollectionInfo ci = nestedNestOp.CollectionInfo[i - 1];
                if (nestOpCollectionOutputs.IsSet(ci.CollectionVar) || newOutputVars.IsSet(ci.CollectionVar)) {
                    newCollectionInfo.Add(ci);
                    newNestInputs.Add(nestedNestNode.Children[i]); 
                    PlanCompiler.Assert(newOutputVars.IsSet(ci.CollectionVar), "collectionVar not in output Vars?"); // I must have missed something...
                } 
            } 

            // Then add in the rest of the inputs to the top level nest node (and 
            // they're collection Infos)
            for (int i = 1; i < nestNode.Children.Count; i++) {
                CollectionInfo ci = nestOp.CollectionInfo[i - 1];
                newCollectionInfo.Add(ci); 
                newNestInputs.Add(nestNode.Children[i]);
                PlanCompiler.Assert(newOutputVars.IsSet(ci.CollectionVar), "collectionVar not in output Vars?"); // I must have missed something... 
            } 

            //The prefix sort keys for the new nest op should include these of the input nestOp followed by the nestedNestOp 
            //(The nestOp-s that are being merged may have prefix sort keys propagated to them by constrainedSortOp-s pushed below them.
            List sortKeys = ConsolidateSortKeys(nestOp.PrefixSortKeys, nestedNestOp.PrefixSortKeys);

            // Make sure we pullup the sort keys in our output too... 
            foreach (InternalTrees.SortKey sk in sortKeys) {
                newOutputVars.Set(sk.Var); 
            } 

            // Ready to go; build the new NestNode, etc. 
            MultiStreamNestOp newNestOp = Command.CreateMultiStreamNestOp(sortKeys, newOutputVars, newCollectionInfo);
            Node newNode = Command.CreateNode(newNestOp, newNestInputs);

            // Finally, recompute node info 
            Command.RecomputeNodeInfo(newNode);
 
#if DEBUG 
            int size = input.Length;// GC.KeepAlive makes FxCop Grumpy.
            string output = Dump.ToXml(Command, newNode); 
#endif //DEBUG
            return newNode;
        }
 
        /// 
        /// ProjectOp(X,{a,CollectOp(PhysicalProjectOp(Y)),b,...}) ==> MsnOp(ProjectOp'(X,{a,b,...}),Y) 
        /// ProjectOp(X,{a,VarRef(ref-to-collect-var-Y),b,...})    ==> MsnOp(ProjectOp'(X,{a,b,...}),copy-of-Y) 
        ///
        /// Remove CollectOps from projection, constructing a NestOp 
        /// over the ProjectOp.
        /// 
        /// 
        ///  
        private Node ProjectOpCase1(Node projectNode) {
#if DEBUG 
            string input = Dump.ToXml(Command, projectNode); 
#endif //DEBUG
 
            ProjectOp op = (ProjectOp)projectNode.Op;

            // Check to see if any of the computed Vars are in fact NestOps, and
            // construct a collection column map for them. 
            List collectionInfoList = new List();
            List newChildren = new List(); 
            List collectionNodes = new List(); 
            VarVec externalReferences = Command.CreateVarVec();
            VarVec collectionReferences = Command.CreateVarVec(); 
            List definedVars = new List();
            List referencedVars = new List();

            foreach (Node chi in projectNode.Child1.Children) { 
                VarDefOp varDefOp = (VarDefOp)chi.Op;
                Node definingExprNode = chi.Child0; 
 
                if (OpType.Collect == definingExprNode.Op.OpType) {
                    PlanCompiler.Assert(definingExprNode.HasChild0, "collect without input?"); 
                    PlanCompiler.Assert(OpType.PhysicalProject == definingExprNode.Child0.Op.OpType, "collect without physicalProject?");
                    Node physicalProjectNode = definingExprNode.Child0;

                    // Update collection var->defining node map; 
                    m_definingNodeMap.Add(varDefOp.Var, physicalProjectNode);
 
                    ConvertToNestOpInput(physicalProjectNode, varDefOp.Var, collectionInfoList, collectionNodes, externalReferences, collectionReferences); 
                }
                else if (OpType.VarRef == definingExprNode.Op.OpType) { 
                    Var refVar = ResolveVarReference(varDefOp.Var);
                    Node physicalProjectNode;

                    if (m_definingNodeMap.TryGetValue(refVar, out physicalProjectNode)) { 
                        physicalProjectNode = CopyCollectionVarDefinition(physicalProjectNode);
                        //SQLBUDT #602888: We need to track the copy too, in case we need to reuse it 
                        m_definingNodeMap.Add(varDefOp.Var, physicalProjectNode); 
                        ConvertToNestOpInput(physicalProjectNode, varDefOp.Var, collectionInfoList, collectionNodes, externalReferences, collectionReferences);
                    } 
                    else {
                        referencedVars.Add(chi);
                        newChildren.Add(chi);
                    } 
                }
                else { 
                    definedVars.Add(chi); 
                    newChildren.Add(chi);
                } 
            }

            // If we haven't identified a set of collection nodes, then we're done.
            if (0 == collectionNodes.Count) { 
                return projectNode;
            } 
 
            // OK, we found something. We have some heavy lifting to perform.
 
            // Then we need to build up a MultiStreamNestOp above the ProjectOp and the
            // new collection nodes to get what we really need.
            // pretend that the keys included everything from the new projectOp
            VarVec outputVars = Command.CreateVarVec(op.Outputs); 

            // First we need to modify this physicalProjectNode to leave out the collection 
            // Vars that we've just seen. 
            VarVec newProjectVars = Command.CreateVarVec(op.Outputs);
            newProjectVars.Minus(collectionReferences); 

            // If there are any external references from any of the collections, add
            // those to the projectOp explicitly. This must be ok because the projectOp
            // could not have had any left-correlation 
            newProjectVars.Or(externalReferences);
 
            // Create the new projectOp, and hook it into this one.  The new projectOp 
            // no longer references the collections in it's children; of course we only
            // construct a new projectOp if it actually projects out some Vars. 
            if (!newProjectVars.IsEmpty) {
                if (IsNestOpNode(projectNode.Child0)) {
                    // If the input is a nest node, we need to figure out what to do with the
                    // rest of the in the VarDefList; we can't just pitch them, but we also 
                    // really want to have the input be a nestop.
                    // 
                    // What we do is essentially push any non-collection VarDef�s down under 
                    // the driving node of the MSN:
                    // 
                    //      Project[Z,Y,W](Msn(X,Y),VarDef(Z=blah),VarDef(W=Collect(etc)) ==> MSN(MSN(Project[Z](X,VarDef(Z=blah)),Y),W)
                    //
                    // An optimization, of course being to not push anything down when there
                    // aren't any extra vars defined. 

                    if (definedVars.Count == 0 && referencedVars.Count == 0) { 
                        // We'll just pick the NestNode; we expect MergeNestedNestOps to merge 
                        // it into what we're about to generate later.
                        projectNode = projectNode.Child0; 
                        EnsureReferencedVarsAreRemoved(referencedVars, outputVars);
                    }
                    else {
                        NestBaseOp nestedNestOp = (NestBaseOp)projectNode.Child0.Op; 

                        // Build the new ProjectOp to be used as input to the new nestedNestOp; 
                        // it's input is the input to the current nestedNestOp and a new 
                        // VarDefList with only the vars that were defined on the top level
                        // ProjectOp. 
                        List newNestedProjectNodeInputs = new List();
                        newNestedProjectNodeInputs.Add(projectNode.Child0.Child0);
                        referencedVars.AddRange(definedVars);
                        newNestedProjectNodeInputs.Add(Command.CreateNode(Command.CreateVarDefListOp(), referencedVars)); 

                        VarVec newNestedProjectOutputs = Command.CreateVarVec(nestedNestOp.Outputs); 
 
                        // SQLBUDT #508722:  We need to remove the collection vars,
                        //  these are not produced by the project 
                        foreach (CollectionInfo ci in nestedNestOp.CollectionInfo) {
                            newNestedProjectOutputs.Clear(ci.CollectionVar);
                        }
 
                        foreach (Node varDefNode in referencedVars) {
                            newNestedProjectOutputs.Set(((VarDefOp)varDefNode.Op).Var); 
                        } 

                        Node newNestedProjectNode = Command.CreateNode(Command.CreateProjectOp(newNestedProjectOutputs), newNestedProjectNodeInputs); 

                        // Now build the new nestedNestedNestOp, with the new nestedProjectOp
                        // as it's input; we have to update the outputs of the NestOp to include
                        // the vars we pushed down. 
                        VarVec newNestedNestOutputs = Command.CreateVarVec(newNestedProjectOutputs);
                        newNestedNestOutputs.Or(nestedNestOp.Outputs); 
 
                        MultiStreamNestOp newNestedNestOp = Command.CreateMultiStreamNestOp(
                                                                    nestedNestOp.PrefixSortKeys, 
                                                                    newNestedNestOutputs,
                                                                    nestedNestOp.CollectionInfo);

                        List newNestedNestNodeInputs = new List(); 
                        newNestedNestNodeInputs.Add(newNestedProjectNode);
                        for (int j = 1; j < projectNode.Child0.Children.Count; j++) { 
                            newNestedNestNodeInputs.Add(projectNode.Child0.Children[j]); 
                        }
                        projectNode = Command.CreateNode(newNestedNestOp, newNestedNestNodeInputs); 
                        // We don't need to remove or remap referenced vars here because
                        // we're including them on the node we create; they won't become
                        // invalid.
                    } 
                }
                else { 
                    ProjectOp newProjectOp = Command.CreateProjectOp(newProjectVars); 
                    projectNode.Child1 = Command.CreateNode(projectNode.Child1.Op, newChildren);
                    projectNode.Op = newProjectOp; 
                    EnsureReferencedVarsAreRemapped(referencedVars);
                }
            }
            else { 
                projectNode = projectNode.Child0;
                EnsureReferencedVarsAreRemoved(referencedVars, outputVars); 
            } 

            // We need to make sure that we project out any external references to the driving 
            // node that the nested collections have, or we're going to end up with unresolvable
            // vars when we pull them up over the current driving node.  Of course, we only
            // want the references that are actually ON the driving node.
            externalReferences.And(projectNode.GetExtendedNodeInfo(Command).Definitions); 
            outputVars.Or(externalReferences);
 
            // There are currently no prefix sortkeys. The processing for a SortOp may later 
            // introduce some prefix sortkeys, but there aren't any now.
            MultiStreamNestOp nestOp = Command.CreateMultiStreamNestOp(new List(), outputVars, collectionInfoList); 

            // Insert the current node at the head of the the list of collections
            collectionNodes.Insert(0, projectNode);
            Node nestNode = Command.CreateNode(nestOp, collectionNodes); 

            // Finally, recompute node info 
            Command.RecomputeNodeInfo(projectNode); 
            Command.RecomputeNodeInfo(nestNode);
 
#if DEBUG
            int size = input.Length;// GC.KeepAlive makes FxCop Grumpy.
            string output = Dump.ToXml(Command, nestNode);
#endif //DEBUG 
            return nestNode;
        } 
 
        /// 
        /// If we're going to eat the ProjectNode, then we at least need to make 
        /// sure we remap any vars it defines as varRefs, and ensure that any
        /// references to them are switched.
        /// 
        ///  
        /// 
        private void EnsureReferencedVarsAreRemoved(List referencedVars, VarVec outputVars) { 
            foreach (Node chi in referencedVars) { 
                VarDefOp varDefOp = (VarDefOp)chi.Op;
                Var defVar = varDefOp.Var; 
                Var refVar = ResolveVarReference(defVar);
                m_varRemapper.AddMapping(defVar, refVar);
                outputVars.Clear(defVar);
                outputVars.Set(refVar); 
            }
        } 
 
        /// 
        /// We need to make sure that we remap the column maps that we're pulling 
        /// up to point to the defined var, not it's reference.
        /// 
        /// 
        private void EnsureReferencedVarsAreRemapped(List referencedVars) { 
            foreach (Node chi in referencedVars) {
                VarDefOp varDefOp = (VarDefOp)chi.Op; 
                Var defVar = varDefOp.Var; 
                Var refVar = ResolveVarReference(defVar);
                m_varRemapper.AddMapping(refVar, defVar); 
            }
        }

        ///  
        /// Convert a CollectOp subtree (when used as the defining expression for a
        /// VarDefOp) into a reasonable input to a NestOp. 
        ///  
        /// 
        /// There are a couple of cases that we handle here: 
        ///
        ///     (a) PhysicalProject(X) ==> X
        ///     (b) PhysicalProject(Sort(X)) ==> Sort(X)
        /// 
        /// 
        /// the child of the CollectOp 
        /// the collectionVar being defined 
        /// where to append the new collectionInfo
        /// where to append the collectionNode 
        /// a bit vector of external references of the physicalProject
        /// a bit vector of collection vars
        private void ConvertToNestOpInput(Node physicalProjectNode, Var collectionVar, List collectionInfoList, List collectionNodes, VarVec externalReferences, VarVec collectionReferences) {
 
            // Keep track of any external references the physicalProjectOp has
            externalReferences.Or(Command.GetNodeInfo(physicalProjectNode).ExternalReferences); 
 

            // Case: (a) PhysicalProject(X) ==> X 
            Node nestOpInput = physicalProjectNode.Child0;

            // Now build the collectionInfo for this input, including the flattened
            // list of vars, which is essentially the outputs from the physicalProject 
            // with the sortKey vars that aren't already in the outputs we already
            // have. 
            PhysicalProjectOp physicalProjectOp = (PhysicalProjectOp)physicalProjectNode.Op; 
            VarList flattenedElementVarList = Command.CreateVarList(physicalProjectOp.Outputs);
            VarVec flattenedElementVarVec = Command.CreateVarVec(flattenedElementVarList); // Use a VarVec to make the lookups faster 
            List sortKeys = null;

            if (OpType.Sort == nestOpInput.Op.OpType) {
                // Case: (b) PhysicalProject(Sort(X)) ==> Sort(X) 
                SortOp sortOp = (SortOp)nestOpInput.Op;
                sortKeys = OpCopier.Copy(Command , sortOp.Keys); 
 
                foreach (InternalTrees.SortKey sk in sortKeys) {
                    if (!flattenedElementVarVec.IsSet(sk.Var)) { 
                        flattenedElementVarList.Add(sk.Var);
                        flattenedElementVarVec.Set(sk.Var);
                    }
                } 
            }
            else { 
                sortKeys = new List(); 
            }
 
            // Get the keys for the collection
            VarVec keys = Command.CreateVarVec();
            foreach (Var key in Command.GetExtendedNodeInfo(nestOpInput).Keys.KeyVars) {
                if (flattenedElementVarVec.IsSet(key)) { 
                    keys.Set(key);
                } 
            } 

            // Create the collectionInfo. 
            CollectionInfo collectionInfo = Command.CreateCollectionInfo(collectionVar, physicalProjectOp.ColumnMap.Element, flattenedElementVarList, keys, sortKeys, null/*discriminatorValue*/);

            // Now update the collections we're tracking.
            collectionInfoList.Add(collectionInfo); 
            collectionNodes.Add(nestOpInput);
            collectionReferences.Set(collectionVar); 
        } 

        ///  
        /// Case 2 for ProjectOp: NestOp is the input:
        ///
        ///     ProjectOp(NestOp(X,Y,...)) => NestOp'(ProjectOp'(X),Y,...)
        /// 
        /// Remove collection references from the ProjectOp and pull the
        /// NestOp over it, adding any outputs that the projectOp added. 
        /// 
        /// The outputs are important here; expanding the above:
        /// 
        ///     P{a,n}(N{x1,x2,x3,y}(X,Y)) => N{a,x1,x2,x3,y}(P{a,x1,x2,x3}(X),Y)
        ///
        /// Strategy:
        /// 
        ///     (1) Determine oldNestOpCollectionOutputs
        ///     (2) oldNestOpNonCollectionOutputs = oldNestOpOutputs - oldNestOpCollectionOutputs; 
        ///     (3) oldProjectOpNonCollectionOutputs = oldProjectOpOutputs - oldNestOpCollectionOutputs 
        ///     (4) oldProjectOpCollectionOutputs = oldProjectOpOutputs - oldProjectOpNonCollectionOutputs
        ///     (5) build a new list of collectionInfo's for the new NestOp, including 
        ///         only oldProjectOpCollectionOutputs.
        ///     (6) leftCorrelationVars = vars that are defined by the left most child of the input nestOpNode
        ///         and used in the subtrees rooted at the other children of the input nestOpNode
        ///     (7) newProjectOpOutputs = oldProjectOpNonCollectionOutputs + oldNestOpNonCollectionOutputs + leftCorrelationVars 
        ///
        ///     (8) newProjectOpChildren = .... 
        /// 
        /// Of course everything needs to be "derefed", that is, expressed in the projectOp Var Ids.
        /// 
        ///     (9) Set ProjectOp's input to NestOp's input
        ///     (10) Set NestOp's input to ProjectOp.
        /// 
        ///  
        /// 
        private Node ProjectOpCase2(Node projectNode) { 
#if DEBUG 
            string input = Dump.ToXml(Command, projectNode);
#endif //DEBUG 
            ProjectOp projectOp = (ProjectOp)projectNode.Op;
            Node nestNode = projectNode.Child0;
            NestBaseOp nestOp = nestNode.Op as NestBaseOp;
#if DEBUG 
            // NOTE: I do not believe that we need to remap the nest op in terms of
            //       the project op, but I can't prove it right now; if the assert 
            //       below fires, I was wrong. 
            //Dictionary projectToNestVarMap = new Dictionary();
 
            Command.RecomputeNodeInfo(projectNode);
            ExtendedNodeInfo projectNodeInfo = Command.GetExtendedNodeInfo(projectNode);
            ExtendedNodeInfo nestNodeInfo = Command.GetExtendedNodeInfo(nestNode);
 
            foreach (Node chi in projectNode.Child1.Children) {
                VarDefOp varDefOp = (VarDefOp)chi.Op; 
                Node definingExprNode = chi.Child0; 

                if (OpType.VarRef == definingExprNode.Op.OpType) { 
                    VarRefOp varRefOp = (VarRefOp)definingExprNode.Op;
                    PlanCompiler.Assert(varRefOp.Var == varDefOp.Var || !projectNodeInfo.LocalDefinitions.IsSet(varRefOp.Var), "need to remap vars!");

                    //if (!projectToNestVarMap.ContainsKey(varRefOp.Var)) { 
                    //    projectToNestVarMap.Add(varRefOp.Var, varDefOp.Var);
                    //} 
                } 
            }
#endif //DEBUG 

            // (1) Determine oldNestOpCollectionOutputs
            VarVec oldNestOpCollectionOutputs = Command.CreateVarVec();
            foreach (CollectionInfo ci in nestOp.CollectionInfo) { 
                oldNestOpCollectionOutputs.Set(ci.CollectionVar);
            } 
 
            // (2) oldNestOpNonCollectionOutputs = oldNestOpOutputs - oldNestOpCollectionOutputs;
            VarVec oldNestOpNonCollectionOutputs = Command.CreateVarVec(nestOp.Outputs); 
            oldNestOpNonCollectionOutputs.Minus(oldNestOpCollectionOutputs);

            // (3) oldProjectOpNonCollectionOutputs = oldProjectOpOutputs - oldNestOpCollectionOutputs
            VarVec oldProjectOpNonCollectionOutputs = Command.CreateVarVec(projectOp.Outputs); 
            oldProjectOpNonCollectionOutputs.Minus(oldNestOpCollectionOutputs);
 
            // (4) oldProjectOpCollectionOutputs = oldProjectOpOutputs - oldProjectOpNonCollectionOutputs 
            VarVec oldProjectOpCollectionOutputs = Command.CreateVarVec(projectOp.Outputs);
            oldProjectOpCollectionOutputs.Minus(oldProjectOpNonCollectionOutputs); 

            // (5) build a new list of collectionInfo's for the new NestOp, including
            //     only oldProjectOpCollectionOutputs.
            VarVec collectionsToRemove = Command.CreateVarVec(oldNestOpCollectionOutputs); 
            collectionsToRemove.Minus(oldProjectOpCollectionOutputs);
            List newCollectionInfoList; 
            List newNestNodeChildren; 

            if (collectionsToRemove.IsEmpty) { 
                newCollectionInfoList = nestOp.CollectionInfo;
                newNestNodeChildren = new List(nestNode.Children);
            }
            else { 
                newCollectionInfoList = new List();
                newNestNodeChildren = new List(); 
                newNestNodeChildren.Add(nestNode.Child0); 
                int i = 1;
                foreach (CollectionInfo ci in nestOp.CollectionInfo) { 
                    if (!collectionsToRemove.IsSet(ci.CollectionVar)) {
                        newCollectionInfoList.Add(ci);
                        newNestNodeChildren.Add(nestNode.Children[i]);
                    } 
                    i++;
                } 
            } 

            // (6) leftCorrelationVars = vars that are defined by the left most child of the input nestOpNode 
            //   and used in the subtrees rooted at the other children of the input nestOpNode
            //   #479547:  These need to be added to the outputs of the project
            VarVec leftCorrelationVars = Command.CreateVarVec();
            for (int i = 1; i < nestNode.Children.Count; i++) { 
                leftCorrelationVars.Or(nestNode.Children[i].GetExtendedNodeInfo(Command).ExternalReferences);
            } 
            leftCorrelationVars.And(nestNode.Child0.GetExtendedNodeInfo(this.Command).Definitions); 

            // (7) newProjectOpOutputs = oldProjectOpNonCollectionOutputs + oldNestOpNonCollectionOutputs + leftCorrelationVars 
            VarVec newProjectOpOutputs = Command.CreateVarVec(oldProjectOpNonCollectionOutputs);
            newProjectOpOutputs.Or(oldNestOpNonCollectionOutputs);
            newProjectOpOutputs.Or(leftCorrelationVars);
 
            // (8) newProjectOpChildren = ....
            List newProjectOpChildren = new List(projectNode.Child1.Children.Count); 
            foreach (Node chi in projectNode.Child1.Children) { 
                VarDefOp varDefOp = (VarDefOp)chi.Op;
 
                if (newProjectOpOutputs.IsSet(varDefOp.Var)) {
                    newProjectOpChildren.Add(chi);
                }
            } 

            // (9) and (10), do the switch. 
            if (0 != newCollectionInfoList.Count) { 
                // In some cases, the only var in the projection is the collection var; so
                // the new projectOp will have an empty projection list; we can't just pullup 
                // the input, so we add a temporary constant op to it, ensuring that we don't
                // have an empty projection list.
                if (newProjectOpOutputs.IsEmpty) {
                    PlanCompiler.Assert(newProjectOpChildren.Count == 0, "outputs is empty with non-zero count of children?"); 

                    NullOp tempOp = Command.CreateNullOp(Command.StringType); 
                    Node tempNode = Command.CreateNode(tempOp); 
                    Var tempVar;
                    Node varDefNode = Command.CreateVarDefNode(tempNode, out tempVar); 
                    newProjectOpChildren.Add(varDefNode);
                    newProjectOpOutputs.Set(tempVar);
                }
            } 

            // Update the projectOp node with the new list of vars and 
            // the new list of children. 
            projectNode.Op = Command.CreateProjectOp(Command.CreateVarVec(newProjectOpOutputs));
            projectNode.Child1 = Command.CreateNode(projectNode.Child1.Op, newProjectOpChildren); 

            if (0 == newCollectionInfoList.Count) {
                // There are no remaining nested collections (because none of them
                // were actually referenced)  We just pullup the driving node of the 
                // nest and eliminate the nestOp entirely.
                projectNode.Child0 = nestNode.Child0; 
                nestNode = projectNode; 
            }
            else { 
                // We need to make sure that we project out any external references to the driving
                // node that the nested collections have, or we're going to end up with unresolvable
                // vars when we pull them up over the current driving node.
                VarVec nestOpOutputs = Command.CreateVarVec(projectOp.Outputs); 

                for (int i = 1; i < newNestNodeChildren.Count; i++) { 
                    nestOpOutputs.Or(newNestNodeChildren[i].GetNodeInfo(Command).ExternalReferences); 
                }
 
                // We need to make sure we project out the sort keys too...
                foreach (InternalTrees.SortKey sk in nestOp.PrefixSortKeys) {
                    nestOpOutputs.Set(sk.Var);
                } 

                nestNode.Op = Command.CreateMultiStreamNestOp(nestOp.PrefixSortKeys, nestOpOutputs, newCollectionInfoList); 
 
                // we need to create a new node because we may have removed some of the collections.
                nestNode = Command.CreateNode(nestNode.Op, newNestNodeChildren); 

                // Pull the nestNode up over the projectNode, and adjust
                // their inputs accordingly.
                projectNode.Child0 = nestNode.Child0; 
                nestNode.Child0 = projectNode;
 
                Command.RecomputeNodeInfo(projectNode); 
            }
 
            // Finally, recompute node info
            Command.RecomputeNodeInfo(nestNode);
#if DEBUG
            int size = input.Length; // GC.KeepAlive makes FxCop Grumpy. 
            string output = Dump.ToXml(Command, nestNode);
#endif //DEBUG 
            return nestNode; 
        }
 
        /// 
        /// SetOp common processing
        /// 
        ///  
        /// The input to an IntersectOp or an ExceptOp cannot be a NestOp � that
        /// would imply that we support distinctness over collections  - which 
        /// we don�t. 
        ///
        /// UnionAllOp is somewhat trickier. We would need a way to percolate keys 
        /// up the UnionAllOp � and I�m ok with not supporting this case for now.
        /// 
        /// 
        ///  
        /// 
        protected override Node VisitSetOp(SetOp op, Node n) { 
            return NestingNotSupported(op, n); 
        }
 
        /// 
        /// SingleRowOp
        ///
        /// SingleRowOp(NestOp(x,...)) => NestOp(SingleRowOp(x),...) 
        /// 
        ///  
        ///  
        /// 
        public override Node Visit(SingleRowOp op, Node n) { 
            VisitChildren(n);

            if (IsNestOpNode(n.Child0)) {
                n = n.Child0; 
                Node newSingleRowOpNode = Command.CreateNode(op, n.Child0);
                n.Child0 = newSingleRowOpNode; 
                Command.RecomputeNodeInfo(n); 
            }
            return n; 
        }

        /// 
        /// SortOp 
        /// 
        ///  
        /// If the input to a SortOp is a NestOp, then none of the sort 
        /// keys can be collection Vars of the NestOp � we don't support
        /// sorts over collections. 
        /// 
        /// 
        /// 
        ///  
        public override Node Visit(SortOp op, Node n) {
            // Visit the children 
            VisitChildren(n); 
            m_varRemapper.RemapNode(n);
 
            // If the child is a NestOp, then simply push the sortkeys into the
            // "prefixKeys" of the nestOp, and return the NestOp itself.
            // The SortOp has now been merged into the NestOp
            NestBaseOp nestOp = n.Child0.Op as NestBaseOp; 
            if (nestOp != null) {
                n.Child0.Op = GetNestOpWithConsolidatedSortKeys(nestOp, op.Keys); 
                return n.Child0; 
            }
 
            return n;
        }

        ///  
        /// ConstrainedSortOp
        ///  
        ///  
        /// Push the ConstrainedSortOp onto the driving node of the NestOp:
        /// 
        ///     ConstrainedSortOp(NestOp(X,Y,...)) ==> NestOp(ConstrainedSortOp(X),Y,...)
        ///
        /// There should not be any need for var renaming, because the ConstrainedSortOp cannot
        /// refer to any vars from the NestOp 
        /// 
        ///  
        ///  
        /// 
        public override Node Visit(ConstrainedSortOp op, Node n) { 
            // Visit the children
            VisitChildren(n);

            // If the input is a nest op, we push the ConstrainedSort onto 
            // the driving node.
            NestBaseOp nestOp = n.Child0.Op as NestBaseOp; 
            if (nestOp != null) { 
                Node nestNode = n.Child0;
                n.Child0 = nestNode.Child0; 
                nestNode.Child0 = n;
                nestNode.Op = GetNestOpWithConsolidatedSortKeys(nestOp, op.Keys);
                n = nestNode;
            } 
            return n;
        } 
 
        /// 
        /// Helper method used by Visit(ConstrainedSortOp, Node)and Visit(SortOp, Node). 
        /// It returns a NestBaseOp equivalent to the inputNestOp, only with the given sortKeys
        /// prepended to the prefix sort keys already on the inputNestOp.
        /// 
        ///  
        /// 
        ///  
        private NestBaseOp GetNestOpWithConsolidatedSortKeys(NestBaseOp inputNestOp, List sortKeys) { 
            NestBaseOp result;
 
            // Include the  sort keys as the prefix sort keys;
            // Note that we can't actually have a SSNest at this point in
            // the tree; they're only introduced once we've processed the
            // entire tree. 

            if (inputNestOp.PrefixSortKeys.Count == 0) { 
                foreach (InternalTrees.SortKey sk in sortKeys) { 
                    //SQLBUDT #507170 - We can't just add the sort keys, we need to copy them,
                    // to avoid changes to one to affect the other 
                    inputNestOp.PrefixSortKeys.Add(Command.CreateSortKey(sk.Var, sk.AscendingSort, sk.Collation));
                }
                result = inputNestOp;
            } 
            else {
 
                VarVec sortVars = Command.CreateVarVec(); 

                // First add the sort keys from the SortBaseOp, then the NestOp keys 
                List sortKeyList = ConsolidateSortKeys(sortKeys, inputNestOp.PrefixSortKeys);

                PlanCompiler.Assert(inputNestOp is MultiStreamNestOp, "Unexpected SingleStreamNestOp?");
 
                // Finally, build a new NestOp with the keys...
                result = Command.CreateMultiStreamNestOp(sortKeyList, inputNestOp.Outputs, inputNestOp.CollectionInfo); 
            } 
            return result;
        } 

        /// 
        /// Helper method that given two lists of sort keys creates a single list of sort keys without duplicates.
        /// First the keys from the first given list are added, then from the second one. 
        /// 
        ///  
        ///  
        /// 
        private List ConsolidateSortKeys(List sortKeyList1, List sortKeyList2) { 
            VarVec sortVars = Command.CreateVarVec();
            List sortKeyList = new List();

            foreach (InternalTrees.SortKey sk in sortKeyList1) { 
                if (!sortVars.IsSet(sk.Var)) {
                    sortVars.Set(sk.Var); 
 
                    //SQLBUDT #507170 - We can't just add the sort keys, we need to copy them,
                    // to avoid changes to one to affect the other 
                    sortKeyList.Add(Command.CreateSortKey(sk.Var, sk.AscendingSort, sk.Collation));
                }
            }
 
            foreach (InternalTrees.SortKey sk in sortKeyList2) {
                if (!sortVars.IsSet(sk.Var)){ 
                    sortVars.Set(sk.Var); 
                    sortKeyList.Add(Command.CreateSortKey(sk.Var, sk.AscendingSort, sk.Collation));
                } 
            }

            return sortKeyList;
        } 

        ///  
        /// UnnestOp 
        /// 
        ///  
        /// Logically, the UnnestOp can simply be replaced with the defining expression
        /// corresponding to the Var property of the UnnestOp. The tricky part is that
        /// the UnnestOp produces a set of ColumnVars which may be referenced in other
        /// parts of the query, and these need to be replaced by the corresponding Vars 
        /// produced by the defining expression.
        /// 
        /// There are essentially four cases: 
        ///
        /// Case 1: The UnnestOps Var is a UDT. Only the store can handle this, so we 
        ///         pass it on without changing it.
        ///
        /// Case 2: The UnnestOp has a Function as its input.  This implies that the
        ///         store has TVFs, which it can Unnest, so we let it handle that and do 
        ///         nothing.
        /// 
        /// Case 3: The UnnestOp Var defines a Nested collection.  We'll just replace 
        ///         the UnnestOp with the Input:
        /// 
        ///             UnnestOp(VarDef(CollectOp(PhysicalProjectOp(input)))) => input
        ///
        /// Case 4: The UnnestOp Var refers to a Nested collection from elsewhere.  As we
        ///         discover NestOps, we maintain a var->PhysicalProject Node map.  When 
        ///         we get this case, we just make a copy of the PhysicalProject node, for
        ///         the referenced Var, and we replace the UnnestOp with it. 
        /// 
        ///             UnnestOp(VarDef(VarRef(v))) ==> copy-of-defining-node-for-v
        /// 
        /// Then, we need to update all references to the output Vars (ColumnVars) produced
        /// by the Unnest to instead refer to the Vars produced by the copy of the subquery.
        /// We produce a map from the Vars of the subquery to the corresponding vars of the
        /// UnnestOp. We then use this map as we walk up the tree, and replace any references 
        /// to the Unnest Vars by the new Vars.
        /// 
        /// To simplify this process, as part of the ITreeGenerator, whenever we generate 
        /// an UnnestOp, we will generate a ProjectOp above it � which simply selects out
        /// all Vars from the UnnestOp; and has no local definitions. This allows us to 
        /// restrict the Var->Var replacement to just ProjectOp.
        /// 
        /// 
        ///  
        /// 
        public override Node Visit(UnnestOp op, Node n) { 
#if DEBUG 
            string input = Dump.ToXml(Command, n);
#endif //DEBUG 
            // First, visit my children
            VisitChildren(n);

            // If we're unnesting a UDT, then simply return - we cannot eliminate this unnest 
            // It must be handled by the store
            md.CollectionType collType = TypeHelpers.GetEdmType(op.Var.Type); 
            if (TypeUtils.IsUdt(collType.TypeUsage)) { 
                return n;
            } 

            // Find the VarDef node for the var we're supposed to unnest.
            PlanCompiler.Assert(n.Child0.Op.OpType == OpType.VarDef, "Unnest without VarDef input?");
            PlanCompiler.Assert(((VarDefOp)n.Child0.Op).Var == op.Var, "Unnest var not found?"); 
            PlanCompiler.Assert(n.Child0.HasChild0, "VarDef without input?");
            Node newNode = n.Child0.Child0; 
 
            if (OpType.Function == newNode.Op.OpType) {
                // If we have an unnest over a function, there's nothing more we can do 
                // This really means that the underlying store has the ability to
                // support TVFs, and therefore unnests, and we simply leave it as is
                return n;
            } 
            else if (OpType.Collect == newNode.Op.OpType) {
                // UnnestOp(VarDef(CollectOp(PhysicalProjectOp(x)))) ==> x 
 
                PlanCompiler.Assert(newNode.HasChild0, "collect without input?");
                newNode = newNode.Child0; 

                PlanCompiler.Assert(newNode.Op.OpType == OpType.PhysicalProject, "collect without physicalProject?");

                // Ensure others that reference my var will know to use me; 
                m_definingNodeMap.Add(op.Var, newNode);
            } 
            else if (OpType.VarRef == newNode.Op.OpType) { 
                // UnnestOp(VarDef(VarRef(v))) ==> copy-of-defining-node-for-v
                // 
                // The Unnest's input is a VarRef; we need to replace it with
                // the defining node, and ensure we fixup the vars.
                Var refVar = ResolveVarReference(((VarRefOp)newNode.Op).Var);
 
                Node refVarDefiningNode;
                bool found = m_definingNodeMap.TryGetValue(refVar, out refVarDefiningNode); 
                PlanCompiler.Assert(found, "Could not find a definition for a referenced collection var"); 

                newNode = CopyCollectionVarDefinition(refVarDefiningNode); 

                PlanCompiler.Assert(newNode.Op.OpType == OpType.PhysicalProject, "driving node is not physicalProject?");
            }
            else { 
                throw EntityUtil.InternalError(EntityUtil.InternalErrorCode.InvalidInternalTree, 2, newNode.Op.OpType);
            } 
 
            IEnumerable inputVars = ((PhysicalProjectOp)newNode.Op).Outputs;
 
            PlanCompiler.Assert(newNode.HasChild0, "physicalProject without input?");
            newNode = newNode.Child0;

            // Bug 480721: a SortOp input to Unnest is irrelevant - eliminate it 
            if (newNode.Op.OpType == OpType.Sort) {
                newNode = newNode.Child0; 
            } 

            // Update the replacement vars to reflect the pulled up operation 
            UpdateReplacementVarMap(op.Table.Columns, inputVars);

#if DEBUG
            int size = input.Length; // GC.KeepAlive makes FxCop Grumpy. 
            string output = Dump.ToXml(Command, newNode);
#endif //DEBUG 
            return newNode; 
        }
 
        /// 
        /// Copies the given defining node for a collection var, but also makes sure to 'register' all newly
        /// created collection vars (i.e. copied).
        /// 
        ///SQLBUDT #557427: The defining node that is being copied may itself contain definitions to other
        /// collection vars. These defintions would be present in m_definingNodeMap. However, after we make a copy 
        /// of the defining node, we need to make sure to also put 'matching' definitions of these other collection 
        /// vars into m_definingNodeMap.
        /// The dictionary collectionVarDefinitions (below) contains the copied definitions of such collection vars. 
        /// but without the wrapping PhysicalProjectOp.
        ///
        /// Example:     m_definingNodeMap contains (var1, definition1) and (var2, definintion2).
        ///              var2 is defined inside the definition of var1. 
        ///              Here we copy definition1 -> definintion1'.
        ///              We need to add to m_definitionNodeMap (var2', definition2'). 
        ///              definition2' should be a copy of definiton2 in the context of to definition1', 
        ///              i.e. definition2' should relate to definition1' in same way that definition2 relates to definition1
        ///         ///  
        /// 
        /// 
        private Node CopyCollectionVarDefinition(Node refVarDefiningNode) {
 
            VarMap varMap;
            Dictionary collectionVarDefinitions; 
            Node newNode = OpCopierTrackingCollectionVars.Copy(Command, refVarDefiningNode, out varMap, out collectionVarDefinitions); 

            if (collectionVarDefinitions.Count != 0) { 
                VarMap reverseMap = varMap.GetReverseMap();

                foreach (KeyValuePair collectionVarDefinitionPair in collectionVarDefinitions) {
                    // 
                    // Getting the matching definition for a collection map (i.e. definition2' from the example above)
                    // 
                    // Definitions of collection vars are rooted at a PhysicalProjectOp, 
                    //      i.e. definition2 = PhysicalProjectOp(output2, columnMap2, definingSubtree2)
                    // 
                    //  The collectionVarDefinitions dictionary gives us the defining nodes rooted at what would a child
                    //  of such PhysicalProjectOp, i.e.  definingSubtree2'.
                    //
                    //  definition2' = PhysicalProjectOp(CopyWithRemap(output2), CopyWithRemap(columnMap2), definingSubtree2') 
                    //
 
                    Node keyDefiningNode; 
                    Var keyDefiningVar = reverseMap[collectionVarDefinitionPair.Key];
                    //Note: we should not call ResolveVarReference(keyDefiningNode), we can only use the exact var 
                    if (m_definingNodeMap.TryGetValue(keyDefiningVar, out keyDefiningNode)) {
                        PhysicalProjectOp originalPhysicalProjectOp = (PhysicalProjectOp)keyDefiningNode.Op;

                        VarList newOutputs = VarRemapper.RemapVarList(Command, varMap, originalPhysicalProjectOp.Outputs); 
                        SimpleCollectionColumnMap newColumnMap = (SimpleCollectionColumnMap)ColumnMapCopier.Copy(originalPhysicalProjectOp.ColumnMap, varMap);
 
                        PhysicalProjectOp newPhysicalProjectOp = Command.CreatePhysicalProjectOp(newOutputs, newColumnMap); 
                        Node newDefiningNode = Command.CreateNode(newPhysicalProjectOp, collectionVarDefinitionPair.Value);
 
                        m_definingNodeMap.Add(collectionVarDefinitionPair.Key, newDefiningNode);
                    }
                }
            } 
            return newNode;
        } 
 
        #endregion
 
        #region PhysicalOp Visitors

        /// 
        /// MultiStreamNestOp/SingleStreamNestOp common processing. 
        ///
        /// Pretty much just verifies that we didn't leave a NestOp behind. 
        ///  
        /// 
        ///  
        /// 
        ///
        protected override Node VisitNestOp(NestBaseOp op, Node n) {
            // First, visit my children 
            VisitChildren(n);
 
            // If any of the children are a nestOp, then we have a 
            // problem; it shouldn't have happened.
            foreach (Node chi in n.Children) { 
                if (IsNestOpNode(chi)) {
                    throw EntityUtil.InternalError(EntityUtil.InternalErrorCode.NestOverNest);
                }
            } 
            return n;
        } 
 
        /// 
        /// PhysicalProjectOp 
        /// 
        /// 
        /// Tranformation:
        /// 
        ///     PhysicalProjectOp(MultiStreamNestOp(...)) => PhysicalProjectOp(SortOp(...))
        /// 
        /// Strategy: 
        ///
        ///     (1) Convert MultiStreamNestOp(...) => SingleStreamNestOp(...) 
        ///     (2) Convert SingleStreamNestOp(...) => SortOp(...)
        ///     (3) Fixup the column maps.
        ///
        ///  
        /// 
        ///  
        ///  
        public override Node Visit(PhysicalProjectOp op, Node n) {
            // cannot be multi-input (not at this point) 
            PlanCompiler.Assert(n.Children.Count == 1, "multiple inputs to physicalProject?");

            // First visit my children
            VisitChildren(n); 
            m_varRemapper.RemapNode(n);
 
            // Wait until we're processing the root physicalProjectNode to convert the nestOp 
            // to sort/union all; it's much easier to unnest them if we don't monkey with them
            // until then. 
            //
            // Also, even if we're the root physicalProjectNode and the children aren't NestOps,
            // then there's nothing further to do.
            if (n != Command.Root || !IsNestOpNode(n.Child0)) { 
                return n;
            } 
 
#if DEBUG
            string input = Dump.ToXml(Command, n); 
#endif //DEBUG

            Node nestNode = n.Child0;
 
            // OK, we're now guaranteed to be processing a root physicalProjectNode with at
            // least one MultiStreamNestOp as it's input.  First step is to convert that into 
            // a single SingleStreamNestOp. 
            //
            // NOTE: if we ever wanted to support MARS, we would probably avoid the conversion 
            //       to SingleStreamNest here, and do something to optimize this a bit
            //       differently for MARS.  But that's a future feature.
            Dictionary varRefReplacementMap = new Dictionary();
            VarList outputVars; 
            SimpleColumnMap[] keyColumnMaps;
 
            nestNode = ConvertToSingleStreamNest(nestNode, varRefReplacementMap, out outputVars, out keyColumnMaps); 
            SingleStreamNestOp ssnOp = (SingleStreamNestOp)nestNode.Op;
 
            // Build up the sort node (if necessary).
            Node sortNode = BuildSortForNestElimination(ssnOp, nestNode);

            // Create a new column map using the columnMapPatcher that was updated by the 
            // conversion to SingleStreamNest process.
            SimpleCollectionColumnMap newProjectColumnMap = (SimpleCollectionColumnMap)ColumnMapTranslator.Translate(((PhysicalProjectOp)n.Op).ColumnMap, varRefReplacementMap); 
            newProjectColumnMap = new SimpleCollectionColumnMap(newProjectColumnMap.Type, newProjectColumnMap.Name, newProjectColumnMap.Element, keyColumnMaps, null, newProjectColumnMap.SortKeys); 

            // Ok, build the new PhysicalProjectOp, slap the sortNode as its input 
            // and we're all done.
            n.Op = Command.CreatePhysicalProjectOp(outputVars, newProjectColumnMap);
            n.Child0 = sortNode;
 
#if DEBUG
            int size = input.Length;// GC.KeepAlive makes FxCop Grumpy. 
            string output = Dump.ToXml(Command, n); 
#endif //DEBUG
 
            return n;
        }

        ///  
        /// Build up a sort node above the nestOp's input - only if there
        /// are any sort keys to produce 
        ///  
        /// 
        ///  
        /// 
        private Node BuildSortForNestElimination(SingleStreamNestOp ssnOp, Node nestNode) {
            Node sortNode;
 
            List sortKeyList = BuildSortKeyList(ssnOp);
 
            // Now if, at this point, there aren't any sort keys then remove the 
            // sort operation, otherwise, build a new SortNode;
            if (sortKeyList.Count > 0) { 
                SortOp sortOp = Command.CreateSortOp(sortKeyList);
                sortNode = Command.CreateNode(sortOp, nestNode.Child0);
            }
            else { 
                // No sort keys => single_row_table => no need to sort
                sortNode = nestNode.Child0; 
            } 
            return sortNode;
        } 

        /// 
        /// Build up the list of sortkeys. This list should comprise (in order):
        /// 
        ///  - Any prefix sort keys (these represent sort operations on the
        ///    driving table, that were logically above the nest) 
        ///  - The keys of the nest operation 
        ///  - The discriminator column for the nest operation
        ///  - the list of postfix sort keys (used to represent nested collections) 
        ///
        /// Note that we only add the first occurrance of a var to the list; further
        /// references to the same variable would be trumped by the first one.
        ///  
        /// 
        ///  
        private List BuildSortKeyList(SingleStreamNestOp ssnOp) { 
            VarVec sortVars = Command.CreateVarVec();
 
            // First add the prefix sort keys
            List sortKeyList = new List();
            foreach (InternalTrees.SortKey sk in ssnOp.PrefixSortKeys) {
                if (!sortVars.IsSet(sk.Var)) { 
                    sortVars.Set(sk.Var);
                    sortKeyList.Add(sk); 
                } 
            }
 
            // Then add the nestop keys
            foreach (Var v in ssnOp.Keys) {
                if (!sortVars.IsSet(v)) {
                    sortVars.Set(v); 
                    InternalTrees.SortKey sk = Command.CreateSortKey(v);
                    sortKeyList.Add(sk); 
                } 
            }
 
            // Then add the discriminator var
            PlanCompiler.Assert(!sortVars.IsSet(ssnOp.Discriminator), "prefix sort on discriminator?");
            sortKeyList.Add(Command.CreateSortKey(ssnOp.Discriminator));
 
            // Finally, add the postfix keys
            foreach (InternalTrees.SortKey sk in ssnOp.PostfixSortKeys) { 
                if (!sortVars.IsSet(sk.Var)) { 
                    sortVars.Set(sk.Var);
                    sortKeyList.Add(sk); 
                }
            }
            return sortKeyList;
        } 

 
        ///  
        /// convert MultiStreamNestOp to SingleStreamNestOp
        ///  
        /// 
        /// A MultiStreamNestOp is typically of the form M(D, N1, N2, ..., Nk)
        /// where D is the driver stream, and N1, N2 etc. represent the collections.
        /// 
        /// In general, this can be converted into a SingleStreamNestOp over:
        /// 
        ///    (D+ outerApply N1) AugmentedUnionAll (D+ outerApply N2) ... 
        ///
        /// Where: 
        ///
        ///     D+ is D with an extra discriminator column that helps to identify
        ///     the specific collection.
        /// 
        ///     AugmentedUnionAll is simply a unionAll where each branch of the
        ///     unionAll is augmented with nulls for the corresponding columns 
        ///     of other tables in the branch 
        ///
        /// The simple case where there is only a single nested collection is easier 
        /// to address, and can be represented by:
        ///
        ///     MultiStreamNest(D, N1) => SingleStreamNest(OuterApply(D, N1))
        /// 
        /// The more complex case, where there is more than one nested column, requires
        /// quite a bit more work: 
        /// 
        ///     MultiStreamNest(D, X, Y,...) => SingleStreamNest(UnionAll(Project{"1", D1...Dn, X1...Xn, nY1...nYn}(OuterApply(D, X)), Project{"2", D1...Dn, nX1...nXn, Y1...Yn}(OuterApply(D, Y)), ...))
        /// 
        /// Where:
        ///      D           is the driving collection
        ///      D1...Dn     are the columns from the driving collection
        ///      X           is the first nested collection 
        ///      X1...Xn     are the columns from the first nested collection
        ///      nX1...nXn   are null values for all columns from the first nested collection 
        ///      Y           is the second nested collection 
        ///      Y1...Yn     are the columns from the second nested collection
        ///      nY1...nYn   are null values for all columns from the second nested collection 
        /// 
        /// 
        /// 
        ///  
        /// 
        ///  
        private Node ConvertToSingleStreamNest(Node nestNode, Dictionary varRefReplacementMap, out VarList flattenedOutputVarList, out SimpleColumnMap[] parentKeyColumnMaps) { 
#if DEBUG
            string input = Dump.ToXml(Command, nestNode); 
#endif //DEBUG
            MultiStreamNestOp nestOp = (MultiStreamNestOp)nestNode.Op;

            // We can't convert this node to a SingleStreamNest until all it's MultiStreamNest 
            // inputs are converted, so do that first.
            for (int i = 1; i < nestNode.Children.Count; i++) { 
                Node chi = nestNode.Children[i]; 

                if (chi.Op.OpType == OpType.MultiStreamNest) { 
                    CollectionInfo chiCi = nestOp.CollectionInfo[i - 1];

                    VarList childFlattenedOutputVars;
                    SimpleColumnMap[] childKeyColumnMaps; 

                    nestNode.Children[i] = ConvertToSingleStreamNest(chi, varRefReplacementMap, out childFlattenedOutputVars, out childKeyColumnMaps); 
 
                    // Now this may seem odd here, and it may look like we should have done this
                    // inside the recursive ConvertToSingleStreamNest call above, but that call 
                    // doesn't have access to the CollectionInfo for it's parent, which is what
                    // we need to manipulate before we enter the loop below where we try and fold
                    // THIS nestOp nodes into a singleStreamNestOp.
                    ColumnMap childColumnMap = ColumnMapTranslator.Translate(chiCi.ColumnMap, varRefReplacementMap); 

                    VarVec childKeys = Command.CreateVarVec(((SingleStreamNestOp)nestNode.Children[i].Op).Keys); 
 
                    nestOp.CollectionInfo[i - 1] = Command.CreateCollectionInfo(chiCi.CollectionVar,
                                                                                  childColumnMap, 
                                                                                  childFlattenedOutputVars,
                                                                                  childKeys,
                                                                                  chiCi.SortKeys,
                                                                                  null /*discriminatorValue*/ 
                                                                                  );
                } 
            } 

            // Make sure that the driving node has keys defined. Otherwise we're in 
            // trouble; we must be able to infer keys from the driving node; If it's
            // available, we'll use the NewGuid canonical function to put a key on the
            // driving node.
            Node drivingNode = nestNode.Child0; 
            KeyVec drivingNodeKeys = Command.PullupKeys(drivingNode);
 
            // Get a deterministic ordering of Vars from this node. 
            // NOTE: we're using the drivingNode's definitions, which is a VarVec so it
            //       won't match the order of the input's columns, but the key thing is 
            //       that we use the same order for all nested children, so it's OK.
            ExtendedNodeInfo drivingNodeInfo = Command.GetExtendedNodeInfo(drivingNode);
            VarVec drivingNodeVarVec = drivingNodeInfo.Definitions;
            VarList drivingNodeVars = Command.CreateVarList(drivingNodeVarVec); 

            if (drivingNodeKeys.NoKeys) { 
                // No Keys.  We'll Transform the statement to add a NewGuid as follows: 
                //
                //      MultiStreamNest(D, X, Y,...) => MultiStreamNest(Project{NewGuid},(D), X, Y,...) 

                md.EdmFunction newGuidFunction;

                if (!TryGetNewGuidFunction(out newGuidFunction)) { 
                    // If we can't get the NewGuid canonical function, we'll just throw
                    // an exception indicating that we need keys. 
                    throw EntityUtil.KeysRequiredForNesting(); 
                }
 
                // Construct the FunctionOp, the VarDef and The VarDefList to call the
                // NewGuid canonical function,
                Var newGuidVar;
                Node newVarDefListNode = Command.CreateVarDefListNode( 
                                            Command.CreateNode(
                                                Command.CreateFunctionOp(newGuidFunction)), out newGuidVar); 
 
                // We need to make a copy of the drivingNodeVarVec, because we don't want
                // to modify the one that's in the extendedNodeInfo.  Once we do, we set 
                // the var from the NewGuid in both it and the varlist.
                drivingNodeVarVec = Command.CreateVarVec(drivingNodeVarVec);
                drivingNodeVarVec.Set(newGuidVar);
                drivingNodeVars.Add(newGuidVar); 

                // While we're at it, make sure we update the KeyVars to include the var from 
                // the NewGuid, effectively solveing our problem. 
                drivingNodeKeys = new KeyVec(Command);
                drivingNodeKeys.KeyVars.Set(newGuidVar); 

                // OK, construct a new projectOp and node to go above the driving node, and
                // hook it into the driving node of the nest op.
                drivingNode = Command.CreateNode(Command.CreateProjectOp(drivingNodeVarVec), drivingNode, newVarDefListNode); 
                nestNode.Child0 = drivingNode;
 
                // Finally, we have to create a new MultiStreamNestOp that includes the key 
                // value we just added in its outputs.
                VarVec newNestopOutputs = Command.CreateVarVec(nestOp.Outputs); 
                newNestopOutputs.Set(newGuidVar);
                nestOp = Command.CreateMultiStreamNestOp(nestOp.PrefixSortKeys, newNestopOutputs, nestOp.CollectionInfo);
                nestNode.Op = nestOp;
                Command.RecomputeNodeInfo(nestNode); 
            }
 
            // Normalize all collection inputs to the nestOp. Specifically, remove any 
            // SortOps (adding the sort keys to the postfix sortkey list). Additionally,
            // add a discriminatorVar to each collection child 
            VarList discriminatorVarList;
            List> postfixSortKeyList;
            NormalizeNestOpInputs(nestOp, nestNode, out discriminatorVarList, out postfixSortKeyList);
 
            // Now build up the union-all subquery
            List> varMapList; 
            Var outputDiscriminatorVar; 
            Node unionAllNode = BuildUnionAllSubqueryForNestOp(nestOp, nestNode, drivingNodeVars, discriminatorVarList, out outputDiscriminatorVar, out varMapList);
            Dictionary drivingNodeVarMap = varMapList[0]; 

            // OK.  We've finally created the UnionAll over each of the project/outerApply
            // combinations.  We know that the output columns will be:
            // 
            //      Discriminator, DrivingColumns, Collection1Columns, Collection2Columns, ...
            // 
            // Now, rebuild the columnMaps, since all of the columns in the original column 
            // maps are now referencing newer variables.  To do that, we'll walk the list of
            // outputs from the unionAll, and construct new VarRefColumnMaps for each one, 
            // and adding it to a ColumnMapPatcher, which we'll use to actually fix everything
            // up.
            //
            // While we're at it, we'll build a new list of top-level output columns, which 
            // should include only the Discriminator, the columns from the driving collection,
            // and and one column for each of the nested collections. 
 
            // Start building the flattenedOutputVarList that the top level PhysicalProjectOp
            // is to output. 
            flattenedOutputVarList = Command.CreateVarList(RemapVars(drivingNodeVars, drivingNodeVarMap));
            VarVec flattenedOutputVarVec = Command.CreateVarVec(flattenedOutputVarList);
            VarVec nestOpOutputs = Command.CreateVarVec(flattenedOutputVarVec);
 
            // Add any adjustments to the driving nodes vars to the column map patcher
            foreach (KeyValuePair kv in drivingNodeVarMap) { 
                if (kv.Key != kv.Value) { 
                    varRefReplacementMap[kv.Key] = new VarRefColumnMap(kv.Value);
                } 
            }

            RemapSortKeys(nestOp.PrefixSortKeys, drivingNodeVarMap);
 
            List newPostfixSortKeys = new List();
            List newCollectionInfoList = new List(); 
 
            // Build the discriminator column map, and ensure it's in the outputs
            VarRefColumnMap discriminatorColumnMap = new VarRefColumnMap(outputDiscriminatorVar); 
            nestOpOutputs.Set(outputDiscriminatorVar);

            if (!flattenedOutputVarVec.IsSet(outputDiscriminatorVar)) {
                flattenedOutputVarList.Add(outputDiscriminatorVar); 
                flattenedOutputVarVec.Set(outputDiscriminatorVar);
            } 
 
            // Build the key column maps, and ensure they're in the outputs as well.
            VarVec parentKeys = RemapVarVec(drivingNodeKeys.KeyVars, drivingNodeVarMap); 
            parentKeyColumnMaps = new SimpleColumnMap[parentKeys.Count];

            int index = 0;
            foreach (Var keyVar in parentKeys) { 
                parentKeyColumnMaps[index] = new VarRefColumnMap(keyVar);
                index++; 
 
                if (!flattenedOutputVarVec.IsSet(keyVar)) {
                    flattenedOutputVarList.Add(keyVar); 
                    flattenedOutputVarVec.Set(keyVar);
                }
            }
 
            // Now that we've handled the driving node, deal with each of the
            // nested inputs, in sequence. 
            for (int i = 1; i < nestNode.Children.Count; i++) { 
                CollectionInfo ci = nestOp.CollectionInfo[i - 1];
                List postfixSortKeys = postfixSortKeyList[i]; 

                RemapSortKeys(postfixSortKeys, varMapList[i]);
                newPostfixSortKeys.AddRange(postfixSortKeys);
 
                ColumnMap newColumnMap = ColumnMapTranslator.Translate(ci.ColumnMap, varMapList[i]);
                VarList newFlattenedElementVars = RemapVarList(ci.FlattenedElementVars, varMapList[i]); 
                VarVec newCollectionKeys = RemapVarVec(ci.Keys, varMapList[i]); 

                RemapSortKeys(ci.SortKeys, varMapList[i]); 

                CollectionInfo newCollectionInfo = Command.CreateCollectionInfo(
                                                                                ci.CollectionVar,
                                                                                newColumnMap, 
                                                                                newFlattenedElementVars,
                                                                                newCollectionKeys, 
                                                                                ci.SortKeys, 
                                                                                i);
                newCollectionInfoList.Add(newCollectionInfo); 

                // For a collection Var, we add the flattened elementVars for the
                // collection in place of the collection Var itself, and we create
                // a new column map to represent all the stuff we've done. 

                foreach (Var v in newFlattenedElementVars) { 
                    if (!flattenedOutputVarVec.IsSet(v)) { 
                        flattenedOutputVarList.Add(v);
                        flattenedOutputVarVec.Set(v); 
                    }
                }

                nestOpOutputs.Set(ci.CollectionVar); 

                int keyColumnMapIndex = 0; 
                SimpleColumnMap[] keyColumnMaps = new SimpleColumnMap[newCollectionInfo.Keys.Count]; 
                foreach (Var keyVar in newCollectionInfo.Keys) {
                    keyColumnMaps[keyColumnMapIndex] = new VarRefColumnMap(keyVar); 
                    keyColumnMapIndex++;
                }

                SortKeyInfo[] sortKeyColumnMaps = new SortKeyInfo[newCollectionInfo.SortKeys.Count]; 
                for(int k = 0; k < sortKeyColumnMaps.Length; ++k) {
                    InternalTrees.SortKey sortKey = newCollectionInfo.SortKeys[k]; 
                    VarRefColumnMap varRefColumnMap = new VarRefColumnMap(sortKey.Var); 
                    sortKeyColumnMaps[k] = new SortKeyInfo(varRefColumnMap, sortKey.AscendingSort, sortKey.Collation);
                } 

                DiscriminatedCollectionColumnMap collectionColumnMap = new DiscriminatedCollectionColumnMap(
                                                                            TypeUtils.CreateCollectionType(newCollectionInfo.ColumnMap.Type),
                                                                            newCollectionInfo.ColumnMap.Name, 
                                                                            newCollectionInfo.ColumnMap,
                                                                            keyColumnMaps, 
                                                                            parentKeyColumnMaps, 
                                                                            sortKeyColumnMaps,
                                                                            discriminatorColumnMap, 
                                                                            newCollectionInfo.DiscriminatorValue
                                                                            );
                varRefReplacementMap[ci.CollectionVar] = collectionColumnMap;
            } 

            // Finally, build up the SingleStreamNest Node 
            SingleStreamNestOp newSsnOp = Command.CreateSingleStreamNestOp( 
                                                            parentKeys,
                                                            nestOp.PrefixSortKeys, 
                                                            newPostfixSortKeys,
                                                            nestOpOutputs,
                                                            newCollectionInfoList,
                                                            outputDiscriminatorVar); 
            Node newNestNode = Command.CreateNode(newSsnOp, unionAllNode);
 
#if DEBUG 
            int size = input.Length;// GC.KeepAlive makes FxCop Grumpy.
            string output = Dump.ToXml(Command, newNestNode); 
#endif //DEBUG

            return newNestNode;
        } 

        ///  
        /// "Normalize" each input to the NestOp. 
        /// We're now in the context of a MultiStreamNestOp, and we're trying to convert this
        /// into a SingleStreamNestOp. 
        ///
        /// Normalization specifically refers to
        ///   - augmenting each input with a discriminator value (that describes the collection)
        ///   - removing the sort node at the root (and capturing this information as part of the sortkeys) 
        /// 
        /// the nestOp 
        /// the nestOp subtree 
        /// Discriminator Vars for each Collection input
        /// SortKeys (postfix) for each Collection input 
        ///
        ///
        private void NormalizeNestOpInputs(NestBaseOp nestOp, Node nestNode, out VarList discriminatorVarList, out List> sortKeys) {
            discriminatorVarList = Command.CreateVarList(); 

            // We insert a dummy var and value at poistion 0 for the deriving node, which 
            // we should never reference; 
            discriminatorVarList.Add(null);
 
            sortKeys = new List>();
            sortKeys.Add(nestOp.PrefixSortKeys);

            for (int i = 1; i < nestNode.Children.Count; i++) { 
                Node inputNode = nestNode.Children[i];
                // Since we're called from ConvertToSingleStreamNest, it is possible that we have a 
                // SingleStreamNest here, because the input to the MultiStreamNest we're converting 
                // may have been a MultiStreamNest that was converted to a SingleStreamNest.
                SingleStreamNestOp ssnOp = inputNode.Op as SingleStreamNestOp; 

                // If this collection is a SingleStreamNest, we pull up the key information
                // in it, and pullup the input;
                if (null != ssnOp) { 
                    // Note that the sortKeys argument is 1:1 with the nestOp inputs, that is
                    // each input may have exactly one entry in the list, so we have to combine 
                    // all of the sort key components (Prefix+Keys+Discriminator+PostFix) into 
                    // one list.
                    List mySortKeys = BuildSortKeyList(ssnOp); 
                    sortKeys.Add(mySortKeys);

                    inputNode = inputNode.Child0;
                } 
                else {
                    // If the current collection has a SortNode specified, then pull that 
                    // out, and add the information to the list of postfix SortColumns 
                    SortOp sortOp = inputNode.Op as SortOp;
                    if (null != sortOp) { 
                        inputNode = inputNode.Child0; // bypass the sort node
                        // Add the sort keys to the list of postfix sort keys
                        sortKeys.Add(sortOp.Keys);
                    } 
                    else {
                        // No postfix sort keys for this case 
                        sortKeys.Add(new List()); 
                    }
                } 

                // #447304: Ensure that any SortKey Vars will be projected from the input in addition to showing up in the postfix sort keys
                // by adding them to the FlattenedElementVars for this NestOp input's CollectionInfo.
                VarList flattenedElementVars = nestOp.CollectionInfo[i - 1].FlattenedElementVars; 
                foreach (InternalTrees.SortKey sortKey in sortKeys[i]) {
                    if (!flattenedElementVars.Contains(sortKey.Var)) { 
                        flattenedElementVars.Add(sortKey.Var); 
                    }
                } 

                // Add a discriminator column to the collection-side - this must
                // happen before the outer-apply is added on; we need to use the value of
                // the discriminator to distinguish between null and empty collections 
                Var discriminatorVar;
                Node augmentedInput = AugmentNodeWithInternalConstant(inputNode, i, out discriminatorVar); 
                nestNode.Children[i] = augmentedInput; 
                discriminatorVarList.Add(discriminatorVar);
            } 
        }

        /// 
        /// Add an internal integer constant to a node. Specifically: 
        ///
        ///     N ==> Project(N,{definitions-from-N, internalConstant}) 
        ///  
        /// the input node to augment
        /// the value for the itnernal constant  
        /// the computed Var for the internal constant
        /// the augmented node
        private Node AugmentNodeWithInternalConstant(Node input, int internalConstantValue, out Var internalConstantVar) {
            // Construct the internal constant value, a ConstantOp node, and 
            // a VarDef node that that defines it.
            InternalConstantOp discriminatorOp = Command.CreateInternalConstantOp(Command.IntegerType, internalConstantValue); 
            Node discriminatorNode = Command.CreateNode(discriminatorOp); 
            Node varDefListNode = Command.CreateVarDefListNode(discriminatorNode, out internalConstantVar);
 
            // Now identify the list of definitions from the input, and project out
            // every one of them and include the internalConstantVar
            ExtendedNodeInfo inputNodeInfo = Command.GetExtendedNodeInfo(input);
            VarVec projectOutputs = Command.CreateVarVec(inputNodeInfo.Definitions); 
            projectOutputs.Set(internalConstantVar);
 
            ProjectOp projectOp = Command.CreateProjectOp(projectOutputs); 
            Node projectNode = Command.CreateNode(projectOp, input, varDefListNode);
 
            return projectNode;
        }

        ///  
        /// Convert a SingleStreamNestOp into a massive UnionAllOp
        ///  
        ///  
        /// 
        ///  
        /// 
        ///
        /// 
        ///  
        /// 
        private Node BuildUnionAllSubqueryForNestOp(NestBaseOp nestOp, Node nestNode, VarList drivingNodeVars, VarList discriminatorVarList, out Var discriminatorVar, out List> varMapList) { 
            Node drivingNode = nestNode.Child0; 

            // For each of the NESTED collections... 
            Node unionAllNode = null;
            VarList unionAllOutputs = null;
            for (int i = 1; i < nestNode.Children.Count; i++) {
                // Ensure we only use the driving collection tree once, so other 
                // transformations do not unintentionally change more than one path.
                // To prevent nodes in the tree from being used in multiple paths, 
                // we copy the driving input on successive nodes. 
                VarList newDrivingNodeVars;
                Node newDrivingNode; 
                VarList newFlattenedElementVars;
                Op op;

                if (i > 1) { 
                    newDrivingNode = OpCopier.Copy(Command, drivingNode, drivingNodeVars, out newDrivingNodeVars);
                    // 
                    // Bug 450245: If we copied the driver node, then references to driver node vars 
                    // from the collection subquery must be patched up
                    // 
                    VarRemapper varRemapper = new VarRemapper(this.Command);
                    for (int j = 0; j < drivingNodeVars.Count; j++) {
                        varRemapper.AddMapping(drivingNodeVars[j], newDrivingNodeVars[j]);
                    } 
                    // Remap all references in the current subquery
                    varRemapper.RemapSubtree(nestNode.Children[i]); 
 
                    // Bug 479183: Remap the flattened element vars
                    newFlattenedElementVars = varRemapper.RemapVarList(nestOp.CollectionInfo[i - 1].FlattenedElementVars); 

                    // Create a cross apply for all but the first collection
                    op = Command.CreateCrossApplyOp();
                } 
                else {
                    newDrivingNode = drivingNode; 
                    newDrivingNodeVars = drivingNodeVars; 
                    newFlattenedElementVars = nestOp.CollectionInfo[i - 1].FlattenedElementVars;
 
                    // Create an outer apply for the first collection,
                    // that way we ensure at least one row for each row in the driver node.
                    op = Command.CreateOuterApplyOp();
                } 

                // Create an outer apply with the driver node and the nested collection. 
                Node applyNode = Command.CreateNode(op, newDrivingNode, nestNode.Children[i]); 

                // Now create a ProjectOp that augments the output from the OuterApplyOp 
                // with nulls for each column from other collections

                // Build the VarDefList (the list of vars) for the Project, starting
                // with the collection discriminator var 
                List varDefListChildren = new List();
                VarList projectOutputs = Command.CreateVarList(); 
 
                // Add the collection discriminator var to the output.
                projectOutputs.Add(discriminatorVarList[i]); 

                // Add all columns from the driving node
                projectOutputs.AddRange(newDrivingNodeVars);
 
                // Add all the vars from all the nested collections;
                for (int j = 1; j < nestNode.Children.Count; j++) { 
                    CollectionInfo otherCollectionInfo = nestOp.CollectionInfo[j - 1]; 
                    // For the current nested collection, we just pick the var that's
                    // coming from there and don't need have a new var defined, but for 
                    // the rest we construct null values.
                    if (i == j) {
                        projectOutputs.AddRange(newFlattenedElementVars);
                    } 
                    else {
                        foreach (Var v in otherCollectionInfo.FlattenedElementVars) { 
                            NullOp nullOp = Command.CreateNullOp(v.Type); 
                            Node nullOpNode = Command.CreateNode(nullOp);
                            Var nullOpVar; 
                            Node nullOpVarDefNode = Command.CreateVarDefNode(nullOpNode, out nullOpVar);
                            varDefListChildren.Add(nullOpVarDefNode);
                            projectOutputs.Add(nullOpVar);
                        } 
                    }
                } 
 
                Node varDefListNode = Command.CreateNode(Command.CreateVarDefListOp(), varDefListChildren);
 
                // Now, build up the projectOp
                VarVec projectOutputsVarSet = Command.CreateVarVec(projectOutputs);
                ProjectOp projectOp = Command.CreateProjectOp(projectOutputsVarSet);
                Node projectNode = Command.CreateNode(projectOp, applyNode, varDefListNode); 

                // finally, build the union all 
                if (unionAllNode == null) { 
                    unionAllNode = projectNode;
                    unionAllOutputs = projectOutputs; 
                }
                else {
                    VarMap unionAllMap = new VarMap();
                    VarMap projectMap = new VarMap(); 
                    for (int idx = 0; idx < unionAllOutputs.Count; idx++) {
                        Var outputVar = Command.CreateSetOpVar(unionAllOutputs[idx].Type); 
                        unionAllMap.Add(outputVar, unionAllOutputs[idx]); 
                        projectMap.Add(outputVar, projectOutputs[idx]);
                    } 
                    UnionAllOp unionAllOp = Command.CreateUnionAllOp(unionAllMap, projectMap);
                    unionAllNode = Command.CreateNode(unionAllOp, unionAllNode, projectNode);

                    // Get the output vars from the union-op. This must be in the same order 
                    // as the original list of Vars
                    unionAllOutputs = GetUnionOutputs(unionAllOp, unionAllOutputs); 
                } 
            }
 
            // We're done building the node, but now we have to build a mapping from
            // the before-Vars to the after-Vars
            varMapList = new List>();
            IEnumerator outputVarsEnumerator = unionAllOutputs.GetEnumerator(); 
            if (!outputVarsEnumerator.MoveNext()) {
                throw EntityUtil.InternalError(EntityUtil.InternalErrorCode.ColumnCountMismatch, 4); // more columns from children than are on the unionAll? 
            } 
            // The discriminator var is always first
            discriminatorVar = outputVarsEnumerator.Current; 

            // Build a map for each input
            for (int i = 0; i < nestNode.Children.Count; i++) {
                Dictionary varMap = new Dictionary(); 
                VarList varList = (i == 0) ? drivingNodeVars : nestOp.CollectionInfo[i - 1].FlattenedElementVars;
                foreach (Var v in varList) { 
                    if (!outputVarsEnumerator.MoveNext()) { 
                        throw EntityUtil.InternalError(EntityUtil.InternalErrorCode.ColumnCountMismatch, 5); // more columns from children than are on the unionAll?
                    } 
                    varMap[v] = outputVarsEnumerator.Current;
                }
                varMapList.Add(varMap);
            } 
            if (outputVarsEnumerator.MoveNext()) {
                throw EntityUtil.InternalError(EntityUtil.InternalErrorCode.ColumnCountMismatch, 6); // at this point, we better be done with both lists... 
            } 

            return unionAllNode; 
        }

        /// 
        /// Get back an ordered list of outputs from a union-all op. The ordering should 
        /// be identical to the ordered list "leftVars" which describes the left input of
        /// the unionAllOp 
        ///  
        /// the unionall Op
        /// vars of the left input 
        /// output vars ordered in the same way as the left input
        private static VarList GetUnionOutputs(UnionAllOp unionOp, VarList leftVars) {
            VarMap varMap = unionOp.VarMap[0];
            Dictionary reverseVarMap = varMap.GetReverseMap(); 

            VarList unionAllVars = Command.CreateVarList(); 
            foreach (Var v in leftVars) { 
                Var newVar = reverseVarMap[v];
                unionAllVars.Add(newVar); 
            }

            return unionAllVars;
        } 

        ///  
        /// Tries to get NewGuid canonical function. 
        /// 
        /// out EdmFunction for NewGuid canonical function 
        /// true if a unique Edm.NewGuid canonical function was found, false otherwise
        private bool TryGetNewGuidFunction(out md.EdmFunction newGuidFunction) {
            newGuidFunction = null;
            System.Collections.ObjectModel.ReadOnlyCollection functions = m_compilerState.MetadataWorkspace.GetFunctions("NewGuid", "Edm", System.Data.Metadata.Edm.DataSpace.CSpace); 
            if (null != functions && 1 == functions.Count) {
                newGuidFunction = functions[0]; 
            } 
            return (null != newGuidFunction);
        } 

        #endregion

        #endregion 
    }
 
    #region Class OpCopierTrackingCollectionVars 
    /// 
    /// Wrapper around OpCopier to keep track of the defining subtrees 
    /// of collection vars defined in the subtree being returned as a copy.
    /// 
    internal class OpCopierTrackingCollectionVars : OpCopier {
        #region Private State 
        private Dictionary m_newCollectionVarDefinitions = new Dictionary();
        #endregion 
 
        #region Private Constructor
        private OpCopierTrackingCollectionVars(Command cmd) : base(cmd) { 
        }
        #endregion

        #region Public Surface 
        /// 
        /// Equivalent to OpCopier.Copy, only in addition it keeps track of the defining subtrees 
        /// of collection vars defined in the subtree rooted at the copy of the input node n. 
        /// 
        ///  
        /// 
        /// 
        /// 
        ///  
        internal static Node Copy(Command cmd, Node n, out VarMap varMap, out Dictionary newCollectionVarDefinitions) {
            OpCopierTrackingCollectionVars oc = new OpCopierTrackingCollectionVars(cmd); 
            Node newNode = oc.CopyNode(n); 
            varMap = oc.m_varMap;
            newCollectionVarDefinitions = oc.m_newCollectionVarDefinitions; 
            return newNode;
        }
        #endregion
 
        #region Visitor Members
        ///  
        /// Tracks the collection vars after calling the base implementation 
        /// 
        ///  
        /// 
        /// 
        public override Node Visit(MultiStreamNestOp op, Node n) {
            Node result = base.Visit(op, n); 
            MultiStreamNestOp newOp = (MultiStreamNestOp)result.Op;
 
            for (int i = 0; i < newOp.CollectionInfo.Count; i++) { 
                m_newCollectionVarDefinitions.Add(newOp.CollectionInfo[i].CollectionVar, result.Children[i + 1]);
            } 
            return result;
        }
        #endregion
    } 
    #endregion
} 

// File provided for Reference Use Only by Microsoft Corporation (c) 2007.
//---------------------------------------------------------------------- 
// 
//      Copyright (c) Microsoft Corporation.  All rights reserved.
// 
// 
// @owner  [....], [....]
//--------------------------------------------------------------------- 
 
using System;
//using System.Diagnostics; // Please use PlanCompiler.Assert instead of Debug.Assert in this class... 
using System.Collections.Generic;
using System.Globalization;

using System.Data.Common; 
using md = System.Data.Metadata.Edm;
using System.Data.Query.InternalTrees; 
using System.Data.Query.PlanCompiler; 

namespace System.Data.Query.PlanCompiler { 

    /// 
    /// This class "pulls" up nest operations to the root of the tree
    ///  
    /// 
    /// The goal of this module is to eliminate nest operations from the query - more 
    /// specifically, the nest operations are pulled up to the root of the query instead. 
    ///
    internal class NestPullup : BasicOpVisitorOfNode { 

        #region private state

        private PlanCompiler m_compilerState; 

        ///  
        /// map from a collection var to the node where it's defined; the node should be 
        /// the node that should be used as the replacement for the var if it is referred
        /// to in an UnnestOp (through a VarRef)  Note that we expect this to contain the 
        /// PhysicalProjectOp of the node, so we can use the VarList when mapping vars to
        /// the copy; (We'll remove the PhysicalProjectOp when we copy it...)
        /// 
        private Dictionary m_definingNodeMap = new Dictionary(); 

        ///  
        /// map from var to the var we're supposed to replace it with 
        /// 
        private VarRemapper m_varRemapper; 

        /// 
        /// Map from VarRef vars to what they're referencing; used to enable the defining
        /// node map to contain only the definitions, not all the references to it. 
        /// 
        private Dictionary m_varRefMap = new Dictionary(); 
 
        #endregion
 
        #region constructor

        private NestPullup(PlanCompiler compilerState) {
            m_compilerState = compilerState; 
            m_varRemapper = new VarRemapper(compilerState.Command);
        } 
 
        #endregion
 
        #region Process Driver

        internal static void Process(PlanCompiler compilerState) {
            NestPullup np = new NestPullup(compilerState); 
            np.Process();
        } 
 
        /// 
        /// The driver routine. Does all the hard work of processing 
        /// 
        private void Process() {
            PlanCompiler.Assert(Command.Root.Op.OpType == OpType.PhysicalProject, "root node is not physicalProject?");
            Command.Root = VisitNode(Command.Root); 
        }
 
        #endregion 

        #region private methods 

        #region VisitorHelpers

        ///  
        /// the iqt we're processing
        ///  
        private Command Command { get { return m_compilerState.Command; } } 

        ///  
        /// is the node a NestOp node?
        /// 
        /// 
        ///  
        private static bool IsNestOpNode(Node n) {
            PlanCompiler.Assert(n.Op.OpType != OpType.SingleStreamNest, "illegal singleStreamNest?"); 
            return (n.Op.OpType == OpType.SingleStreamNest || n.Op.OpType == OpType.MultiStreamNest); 
        }
 
        /// 
        /// Not Supported common processing
        ///
        /// For all those cases where we don't intend to support 
        /// a nest operation as a child, we have this routine to
        /// do the work. 
        ///  
        /// 
        ///  
        /// 
        private Node NestingNotSupported(Op op, Node n) {
            // First, visit my children
            VisitChildren(n); 
            m_varRemapper.RemapNode(n);
 
            // Make sure we don't have a child that is a nest op. 
            foreach (Node chi in n.Children) {
                if (IsNestOpNode(chi)) { 
                    throw EntityUtil.NestingNotSupported(op, chi.Op);
                }
            }
            return n; 
        }
 
        ///  
        /// Follow the VarRef chain to the defining var
        ///  
        /// 
        /// 
        private Var ResolveVarReference(Var refVar) {
            Var x = refVar; 
            while (m_varRefMap.TryGetValue(x, out x)) {
                refVar = x; 
            } 
            return refVar;
        } 

        /// 
        /// Update the replacement Var map with the vars from the pulled-up
        /// operation; the shape is supposed to be identical, so we should not 
        /// have more vars on either side, and the order is guaranteed to be
        /// the same. 
        ///  
        /// 
        ///  
        private void UpdateReplacementVarMap(IEnumerable fromVars, IEnumerable toVars) {
            IEnumerator toVarEnumerator = toVars.GetEnumerator();

            foreach (Var v in fromVars) { 
                if (!toVarEnumerator.MoveNext()) {
                    throw EntityUtil.InternalError(EntityUtil.InternalErrorCode.ColumnCountMismatch, 2); 
                } 
                m_varRemapper.AddMapping(v, toVarEnumerator.Current);
            } 

            if (toVarEnumerator.MoveNext()) {
                throw EntityUtil.InternalError(EntityUtil.InternalErrorCode.ColumnCountMismatch, 3);
            } 
        }
 
        #region remapping helpers 

        ///  
        /// Replace a list of sortkeys *IN-PLACE* with the corresponding "mapped" Vars
        /// 
        /// sortkeys
        /// the mapping info for Vars 
        private static void RemapSortKeys(List sortKeys, Dictionary varMap) {
            if (sortKeys != null) { 
                foreach (InternalTrees.SortKey sortKey in sortKeys) { 
                    Var replacementVar;
                    if (varMap.TryGetValue(sortKey.Var, out replacementVar)) { 
                        sortKey.Var = replacementVar;
                    }
                }
            } 
        }
 
        ///  
        /// Produce a "mapped" sequence of the input Var sequence - based on the supplied
        /// map 
        /// 
        /// input var sequence
        /// var->var map
        /// the mapped var sequence 
        private IEnumerable RemapVars(IEnumerable vars, Dictionary varMap) {
            foreach (Var v in vars) { 
                Var mappedVar; 
                if (varMap.TryGetValue(v, out mappedVar)) {
                    yield return mappedVar; 
                }
                else {
                    yield return v;
                } 
            }
        } 
 
        /// 
        /// Produce a "mapped" varList 
        /// 
        /// 
        /// 
        ///  
        private VarList RemapVarList(VarList varList, Dictionary varMap) {
            VarList newVarList = Command.CreateVarList(RemapVars(varList, varMap)); 
            return newVarList; 
        }
 
        /// 
        /// Produce a "mapped" varVec
        /// 
        ///  
        /// 
        ///  
        private VarVec RemapVarVec(VarVec varVec, Dictionary varMap) { 
            VarVec newVarVec = Command.CreateVarVec(RemapVars(varVec, varMap));
            return newVarVec; 
        }

        #endregion
 
        #endregion
 
        #region AncillaryOp Visitors 

        ///  
        /// VarDefOp
        ///
        /// Essentially, maintains m_varRefMap, adding an entry for each VarDef that has a
        /// VarRef on it. 
        /// 
        ///  
        ///  
        /// 
        public override Node Visit(VarDefOp op, Node n) { 
            VisitChildren(n);

            // perform any "remapping"
            m_varRemapper.RemapNode(n); 

            if (n.Child0.Op.OpType == OpType.VarRef) { 
                m_varRefMap.Add(op.Var, ((VarRefOp)n.Child0.Op).Var); 
            }
            return n; 
        }

        /// 
        /// VarRefOp 
        /// 
        ///  
        /// When we remove the UnnestOp, we are left with references to it's column vars that 
        /// need to be fixed up; we do this by creating a var replacement map when we remove the
        /// UnnestOp and whenever we find a reference to a var in the map, we replace it with a 
        /// reference to the replacement var instead;
        /// 
        /// 
        ///  
        /// 
        public override Node Visit(VarRefOp op, Node n) { 
            // First, visit my children (do I have children?) 
            VisitChildren(n);
            // perform any "remapping" 
            m_varRemapper.RemapNode(n);
            return n;
        }
 
        #endregion
 
        #region ScalarOp Visitors 
        /// 
        /// We don't yet support nest pullups over Case 
        /// 
        /// 
        /// 
        ///  
        public override Node Visit(CaseOp op, Node n) {
            // Make sure we don't have a child that is a nest op. 
            foreach (Node chi in n.Children) { 
                if (chi.Op.OpType == OpType.Collect) {
                    throw EntityUtil.NestingNotSupported(op, chi.Op); 
                }
                else if (chi.Op.OpType == OpType.VarRef) {
                    Var refVar = ResolveVarReference(((VarRefOp)chi.Op).Var);
                    if (m_definingNodeMap.ContainsKey(refVar)) { 
                        throw EntityUtil.NestingNotSupported(op, chi.Op);
                    } 
                } 
            }
 
            return VisitDefault(n);
        }
        #endregion
 
        #region RelOp Visitors
 
        ///  
        /// Default RelOp processing:
        /// 
        /// We really don't want to allow any NestOps through; just fail if we don't have
        /// something coded.
        /// 
        ///  
        /// 
        ///  
        protected override Node VisitRelOpDefault(RelOp op, Node n) { 
            return NestingNotSupported(op, n);
        } 

        /// 
        /// ApplyOp/JoinOp common processing
        ///  
        /// 
        /// If one of the inputs to any JoinOp/ApplyOp is a NestOp, then the NestOp 
        /// can be pulled above the join/apply if every input to the join/apply has 
        /// a key(s). The keys of the NestOp are augmented with the keys of the
        /// other join inputs: 
        ///
        ///     JoinOp/ApplyOp(NestOp(X, ...), Y) => NestOp(JoinOp/ApplyOp(X, Y), ...)
        ///
        /// In addition, if the NestOp is on a 'nullable' side of a join (i.e. right side of 
        /// LeftOuterJoin/OuterApply or either side of FullOuterJoin), the driving node
        /// of that NestOp (X) is capped with a project with a null sentinel and 
        /// the dependant collection nodes (the rest of the NestOp children) 
        /// are filtered based on that sentinel:
        /// 
        ///  LOJ/OA/FOJ (X, NestOp(Y, Z1, Z2, ..ZN))  =>  NestOp( LOJ/OA/FOJ (X, PROJECT (Y, v = 1)), FILTER(Z1, v!=null), FILTER(Z2, v!=null), ... FILTER(ZN, v!=null))
        ///         FOJ (NestOp(Y, Z1, Z2, ..ZN), X)  =>  NestOp( LOJ/OA/FOJ (PROJECT (Y, v = 1), X), FILTER(Z1, v!=null), FILTER(Z2, v!=null), ... FILTER(ZN, v!=null))
        ///
        ///  Also, FILTER(Zi, v != null) may be transformed to push the filter below any NestOps. 
        ///
        /// Requires: Every input to the join/apply must have a key. 
        ///  
        /// 
        ///  
        /// 
        private Node ApplyOpJoinOp(Op op, Node n) {
            // First, visit my children
            VisitChildren(n); 

            // Now determine if any of the input nodes are a nestOp. 
            int countOfNestInputs = 0; 

            foreach (Node chi in n.Children) { 
                NestBaseOp nestOp = chi.Op as NestBaseOp;
                if (null != nestOp) {
                    countOfNestInputs++;
 
                    if (OpType.SingleStreamNest == chi.Op.OpType) {
                        // There should not be a SingleStreamNest in the tree, because we made a decision 
                        // that in essence means the only way to get a SingleStreamNest is to have a 
                        // PhysicalProject over something with an underlying NestOp.   Having
                        // 
                        //      Project(Collect(PhysicalProject(...)))
                        //
                        // isn�t good enough, because that will get converted to a MultiStreamNest, with
                        // the SingleStreamNest as the input to the MultiStreamNest. 
                        throw EntityUtil.InternalError(EntityUtil.InternalErrorCode.JoinOverSingleStreamNest);
                    } 
                } 
            }
 
            // If none of the inputs are a nest, then we don't really need to do anything.
            if (0 == countOfNestInputs) {
                return n;
            } 

            // We can only pull the nest over a Join/Apply if it has keys, so 
            // we can order things; if it doesn't have keys, we throw a NotSupported 
            // exception.
            foreach (Node chi in n.Children) { 
                if (op.OpType != OpType.MultiStreamNest && chi.Op.IsRelOp) {
                    KeyVec keys = Command.PullupKeys(chi);

                    if (null == keys || keys.NoKeys) { 
                        throw EntityUtil.KeysRequiredForJoinOverNest(op);
                    } 
                } 
            }
 
            // Alright, we're OK to pull the nestOp over the joinOp/applyOp.
            //
            // That means:
            // 
            // (1) build a new list of children for the nestOp and for the joinOp/applyOp
            // (2) build the new list of collectionInfos for the new nestOp. 
            List newNestChildren = new List(); 
            List newJoinApplyChildren = new List();
            List newCollectionInfoList = new List(); 

            foreach (Node chi in n.Children) {
                if (chi.Op.OpType == OpType.MultiStreamNest) {
                    newCollectionInfoList.AddRange(((MultiStreamNestOp)chi.Op).CollectionInfo); 

                    // SQLBUDT #615513: If the nest op is on a 'nullable' side of join 
                    // (i.e. right side of LeftOuterJoin/OuterApply or either side of FullOuterJoin) 
                    // the driving node of that nest operation needs to be capped with a project with
                    // a null sentinel and the dependant collection nodes need to be filtered based on that sentinel. 
                    //
                    //  LOJ/OA/FOJ (X, MSN(Y, Z1, Z2, ..ZN))  =>  MSN( LOJ/OA/FOJ (X, PROJECT (Y, v = 1)), FILTER(Z1, v!=null), FILTER(Z2, v!=null), ... FILTER(ZN, v!=null))
                    //         FOJ (MSN(Y, Z1, Z2, ..ZN), X)  =>  MSN( LOJ/OA/FOJ (PROJECT (Y, v = 1), X), FILTER(Z1, v!=null), FILTER(Z2, v!=null), ... FILTER(ZN, v!=null))
                    // 
                    //  Note: we transform FILTER(Zi, v != null) to push the filter below any MSNs.
                    if ((op.OpType == OpType.FullOuterJoin) 
                        || ((op.OpType == OpType.LeftOuterJoin || op.OpType == OpType.OuterApply) && n.Child1.Op.OpType == OpType.MultiStreamNest)) { 
                        Var sentinelVar = null;
                        newJoinApplyChildren.Add(AugmentNodeWithInternalConstant(chi.Child0, 1, out sentinelVar)); 
                        for (int i = 1; i < chi.Children.Count; i++) {
                            // Push the filter bellow all MultiStremNest-s, because this part of the tree has
                            // already been visited and it is expected that the MultiStreamNests have bubbled up
                            // above the filters. 
                            Node newFilterChild = chi.Children[i];
                            Node newFilterParent = null; 
                            while (newFilterChild.Op.OpType == OpType.MultiStreamNest) { 
                                newFilterParent = newFilterChild;
                                newFilterChild = newFilterChild.Child0; 
                            }

                            Node newFilterNode = CapWithIsNotNullFilter(newFilterChild, sentinelVar);
                            Node newNestChild; // the original child or the filter, depending on whether the filter got pushed 

                            if (newFilterParent != null) { 
                                newFilterParent.Child0 = newFilterNode; 
                                newNestChild = chi.Children[i];
                            } 
                            else {
                                newNestChild = newFilterNode;
                            }
                            newNestChildren.Add(newNestChild); 
                        }
                    } 
                    else { 
                        newJoinApplyChildren.Add(chi.Child0);
                        for (int i = 1; i < chi.Children.Count; i++) { 
                            newNestChildren.Add(chi.Children[i]);
                        }
                    }
                } 
                else {
                    newJoinApplyChildren.Add(chi); 
                } 
            }
 
            // (3) create the new Join/Apply node using the existing op and the
            //     new list of children from (1).
            Node newJoinApplyNode = Command.CreateNode(op, newJoinApplyChildren);
 
            // (4) insert the apply op as the driving node of the nestOp (put it
            //     at the beginning of the new nestOps' children. 
            newNestChildren.Insert(0, newJoinApplyNode); 

            // (5) build an updated list of output vars based upon the new join/apply 
            //     node, and ensure all the collection vars from the nestOp(s) are
            //     included.
            ExtendedNodeInfo xni = newJoinApplyNode.GetExtendedNodeInfo(Command);
            VarVec newOutputVars = Command.CreateVarVec(xni.Definitions); 

            foreach (CollectionInfo ci in newCollectionInfoList) { 
                newOutputVars.Set(ci.CollectionVar); 
            }
 
            // (6) create the new nestop
            NestBaseOp newNestOp = Command.CreateMultiStreamNestOp(new List(), newOutputVars, newCollectionInfoList);
            Node newNode = Command.CreateNode(newNestOp, newNestChildren);
            return newNode; 
        }
 
        ///  
        /// Input =>  Filter(input, Ref(var) is not null)
        ///  
        /// 
        /// 
        /// 
        private Node CapWithIsNotNullFilter(Node input, Var var) { 
            Node varRefNode = Command.CreateNode(Command.CreateVarRefOp(var));
            Node predicateNode = Command.CreateNode( 
                                            Command.CreateConditionalOp(OpType.Not), 
                                            Command.CreateNode(
                                                    Command.CreateConditionalOp(OpType.IsNull), 
                                                    varRefNode));


            Node filterNode = Command.CreateNode(Command.CreateFilterOp(), input, predicateNode); 
            return filterNode;
        } 
 
        /// 
        /// ApplyOp common processing 
        /// 
        /// 
        /// 
        ///  
        protected override Node VisitApplyOp(ApplyBaseOp op, Node n) {
            return ApplyOpJoinOp(op, n); 
        } 

        ///  
        /// DistinctOp
        /// 
        /// 
        /// The input to a DistinctOp cannot be a NestOp � that would imply that 
        /// we support distinctness over collections - which we don�t.
        ///  
        ///  
        /// 
        ///  
        public override Node Visit(DistinctOp op, Node n) {
            return NestingNotSupported(op, n);
        }
 
        /// 
        /// FilterOp 
        ///  
        /// 
        /// If the input to the FilterOp is a NestOp, and if the filter predicate 
        /// does not reference any of the collection Vars of the nestOp, then the
        /// FilterOp can be simply pushed below the NestOp:
        ///
        ///     Filter(Nest(X, ...), pred) => Nest(Filter(X, pred), ...) 
        ///
        /// Note: even if the filter predicate originally referenced one of the 
        /// collection vars, as part of our bottom up traversal, the appropriate 
        /// Var was replaced by a copy of the source of the collection. So, this
        /// transformation should always be legal. 
        /// 
        /// 
        /// 
        ///  
        public override Node Visit(FilterOp op, Node n) {
            // First, visit my children 
            VisitChildren(n); 

            // see if the child is a nestOp 
            NestBaseOp nestOp = n.Child0.Op as NestBaseOp;

            if (null != nestOp) {
#if DEBUG 
                // check to see if the predicate references any of the collection
                // expressions. If it doesn't, then we can push the filter down, but 
                // even if it does it's probably OK. 
                NodeInfo predicateNodeInfo = Command.GetNodeInfo(n.Child1);
                foreach (CollectionInfo ci in nestOp.CollectionInfo) { 
                    PlanCompiler.Assert(!predicateNodeInfo.ExternalReferences.IsSet(ci.CollectionVar), "predicate references collection?");
                }
#endif //DEBUG
 
                // simply pull up the nest child above ourself.
                Node nestOpNode = n.Child0; 
                Node nestOpInputNode = nestOpNode.Child0; 
                n.Child0 = nestOpInputNode;
                nestOpNode.Child0 = n; 

                // recompute node info - no need to perform anything for the predicate
                Command.RecomputeNodeInfo(n);
                Command.RecomputeNodeInfo(nestOpNode); 
                return nestOpNode;
            } 
 
            return n;
        } 

        /// 
        /// GroupByOp
        ///  
        /// 
        /// At this point in the process, there really isn't a way we should actually 
        /// have a NestOp as an input to the GroupByOp, and we currently aren't allowing 
        /// you to specify a collection as an aggregation Var or key, so if we find a
        /// NestOp anywhere on the inputs, it's a NotSupported situation. 
        /// 
        /// 
        /// 
        ///  
        public override Node Visit(GroupByOp op, Node n) {
            return NestingNotSupported(op, n); 
        } 

        ///  
        /// JoinOp common processing
        /// 
        /// 
        ///  
        /// 
        protected override Node VisitJoinOp(JoinBaseOp op, Node n) { 
            return ApplyOpJoinOp(op, n); 
        }
 
        /// 
        /// ProjectOp
        /// 
        ///  
        /// There are three cases to handle in ProjectOp:
        /// 
        ///  (1) The input is not a NestOp; but the ProjectOp locally defines some Vars 
        ///      as collections:
        /// 
        ///         ProjectOp(X,{a,CollectOp(PhysicalProjectOp(Y)),b,...}) ==> MsnOp(ProjectOp'(X,{a,b,...}),Y)
        ///         ProjectOp(X,{a,VarRef(ref-to-collect-var-Y),b,...})    ==> MsnOp(ProjectOp'(X,{a,b,...}),copy-of-Y)
        ///
        ///     Where: 
        ///         ProjectOp' is ProjectOp less any vars that were collection vars, plus
        ///                    any additional Vars needed by the collection. 
        /// 
        ///  (2) The input is a NestOp, but the ProjectOp does not local define some Vars
        ///      as collections: 
        ///
        ///         ProjectOp(MsnOp(X,Y,...)) => MsnOp'(ProjectOp'(X),Y,...)
        ///
        ///      Where: 
        ///         ProjectOp' is ProjectOp plus any additional Vars needed by NestOp
        ///                    (see NestOp.Outputs � except the collection vars) 
        /// 
        ///         MsnOp'     should be MsnOp. Additionally, its Outputs should be enhanced
        ///                    to include any Vars produced by the ProjectOp 
        ///
        ///  (3) The combination of both (1) and (2) -- both the vars define a collection,
        ///      and the input is also a nestOp.  we handle this by first processing Case1,
        ///      then processing Case2. 
        /// 
        ///  
        ///  
        /// 
        public override Node Visit(ProjectOp op, Node n) { 
#if DEBUG
            string input = Dump.ToXml(Command, n);
#endif //DEBUG
 
            // First, visit my children
            VisitChildren(n); 
            m_varRemapper.RemapNode(n); 

            // First, convert any nestOp inputs; 
            Node newNode = ProjectOpCase1(n);

            // Then, if we have a NestOp as an input (and we didn't
            // produce a NestOp when handling Case1) pull it over our 
            // ProjectOp.
            if (newNode.Op.OpType == OpType.Project && IsNestOpNode(newNode.Child0)) { 
                newNode = ProjectOpCase2(newNode); 
            }
 
            // Finally we fold any nested NestOps into one.
            newNode = MergeNestedNestOps(newNode);

#if DEBUG 
            int size = input.Length;// GC.KeepAlive makes FxCop Grumpy.
            string output = Dump.ToXml(Command, newNode); 
#endif //DEBUG 
            return newNode;
        } 

        /// 
        /// Fold nested MultiStreamNestOps into one:
        /// 
        ///     MSN(MSN(X,Y),Z) ==> MSN(X,Y,Z)
        /// 
        /// NOTE: It would be incorrect to merge NestOps from the non-driving node 
        ///       into one nest op, because that would change the intent.  Instead,
        ///       we let those go through the tree and wait until we get to the top 
        ///       level PhysicalProject, when we'll use the ConvertToSingleStreamNest
        ///       process to handle them.
        ///
        /// NOTE: We should never have three levels of nestOps, because we should 
        ///       have folded the lower two together when we constructed one of them.
        /// 
        /// We also remove unreferenced collections, that is, if any collection is 
        /// not referred to by the top level-NestOp, we can safely remove it from
        /// the merged NestOp we produce. 
        /// 
        /// 
        /// 
        private Node MergeNestedNestOps(Node nestNode) { 

            // First, determine if there is anything we can actually do.  If we 
            // aren't given a NestOp or if it's driving node isn't a NestOp we 
            // can just ignore this.
            if (!IsNestOpNode(nestNode) || !IsNestOpNode(nestNode.Child0)) { 
                return nestNode;
            }

#if DEBUG 
            string input = Dump.ToXml(Command, nestNode);
#endif //DEBUG 
            NestBaseOp nestOp = (NestBaseOp)nestNode.Op; 
            Node nestedNestNode = nestNode.Child0;
            NestBaseOp nestedNestOp = (NestBaseOp)nestedNestNode.Op; 

            // Get the collection Vars from the top level NestOp
            VarVec nestOpCollectionOutputs = Command.CreateVarVec();
            foreach (CollectionInfo ci in nestOp.CollectionInfo) { 
                nestOpCollectionOutputs.Set(ci.CollectionVar);
            } 
 
            // Now construct a new list of inputs, collections; and output vars.
            List newNestInputs = new List(); 
            List newCollectionInfo = new List();
            VarVec newOutputVars = Command.CreateVarVec(nestOp.Outputs);

            // Add the new DrivingNode; 
            newNestInputs.Add(nestedNestNode.Child0);
 
            // Now add each of the nested nodes collections, but only when they're 
            // referenced by the top level nestOp's outputs.
            for (int i = 1; i < nestedNestNode.Children.Count; i++) { 
                CollectionInfo ci = nestedNestOp.CollectionInfo[i - 1];
                if (nestOpCollectionOutputs.IsSet(ci.CollectionVar) || newOutputVars.IsSet(ci.CollectionVar)) {
                    newCollectionInfo.Add(ci);
                    newNestInputs.Add(nestedNestNode.Children[i]); 
                    PlanCompiler.Assert(newOutputVars.IsSet(ci.CollectionVar), "collectionVar not in output Vars?"); // I must have missed something...
                } 
            } 

            // Then add in the rest of the inputs to the top level nest node (and 
            // they're collection Infos)
            for (int i = 1; i < nestNode.Children.Count; i++) {
                CollectionInfo ci = nestOp.CollectionInfo[i - 1];
                newCollectionInfo.Add(ci); 
                newNestInputs.Add(nestNode.Children[i]);
                PlanCompiler.Assert(newOutputVars.IsSet(ci.CollectionVar), "collectionVar not in output Vars?"); // I must have missed something... 
            } 

            //The prefix sort keys for the new nest op should include these of the input nestOp followed by the nestedNestOp 
            //(The nestOp-s that are being merged may have prefix sort keys propagated to them by constrainedSortOp-s pushed below them.
            List sortKeys = ConsolidateSortKeys(nestOp.PrefixSortKeys, nestedNestOp.PrefixSortKeys);

            // Make sure we pullup the sort keys in our output too... 
            foreach (InternalTrees.SortKey sk in sortKeys) {
                newOutputVars.Set(sk.Var); 
            } 

            // Ready to go; build the new NestNode, etc. 
            MultiStreamNestOp newNestOp = Command.CreateMultiStreamNestOp(sortKeys, newOutputVars, newCollectionInfo);
            Node newNode = Command.CreateNode(newNestOp, newNestInputs);

            // Finally, recompute node info 
            Command.RecomputeNodeInfo(newNode);
 
#if DEBUG 
            int size = input.Length;// GC.KeepAlive makes FxCop Grumpy.
            string output = Dump.ToXml(Command, newNode); 
#endif //DEBUG
            return newNode;
        }
 
        /// 
        /// ProjectOp(X,{a,CollectOp(PhysicalProjectOp(Y)),b,...}) ==> MsnOp(ProjectOp'(X,{a,b,...}),Y) 
        /// ProjectOp(X,{a,VarRef(ref-to-collect-var-Y),b,...})    ==> MsnOp(ProjectOp'(X,{a,b,...}),copy-of-Y) 
        ///
        /// Remove CollectOps from projection, constructing a NestOp 
        /// over the ProjectOp.
        /// 
        /// 
        ///  
        private Node ProjectOpCase1(Node projectNode) {
#if DEBUG 
            string input = Dump.ToXml(Command, projectNode); 
#endif //DEBUG
 
            ProjectOp op = (ProjectOp)projectNode.Op;

            // Check to see if any of the computed Vars are in fact NestOps, and
            // construct a collection column map for them. 
            List collectionInfoList = new List();
            List newChildren = new List(); 
            List collectionNodes = new List(); 
            VarVec externalReferences = Command.CreateVarVec();
            VarVec collectionReferences = Command.CreateVarVec(); 
            List definedVars = new List();
            List referencedVars = new List();

            foreach (Node chi in projectNode.Child1.Children) { 
                VarDefOp varDefOp = (VarDefOp)chi.Op;
                Node definingExprNode = chi.Child0; 
 
                if (OpType.Collect == definingExprNode.Op.OpType) {
                    PlanCompiler.Assert(definingExprNode.HasChild0, "collect without input?"); 
                    PlanCompiler.Assert(OpType.PhysicalProject == definingExprNode.Child0.Op.OpType, "collect without physicalProject?");
                    Node physicalProjectNode = definingExprNode.Child0;

                    // Update collection var->defining node map; 
                    m_definingNodeMap.Add(varDefOp.Var, physicalProjectNode);
 
                    ConvertToNestOpInput(physicalProjectNode, varDefOp.Var, collectionInfoList, collectionNodes, externalReferences, collectionReferences); 
                }
                else if (OpType.VarRef == definingExprNode.Op.OpType) { 
                    Var refVar = ResolveVarReference(varDefOp.Var);
                    Node physicalProjectNode;

                    if (m_definingNodeMap.TryGetValue(refVar, out physicalProjectNode)) { 
                        physicalProjectNode = CopyCollectionVarDefinition(physicalProjectNode);
                        //SQLBUDT #602888: We need to track the copy too, in case we need to reuse it 
                        m_definingNodeMap.Add(varDefOp.Var, physicalProjectNode); 
                        ConvertToNestOpInput(physicalProjectNode, varDefOp.Var, collectionInfoList, collectionNodes, externalReferences, collectionReferences);
                    } 
                    else {
                        referencedVars.Add(chi);
                        newChildren.Add(chi);
                    } 
                }
                else { 
                    definedVars.Add(chi); 
                    newChildren.Add(chi);
                } 
            }

            // If we haven't identified a set of collection nodes, then we're done.
            if (0 == collectionNodes.Count) { 
                return projectNode;
            } 
 
            // OK, we found something. We have some heavy lifting to perform.
 
            // Then we need to build up a MultiStreamNestOp above the ProjectOp and the
            // new collection nodes to get what we really need.
            // pretend that the keys included everything from the new projectOp
            VarVec outputVars = Command.CreateVarVec(op.Outputs); 

            // First we need to modify this physicalProjectNode to leave out the collection 
            // Vars that we've just seen. 
            VarVec newProjectVars = Command.CreateVarVec(op.Outputs);
            newProjectVars.Minus(collectionReferences); 

            // If there are any external references from any of the collections, add
            // those to the projectOp explicitly. This must be ok because the projectOp
            // could not have had any left-correlation 
            newProjectVars.Or(externalReferences);
 
            // Create the new projectOp, and hook it into this one.  The new projectOp 
            // no longer references the collections in it's children; of course we only
            // construct a new projectOp if it actually projects out some Vars. 
            if (!newProjectVars.IsEmpty) {
                if (IsNestOpNode(projectNode.Child0)) {
                    // If the input is a nest node, we need to figure out what to do with the
                    // rest of the in the VarDefList; we can't just pitch them, but we also 
                    // really want to have the input be a nestop.
                    // 
                    // What we do is essentially push any non-collection VarDef�s down under 
                    // the driving node of the MSN:
                    // 
                    //      Project[Z,Y,W](Msn(X,Y),VarDef(Z=blah),VarDef(W=Collect(etc)) ==> MSN(MSN(Project[Z](X,VarDef(Z=blah)),Y),W)
                    //
                    // An optimization, of course being to not push anything down when there
                    // aren't any extra vars defined. 

                    if (definedVars.Count == 0 && referencedVars.Count == 0) { 
                        // We'll just pick the NestNode; we expect MergeNestedNestOps to merge 
                        // it into what we're about to generate later.
                        projectNode = projectNode.Child0; 
                        EnsureReferencedVarsAreRemoved(referencedVars, outputVars);
                    }
                    else {
                        NestBaseOp nestedNestOp = (NestBaseOp)projectNode.Child0.Op; 

                        // Build the new ProjectOp to be used as input to the new nestedNestOp; 
                        // it's input is the input to the current nestedNestOp and a new 
                        // VarDefList with only the vars that were defined on the top level
                        // ProjectOp. 
                        List newNestedProjectNodeInputs = new List();
                        newNestedProjectNodeInputs.Add(projectNode.Child0.Child0);
                        referencedVars.AddRange(definedVars);
                        newNestedProjectNodeInputs.Add(Command.CreateNode(Command.CreateVarDefListOp(), referencedVars)); 

                        VarVec newNestedProjectOutputs = Command.CreateVarVec(nestedNestOp.Outputs); 
 
                        // SQLBUDT #508722:  We need to remove the collection vars,
                        //  these are not produced by the project 
                        foreach (CollectionInfo ci in nestedNestOp.CollectionInfo) {
                            newNestedProjectOutputs.Clear(ci.CollectionVar);
                        }
 
                        foreach (Node varDefNode in referencedVars) {
                            newNestedProjectOutputs.Set(((VarDefOp)varDefNode.Op).Var); 
                        } 

                        Node newNestedProjectNode = Command.CreateNode(Command.CreateProjectOp(newNestedProjectOutputs), newNestedProjectNodeInputs); 

                        // Now build the new nestedNestedNestOp, with the new nestedProjectOp
                        // as it's input; we have to update the outputs of the NestOp to include
                        // the vars we pushed down. 
                        VarVec newNestedNestOutputs = Command.CreateVarVec(newNestedProjectOutputs);
                        newNestedNestOutputs.Or(nestedNestOp.Outputs); 
 
                        MultiStreamNestOp newNestedNestOp = Command.CreateMultiStreamNestOp(
                                                                    nestedNestOp.PrefixSortKeys, 
                                                                    newNestedNestOutputs,
                                                                    nestedNestOp.CollectionInfo);

                        List newNestedNestNodeInputs = new List(); 
                        newNestedNestNodeInputs.Add(newNestedProjectNode);
                        for (int j = 1; j < projectNode.Child0.Children.Count; j++) { 
                            newNestedNestNodeInputs.Add(projectNode.Child0.Children[j]); 
                        }
                        projectNode = Command.CreateNode(newNestedNestOp, newNestedNestNodeInputs); 
                        // We don't need to remove or remap referenced vars here because
                        // we're including them on the node we create; they won't become
                        // invalid.
                    } 
                }
                else { 
                    ProjectOp newProjectOp = Command.CreateProjectOp(newProjectVars); 
                    projectNode.Child1 = Command.CreateNode(projectNode.Child1.Op, newChildren);
                    projectNode.Op = newProjectOp; 
                    EnsureReferencedVarsAreRemapped(referencedVars);
                }
            }
            else { 
                projectNode = projectNode.Child0;
                EnsureReferencedVarsAreRemoved(referencedVars, outputVars); 
            } 

            // We need to make sure that we project out any external references to the driving 
            // node that the nested collections have, or we're going to end up with unresolvable
            // vars when we pull them up over the current driving node.  Of course, we only
            // want the references that are actually ON the driving node.
            externalReferences.And(projectNode.GetExtendedNodeInfo(Command).Definitions); 
            outputVars.Or(externalReferences);
 
            // There are currently no prefix sortkeys. The processing for a SortOp may later 
            // introduce some prefix sortkeys, but there aren't any now.
            MultiStreamNestOp nestOp = Command.CreateMultiStreamNestOp(new List(), outputVars, collectionInfoList); 

            // Insert the current node at the head of the the list of collections
            collectionNodes.Insert(0, projectNode);
            Node nestNode = Command.CreateNode(nestOp, collectionNodes); 

            // Finally, recompute node info 
            Command.RecomputeNodeInfo(projectNode); 
            Command.RecomputeNodeInfo(nestNode);
 
#if DEBUG
            int size = input.Length;// GC.KeepAlive makes FxCop Grumpy.
            string output = Dump.ToXml(Command, nestNode);
#endif //DEBUG 
            return nestNode;
        } 
 
        /// 
        /// If we're going to eat the ProjectNode, then we at least need to make 
        /// sure we remap any vars it defines as varRefs, and ensure that any
        /// references to them are switched.
        /// 
        ///  
        /// 
        private void EnsureReferencedVarsAreRemoved(List referencedVars, VarVec outputVars) { 
            foreach (Node chi in referencedVars) { 
                VarDefOp varDefOp = (VarDefOp)chi.Op;
                Var defVar = varDefOp.Var; 
                Var refVar = ResolveVarReference(defVar);
                m_varRemapper.AddMapping(defVar, refVar);
                outputVars.Clear(defVar);
                outputVars.Set(refVar); 
            }
        } 
 
        /// 
        /// We need to make sure that we remap the column maps that we're pulling 
        /// up to point to the defined var, not it's reference.
        /// 
        /// 
        private void EnsureReferencedVarsAreRemapped(List referencedVars) { 
            foreach (Node chi in referencedVars) {
                VarDefOp varDefOp = (VarDefOp)chi.Op; 
                Var defVar = varDefOp.Var; 
                Var refVar = ResolveVarReference(defVar);
                m_varRemapper.AddMapping(refVar, defVar); 
            }
        }

        ///  
        /// Convert a CollectOp subtree (when used as the defining expression for a
        /// VarDefOp) into a reasonable input to a NestOp. 
        ///  
        /// 
        /// There are a couple of cases that we handle here: 
        ///
        ///     (a) PhysicalProject(X) ==> X
        ///     (b) PhysicalProject(Sort(X)) ==> Sort(X)
        /// 
        /// 
        /// the child of the CollectOp 
        /// the collectionVar being defined 
        /// where to append the new collectionInfo
        /// where to append the collectionNode 
        /// a bit vector of external references of the physicalProject
        /// a bit vector of collection vars
        private void ConvertToNestOpInput(Node physicalProjectNode, Var collectionVar, List collectionInfoList, List collectionNodes, VarVec externalReferences, VarVec collectionReferences) {
 
            // Keep track of any external references the physicalProjectOp has
            externalReferences.Or(Command.GetNodeInfo(physicalProjectNode).ExternalReferences); 
 

            // Case: (a) PhysicalProject(X) ==> X 
            Node nestOpInput = physicalProjectNode.Child0;

            // Now build the collectionInfo for this input, including the flattened
            // list of vars, which is essentially the outputs from the physicalProject 
            // with the sortKey vars that aren't already in the outputs we already
            // have. 
            PhysicalProjectOp physicalProjectOp = (PhysicalProjectOp)physicalProjectNode.Op; 
            VarList flattenedElementVarList = Command.CreateVarList(physicalProjectOp.Outputs);
            VarVec flattenedElementVarVec = Command.CreateVarVec(flattenedElementVarList); // Use a VarVec to make the lookups faster 
            List sortKeys = null;

            if (OpType.Sort == nestOpInput.Op.OpType) {
                // Case: (b) PhysicalProject(Sort(X)) ==> Sort(X) 
                SortOp sortOp = (SortOp)nestOpInput.Op;
                sortKeys = OpCopier.Copy(Command , sortOp.Keys); 
 
                foreach (InternalTrees.SortKey sk in sortKeys) {
                    if (!flattenedElementVarVec.IsSet(sk.Var)) { 
                        flattenedElementVarList.Add(sk.Var);
                        flattenedElementVarVec.Set(sk.Var);
                    }
                } 
            }
            else { 
                sortKeys = new List(); 
            }
 
            // Get the keys for the collection
            VarVec keys = Command.CreateVarVec();
            foreach (Var key in Command.GetExtendedNodeInfo(nestOpInput).Keys.KeyVars) {
                if (flattenedElementVarVec.IsSet(key)) { 
                    keys.Set(key);
                } 
            } 

            // Create the collectionInfo. 
            CollectionInfo collectionInfo = Command.CreateCollectionInfo(collectionVar, physicalProjectOp.ColumnMap.Element, flattenedElementVarList, keys, sortKeys, null/*discriminatorValue*/);

            // Now update the collections we're tracking.
            collectionInfoList.Add(collectionInfo); 
            collectionNodes.Add(nestOpInput);
            collectionReferences.Set(collectionVar); 
        } 

        ///  
        /// Case 2 for ProjectOp: NestOp is the input:
        ///
        ///     ProjectOp(NestOp(X,Y,...)) => NestOp'(ProjectOp'(X),Y,...)
        /// 
        /// Remove collection references from the ProjectOp and pull the
        /// NestOp over it, adding any outputs that the projectOp added. 
        /// 
        /// The outputs are important here; expanding the above:
        /// 
        ///     P{a,n}(N{x1,x2,x3,y}(X,Y)) => N{a,x1,x2,x3,y}(P{a,x1,x2,x3}(X),Y)
        ///
        /// Strategy:
        /// 
        ///     (1) Determine oldNestOpCollectionOutputs
        ///     (2) oldNestOpNonCollectionOutputs = oldNestOpOutputs - oldNestOpCollectionOutputs; 
        ///     (3) oldProjectOpNonCollectionOutputs = oldProjectOpOutputs - oldNestOpCollectionOutputs 
        ///     (4) oldProjectOpCollectionOutputs = oldProjectOpOutputs - oldProjectOpNonCollectionOutputs
        ///     (5) build a new list of collectionInfo's for the new NestOp, including 
        ///         only oldProjectOpCollectionOutputs.
        ///     (6) leftCorrelationVars = vars that are defined by the left most child of the input nestOpNode
        ///         and used in the subtrees rooted at the other children of the input nestOpNode
        ///     (7) newProjectOpOutputs = oldProjectOpNonCollectionOutputs + oldNestOpNonCollectionOutputs + leftCorrelationVars 
        ///
        ///     (8) newProjectOpChildren = .... 
        /// 
        /// Of course everything needs to be "derefed", that is, expressed in the projectOp Var Ids.
        /// 
        ///     (9) Set ProjectOp's input to NestOp's input
        ///     (10) Set NestOp's input to ProjectOp.
        /// 
        ///  
        /// 
        private Node ProjectOpCase2(Node projectNode) { 
#if DEBUG 
            string input = Dump.ToXml(Command, projectNode);
#endif //DEBUG 
            ProjectOp projectOp = (ProjectOp)projectNode.Op;
            Node nestNode = projectNode.Child0;
            NestBaseOp nestOp = nestNode.Op as NestBaseOp;
#if DEBUG 
            // NOTE: I do not believe that we need to remap the nest op in terms of
            //       the project op, but I can't prove it right now; if the assert 
            //       below fires, I was wrong. 
            //Dictionary projectToNestVarMap = new Dictionary();
 
            Command.RecomputeNodeInfo(projectNode);
            ExtendedNodeInfo projectNodeInfo = Command.GetExtendedNodeInfo(projectNode);
            ExtendedNodeInfo nestNodeInfo = Command.GetExtendedNodeInfo(nestNode);
 
            foreach (Node chi in projectNode.Child1.Children) {
                VarDefOp varDefOp = (VarDefOp)chi.Op; 
                Node definingExprNode = chi.Child0; 

                if (OpType.VarRef == definingExprNode.Op.OpType) { 
                    VarRefOp varRefOp = (VarRefOp)definingExprNode.Op;
                    PlanCompiler.Assert(varRefOp.Var == varDefOp.Var || !projectNodeInfo.LocalDefinitions.IsSet(varRefOp.Var), "need to remap vars!");

                    //if (!projectToNestVarMap.ContainsKey(varRefOp.Var)) { 
                    //    projectToNestVarMap.Add(varRefOp.Var, varDefOp.Var);
                    //} 
                } 
            }
#endif //DEBUG 

            // (1) Determine oldNestOpCollectionOutputs
            VarVec oldNestOpCollectionOutputs = Command.CreateVarVec();
            foreach (CollectionInfo ci in nestOp.CollectionInfo) { 
                oldNestOpCollectionOutputs.Set(ci.CollectionVar);
            } 
 
            // (2) oldNestOpNonCollectionOutputs = oldNestOpOutputs - oldNestOpCollectionOutputs;
            VarVec oldNestOpNonCollectionOutputs = Command.CreateVarVec(nestOp.Outputs); 
            oldNestOpNonCollectionOutputs.Minus(oldNestOpCollectionOutputs);

            // (3) oldProjectOpNonCollectionOutputs = oldProjectOpOutputs - oldNestOpCollectionOutputs
            VarVec oldProjectOpNonCollectionOutputs = Command.CreateVarVec(projectOp.Outputs); 
            oldProjectOpNonCollectionOutputs.Minus(oldNestOpCollectionOutputs);
 
            // (4) oldProjectOpCollectionOutputs = oldProjectOpOutputs - oldProjectOpNonCollectionOutputs 
            VarVec oldProjectOpCollectionOutputs = Command.CreateVarVec(projectOp.Outputs);
            oldProjectOpCollectionOutputs.Minus(oldProjectOpNonCollectionOutputs); 

            // (5) build a new list of collectionInfo's for the new NestOp, including
            //     only oldProjectOpCollectionOutputs.
            VarVec collectionsToRemove = Command.CreateVarVec(oldNestOpCollectionOutputs); 
            collectionsToRemove.Minus(oldProjectOpCollectionOutputs);
            List newCollectionInfoList; 
            List newNestNodeChildren; 

            if (collectionsToRemove.IsEmpty) { 
                newCollectionInfoList = nestOp.CollectionInfo;
                newNestNodeChildren = new List(nestNode.Children);
            }
            else { 
                newCollectionInfoList = new List();
                newNestNodeChildren = new List(); 
                newNestNodeChildren.Add(nestNode.Child0); 
                int i = 1;
                foreach (CollectionInfo ci in nestOp.CollectionInfo) { 
                    if (!collectionsToRemove.IsSet(ci.CollectionVar)) {
                        newCollectionInfoList.Add(ci);
                        newNestNodeChildren.Add(nestNode.Children[i]);
                    } 
                    i++;
                } 
            } 

            // (6) leftCorrelationVars = vars that are defined by the left most child of the input nestOpNode 
            //   and used in the subtrees rooted at the other children of the input nestOpNode
            //   #479547:  These need to be added to the outputs of the project
            VarVec leftCorrelationVars = Command.CreateVarVec();
            for (int i = 1; i < nestNode.Children.Count; i++) { 
                leftCorrelationVars.Or(nestNode.Children[i].GetExtendedNodeInfo(Command).ExternalReferences);
            } 
            leftCorrelationVars.And(nestNode.Child0.GetExtendedNodeInfo(this.Command).Definitions); 

            // (7) newProjectOpOutputs = oldProjectOpNonCollectionOutputs + oldNestOpNonCollectionOutputs + leftCorrelationVars 
            VarVec newProjectOpOutputs = Command.CreateVarVec(oldProjectOpNonCollectionOutputs);
            newProjectOpOutputs.Or(oldNestOpNonCollectionOutputs);
            newProjectOpOutputs.Or(leftCorrelationVars);
 
            // (8) newProjectOpChildren = ....
            List newProjectOpChildren = new List(projectNode.Child1.Children.Count); 
            foreach (Node chi in projectNode.Child1.Children) { 
                VarDefOp varDefOp = (VarDefOp)chi.Op;
 
                if (newProjectOpOutputs.IsSet(varDefOp.Var)) {
                    newProjectOpChildren.Add(chi);
                }
            } 

            // (9) and (10), do the switch. 
            if (0 != newCollectionInfoList.Count) { 
                // In some cases, the only var in the projection is the collection var; so
                // the new projectOp will have an empty projection list; we can't just pullup 
                // the input, so we add a temporary constant op to it, ensuring that we don't
                // have an empty projection list.
                if (newProjectOpOutputs.IsEmpty) {
                    PlanCompiler.Assert(newProjectOpChildren.Count == 0, "outputs is empty with non-zero count of children?"); 

                    NullOp tempOp = Command.CreateNullOp(Command.StringType); 
                    Node tempNode = Command.CreateNode(tempOp); 
                    Var tempVar;
                    Node varDefNode = Command.CreateVarDefNode(tempNode, out tempVar); 
                    newProjectOpChildren.Add(varDefNode);
                    newProjectOpOutputs.Set(tempVar);
                }
            } 

            // Update the projectOp node with the new list of vars and 
            // the new list of children. 
            projectNode.Op = Command.CreateProjectOp(Command.CreateVarVec(newProjectOpOutputs));
            projectNode.Child1 = Command.CreateNode(projectNode.Child1.Op, newProjectOpChildren); 

            if (0 == newCollectionInfoList.Count) {
                // There are no remaining nested collections (because none of them
                // were actually referenced)  We just pullup the driving node of the 
                // nest and eliminate the nestOp entirely.
                projectNode.Child0 = nestNode.Child0; 
                nestNode = projectNode; 
            }
            else { 
                // We need to make sure that we project out any external references to the driving
                // node that the nested collections have, or we're going to end up with unresolvable
                // vars when we pull them up over the current driving node.
                VarVec nestOpOutputs = Command.CreateVarVec(projectOp.Outputs); 

                for (int i = 1; i < newNestNodeChildren.Count; i++) { 
                    nestOpOutputs.Or(newNestNodeChildren[i].GetNodeInfo(Command).ExternalReferences); 
                }
 
                // We need to make sure we project out the sort keys too...
                foreach (InternalTrees.SortKey sk in nestOp.PrefixSortKeys) {
                    nestOpOutputs.Set(sk.Var);
                } 

                nestNode.Op = Command.CreateMultiStreamNestOp(nestOp.PrefixSortKeys, nestOpOutputs, newCollectionInfoList); 
 
                // we need to create a new node because we may have removed some of the collections.
                nestNode = Command.CreateNode(nestNode.Op, newNestNodeChildren); 

                // Pull the nestNode up over the projectNode, and adjust
                // their inputs accordingly.
                projectNode.Child0 = nestNode.Child0; 
                nestNode.Child0 = projectNode;
 
                Command.RecomputeNodeInfo(projectNode); 
            }
 
            // Finally, recompute node info
            Command.RecomputeNodeInfo(nestNode);
#if DEBUG
            int size = input.Length; // GC.KeepAlive makes FxCop Grumpy. 
            string output = Dump.ToXml(Command, nestNode);
#endif //DEBUG 
            return nestNode; 
        }
 
        /// 
        /// SetOp common processing
        /// 
        ///  
        /// The input to an IntersectOp or an ExceptOp cannot be a NestOp � that
        /// would imply that we support distinctness over collections  - which 
        /// we don�t. 
        ///
        /// UnionAllOp is somewhat trickier. We would need a way to percolate keys 
        /// up the UnionAllOp � and I�m ok with not supporting this case for now.
        /// 
        /// 
        ///  
        /// 
        protected override Node VisitSetOp(SetOp op, Node n) { 
            return NestingNotSupported(op, n); 
        }
 
        /// 
        /// SingleRowOp
        ///
        /// SingleRowOp(NestOp(x,...)) => NestOp(SingleRowOp(x),...) 
        /// 
        ///  
        ///  
        /// 
        public override Node Visit(SingleRowOp op, Node n) { 
            VisitChildren(n);

            if (IsNestOpNode(n.Child0)) {
                n = n.Child0; 
                Node newSingleRowOpNode = Command.CreateNode(op, n.Child0);
                n.Child0 = newSingleRowOpNode; 
                Command.RecomputeNodeInfo(n); 
            }
            return n; 
        }

        /// 
        /// SortOp 
        /// 
        ///  
        /// If the input to a SortOp is a NestOp, then none of the sort 
        /// keys can be collection Vars of the NestOp � we don't support
        /// sorts over collections. 
        /// 
        /// 
        /// 
        ///  
        public override Node Visit(SortOp op, Node n) {
            // Visit the children 
            VisitChildren(n); 
            m_varRemapper.RemapNode(n);
 
            // If the child is a NestOp, then simply push the sortkeys into the
            // "prefixKeys" of the nestOp, and return the NestOp itself.
            // The SortOp has now been merged into the NestOp
            NestBaseOp nestOp = n.Child0.Op as NestBaseOp; 
            if (nestOp != null) {
                n.Child0.Op = GetNestOpWithConsolidatedSortKeys(nestOp, op.Keys); 
                return n.Child0; 
            }
 
            return n;
        }

        ///  
        /// ConstrainedSortOp
        ///  
        ///  
        /// Push the ConstrainedSortOp onto the driving node of the NestOp:
        /// 
        ///     ConstrainedSortOp(NestOp(X,Y,...)) ==> NestOp(ConstrainedSortOp(X),Y,...)
        ///
        /// There should not be any need for var renaming, because the ConstrainedSortOp cannot
        /// refer to any vars from the NestOp 
        /// 
        ///  
        ///  
        /// 
        public override Node Visit(ConstrainedSortOp op, Node n) { 
            // Visit the children
            VisitChildren(n);

            // If the input is a nest op, we push the ConstrainedSort onto 
            // the driving node.
            NestBaseOp nestOp = n.Child0.Op as NestBaseOp; 
            if (nestOp != null) { 
                Node nestNode = n.Child0;
                n.Child0 = nestNode.Child0; 
                nestNode.Child0 = n;
                nestNode.Op = GetNestOpWithConsolidatedSortKeys(nestOp, op.Keys);
                n = nestNode;
            } 
            return n;
        } 
 
        /// 
        /// Helper method used by Visit(ConstrainedSortOp, Node)and Visit(SortOp, Node). 
        /// It returns a NestBaseOp equivalent to the inputNestOp, only with the given sortKeys
        /// prepended to the prefix sort keys already on the inputNestOp.
        /// 
        ///  
        /// 
        ///  
        private NestBaseOp GetNestOpWithConsolidatedSortKeys(NestBaseOp inputNestOp, List sortKeys) { 
            NestBaseOp result;
 
            // Include the  sort keys as the prefix sort keys;
            // Note that we can't actually have a SSNest at this point in
            // the tree; they're only introduced once we've processed the
            // entire tree. 

            if (inputNestOp.PrefixSortKeys.Count == 0) { 
                foreach (InternalTrees.SortKey sk in sortKeys) { 
                    //SQLBUDT #507170 - We can't just add the sort keys, we need to copy them,
                    // to avoid changes to one to affect the other 
                    inputNestOp.PrefixSortKeys.Add(Command.CreateSortKey(sk.Var, sk.AscendingSort, sk.Collation));
                }
                result = inputNestOp;
            } 
            else {
 
                VarVec sortVars = Command.CreateVarVec(); 

                // First add the sort keys from the SortBaseOp, then the NestOp keys 
                List sortKeyList = ConsolidateSortKeys(sortKeys, inputNestOp.PrefixSortKeys);

                PlanCompiler.Assert(inputNestOp is MultiStreamNestOp, "Unexpected SingleStreamNestOp?");
 
                // Finally, build a new NestOp with the keys...
                result = Command.CreateMultiStreamNestOp(sortKeyList, inputNestOp.Outputs, inputNestOp.CollectionInfo); 
            } 
            return result;
        } 

        /// 
        /// Helper method that given two lists of sort keys creates a single list of sort keys without duplicates.
        /// First the keys from the first given list are added, then from the second one. 
        /// 
        ///  
        ///  
        /// 
        private List ConsolidateSortKeys(List sortKeyList1, List sortKeyList2) { 
            VarVec sortVars = Command.CreateVarVec();
            List sortKeyList = new List();

            foreach (InternalTrees.SortKey sk in sortKeyList1) { 
                if (!sortVars.IsSet(sk.Var)) {
                    sortVars.Set(sk.Var); 
 
                    //SQLBUDT #507170 - We can't just add the sort keys, we need to copy them,
                    // to avoid changes to one to affect the other 
                    sortKeyList.Add(Command.CreateSortKey(sk.Var, sk.AscendingSort, sk.Collation));
                }
            }
 
            foreach (InternalTrees.SortKey sk in sortKeyList2) {
                if (!sortVars.IsSet(sk.Var)){ 
                    sortVars.Set(sk.Var); 
                    sortKeyList.Add(Command.CreateSortKey(sk.Var, sk.AscendingSort, sk.Collation));
                } 
            }

            return sortKeyList;
        } 

        ///  
        /// UnnestOp 
        /// 
        ///  
        /// Logically, the UnnestOp can simply be replaced with the defining expression
        /// corresponding to the Var property of the UnnestOp. The tricky part is that
        /// the UnnestOp produces a set of ColumnVars which may be referenced in other
        /// parts of the query, and these need to be replaced by the corresponding Vars 
        /// produced by the defining expression.
        /// 
        /// There are essentially four cases: 
        ///
        /// Case 1: The UnnestOps Var is a UDT. Only the store can handle this, so we 
        ///         pass it on without changing it.
        ///
        /// Case 2: The UnnestOp has a Function as its input.  This implies that the
        ///         store has TVFs, which it can Unnest, so we let it handle that and do 
        ///         nothing.
        /// 
        /// Case 3: The UnnestOp Var defines a Nested collection.  We'll just replace 
        ///         the UnnestOp with the Input:
        /// 
        ///             UnnestOp(VarDef(CollectOp(PhysicalProjectOp(input)))) => input
        ///
        /// Case 4: The UnnestOp Var refers to a Nested collection from elsewhere.  As we
        ///         discover NestOps, we maintain a var->PhysicalProject Node map.  When 
        ///         we get this case, we just make a copy of the PhysicalProject node, for
        ///         the referenced Var, and we replace the UnnestOp with it. 
        /// 
        ///             UnnestOp(VarDef(VarRef(v))) ==> copy-of-defining-node-for-v
        /// 
        /// Then, we need to update all references to the output Vars (ColumnVars) produced
        /// by the Unnest to instead refer to the Vars produced by the copy of the subquery.
        /// We produce a map from the Vars of the subquery to the corresponding vars of the
        /// UnnestOp. We then use this map as we walk up the tree, and replace any references 
        /// to the Unnest Vars by the new Vars.
        /// 
        /// To simplify this process, as part of the ITreeGenerator, whenever we generate 
        /// an UnnestOp, we will generate a ProjectOp above it � which simply selects out
        /// all Vars from the UnnestOp; and has no local definitions. This allows us to 
        /// restrict the Var->Var replacement to just ProjectOp.
        /// 
        /// 
        ///  
        /// 
        public override Node Visit(UnnestOp op, Node n) { 
#if DEBUG 
            string input = Dump.ToXml(Command, n);
#endif //DEBUG 
            // First, visit my children
            VisitChildren(n);

            // If we're unnesting a UDT, then simply return - we cannot eliminate this unnest 
            // It must be handled by the store
            md.CollectionType collType = TypeHelpers.GetEdmType(op.Var.Type); 
            if (TypeUtils.IsUdt(collType.TypeUsage)) { 
                return n;
            } 

            // Find the VarDef node for the var we're supposed to unnest.
            PlanCompiler.Assert(n.Child0.Op.OpType == OpType.VarDef, "Unnest without VarDef input?");
            PlanCompiler.Assert(((VarDefOp)n.Child0.Op).Var == op.Var, "Unnest var not found?"); 
            PlanCompiler.Assert(n.Child0.HasChild0, "VarDef without input?");
            Node newNode = n.Child0.Child0; 
 
            if (OpType.Function == newNode.Op.OpType) {
                // If we have an unnest over a function, there's nothing more we can do 
                // This really means that the underlying store has the ability to
                // support TVFs, and therefore unnests, and we simply leave it as is
                return n;
            } 
            else if (OpType.Collect == newNode.Op.OpType) {
                // UnnestOp(VarDef(CollectOp(PhysicalProjectOp(x)))) ==> x 
 
                PlanCompiler.Assert(newNode.HasChild0, "collect without input?");
                newNode = newNode.Child0; 

                PlanCompiler.Assert(newNode.Op.OpType == OpType.PhysicalProject, "collect without physicalProject?");

                // Ensure others that reference my var will know to use me; 
                m_definingNodeMap.Add(op.Var, newNode);
            } 
            else if (OpType.VarRef == newNode.Op.OpType) { 
                // UnnestOp(VarDef(VarRef(v))) ==> copy-of-defining-node-for-v
                // 
                // The Unnest's input is a VarRef; we need to replace it with
                // the defining node, and ensure we fixup the vars.
                Var refVar = ResolveVarReference(((VarRefOp)newNode.Op).Var);
 
                Node refVarDefiningNode;
                bool found = m_definingNodeMap.TryGetValue(refVar, out refVarDefiningNode); 
                PlanCompiler.Assert(found, "Could not find a definition for a referenced collection var"); 

                newNode = CopyCollectionVarDefinition(refVarDefiningNode); 

                PlanCompiler.Assert(newNode.Op.OpType == OpType.PhysicalProject, "driving node is not physicalProject?");
            }
            else { 
                throw EntityUtil.InternalError(EntityUtil.InternalErrorCode.InvalidInternalTree, 2, newNode.Op.OpType);
            } 
 
            IEnumerable inputVars = ((PhysicalProjectOp)newNode.Op).Outputs;
 
            PlanCompiler.Assert(newNode.HasChild0, "physicalProject without input?");
            newNode = newNode.Child0;

            // Bug 480721: a SortOp input to Unnest is irrelevant - eliminate it 
            if (newNode.Op.OpType == OpType.Sort) {
                newNode = newNode.Child0; 
            } 

            // Update the replacement vars to reflect the pulled up operation 
            UpdateReplacementVarMap(op.Table.Columns, inputVars);

#if DEBUG
            int size = input.Length; // GC.KeepAlive makes FxCop Grumpy. 
            string output = Dump.ToXml(Command, newNode);
#endif //DEBUG 
            return newNode; 
        }
 
        /// 
        /// Copies the given defining node for a collection var, but also makes sure to 'register' all newly
        /// created collection vars (i.e. copied).
        /// 
        ///SQLBUDT #557427: The defining node that is being copied may itself contain definitions to other
        /// collection vars. These defintions would be present in m_definingNodeMap. However, after we make a copy 
        /// of the defining node, we need to make sure to also put 'matching' definitions of these other collection 
        /// vars into m_definingNodeMap.
        /// The dictionary collectionVarDefinitions (below) contains the copied definitions of such collection vars. 
        /// but without the wrapping PhysicalProjectOp.
        ///
        /// Example:     m_definingNodeMap contains (var1, definition1) and (var2, definintion2).
        ///              var2 is defined inside the definition of var1. 
        ///              Here we copy definition1 -> definintion1'.
        ///              We need to add to m_definitionNodeMap (var2', definition2'). 
        ///              definition2' should be a copy of definiton2 in the context of to definition1', 
        ///              i.e. definition2' should relate to definition1' in same way that definition2 relates to definition1
        ///         ///  
        /// 
        /// 
        private Node CopyCollectionVarDefinition(Node refVarDefiningNode) {
 
            VarMap varMap;
            Dictionary collectionVarDefinitions; 
            Node newNode = OpCopierTrackingCollectionVars.Copy(Command, refVarDefiningNode, out varMap, out collectionVarDefinitions); 

            if (collectionVarDefinitions.Count != 0) { 
                VarMap reverseMap = varMap.GetReverseMap();

                foreach (KeyValuePair collectionVarDefinitionPair in collectionVarDefinitions) {
                    // 
                    // Getting the matching definition for a collection map (i.e. definition2' from the example above)
                    // 
                    // Definitions of collection vars are rooted at a PhysicalProjectOp, 
                    //      i.e. definition2 = PhysicalProjectOp(output2, columnMap2, definingSubtree2)
                    // 
                    //  The collectionVarDefinitions dictionary gives us the defining nodes rooted at what would a child
                    //  of such PhysicalProjectOp, i.e.  definingSubtree2'.
                    //
                    //  definition2' = PhysicalProjectOp(CopyWithRemap(output2), CopyWithRemap(columnMap2), definingSubtree2') 
                    //
 
                    Node keyDefiningNode; 
                    Var keyDefiningVar = reverseMap[collectionVarDefinitionPair.Key];
                    //Note: we should not call ResolveVarReference(keyDefiningNode), we can only use the exact var 
                    if (m_definingNodeMap.TryGetValue(keyDefiningVar, out keyDefiningNode)) {
                        PhysicalProjectOp originalPhysicalProjectOp = (PhysicalProjectOp)keyDefiningNode.Op;

                        VarList newOutputs = VarRemapper.RemapVarList(Command, varMap, originalPhysicalProjectOp.Outputs); 
                        SimpleCollectionColumnMap newColumnMap = (SimpleCollectionColumnMap)ColumnMapCopier.Copy(originalPhysicalProjectOp.ColumnMap, varMap);
 
                        PhysicalProjectOp newPhysicalProjectOp = Command.CreatePhysicalProjectOp(newOutputs, newColumnMap); 
                        Node newDefiningNode = Command.CreateNode(newPhysicalProjectOp, collectionVarDefinitionPair.Value);
 
                        m_definingNodeMap.Add(collectionVarDefinitionPair.Key, newDefiningNode);
                    }
                }
            } 
            return newNode;
        } 
 
        #endregion
 
        #region PhysicalOp Visitors

        /// 
        /// MultiStreamNestOp/SingleStreamNestOp common processing. 
        ///
        /// Pretty much just verifies that we didn't leave a NestOp behind. 
        ///  
        /// 
        ///  
        /// 
        ///
        protected override Node VisitNestOp(NestBaseOp op, Node n) {
            // First, visit my children 
            VisitChildren(n);
 
            // If any of the children are a nestOp, then we have a 
            // problem; it shouldn't have happened.
            foreach (Node chi in n.Children) { 
                if (IsNestOpNode(chi)) {
                    throw EntityUtil.InternalError(EntityUtil.InternalErrorCode.NestOverNest);
                }
            } 
            return n;
        } 
 
        /// 
        /// PhysicalProjectOp 
        /// 
        /// 
        /// Tranformation:
        /// 
        ///     PhysicalProjectOp(MultiStreamNestOp(...)) => PhysicalProjectOp(SortOp(...))
        /// 
        /// Strategy: 
        ///
        ///     (1) Convert MultiStreamNestOp(...) => SingleStreamNestOp(...) 
        ///     (2) Convert SingleStreamNestOp(...) => SortOp(...)
        ///     (3) Fixup the column maps.
        ///
        ///  
        /// 
        ///  
        ///  
        public override Node Visit(PhysicalProjectOp op, Node n) {
            // cannot be multi-input (not at this point) 
            PlanCompiler.Assert(n.Children.Count == 1, "multiple inputs to physicalProject?");

            // First visit my children
            VisitChildren(n); 
            m_varRemapper.RemapNode(n);
 
            // Wait until we're processing the root physicalProjectNode to convert the nestOp 
            // to sort/union all; it's much easier to unnest them if we don't monkey with them
            // until then. 
            //
            // Also, even if we're the root physicalProjectNode and the children aren't NestOps,
            // then there's nothing further to do.
            if (n != Command.Root || !IsNestOpNode(n.Child0)) { 
                return n;
            } 
 
#if DEBUG
            string input = Dump.ToXml(Command, n); 
#endif //DEBUG

            Node nestNode = n.Child0;
 
            // OK, we're now guaranteed to be processing a root physicalProjectNode with at
            // least one MultiStreamNestOp as it's input.  First step is to convert that into 
            // a single SingleStreamNestOp. 
            //
            // NOTE: if we ever wanted to support MARS, we would probably avoid the conversion 
            //       to SingleStreamNest here, and do something to optimize this a bit
            //       differently for MARS.  But that's a future feature.
            Dictionary varRefReplacementMap = new Dictionary();
            VarList outputVars; 
            SimpleColumnMap[] keyColumnMaps;
 
            nestNode = ConvertToSingleStreamNest(nestNode, varRefReplacementMap, out outputVars, out keyColumnMaps); 
            SingleStreamNestOp ssnOp = (SingleStreamNestOp)nestNode.Op;
 
            // Build up the sort node (if necessary).
            Node sortNode = BuildSortForNestElimination(ssnOp, nestNode);

            // Create a new column map using the columnMapPatcher that was updated by the 
            // conversion to SingleStreamNest process.
            SimpleCollectionColumnMap newProjectColumnMap = (SimpleCollectionColumnMap)ColumnMapTranslator.Translate(((PhysicalProjectOp)n.Op).ColumnMap, varRefReplacementMap); 
            newProjectColumnMap = new SimpleCollectionColumnMap(newProjectColumnMap.Type, newProjectColumnMap.Name, newProjectColumnMap.Element, keyColumnMaps, null, newProjectColumnMap.SortKeys); 

            // Ok, build the new PhysicalProjectOp, slap the sortNode as its input 
            // and we're all done.
            n.Op = Command.CreatePhysicalProjectOp(outputVars, newProjectColumnMap);
            n.Child0 = sortNode;
 
#if DEBUG
            int size = input.Length;// GC.KeepAlive makes FxCop Grumpy. 
            string output = Dump.ToXml(Command, n); 
#endif //DEBUG
 
            return n;
        }

        ///  
        /// Build up a sort node above the nestOp's input - only if there
        /// are any sort keys to produce 
        ///  
        /// 
        ///  
        /// 
        private Node BuildSortForNestElimination(SingleStreamNestOp ssnOp, Node nestNode) {
            Node sortNode;
 
            List sortKeyList = BuildSortKeyList(ssnOp);
 
            // Now if, at this point, there aren't any sort keys then remove the 
            // sort operation, otherwise, build a new SortNode;
            if (sortKeyList.Count > 0) { 
                SortOp sortOp = Command.CreateSortOp(sortKeyList);
                sortNode = Command.CreateNode(sortOp, nestNode.Child0);
            }
            else { 
                // No sort keys => single_row_table => no need to sort
                sortNode = nestNode.Child0; 
            } 
            return sortNode;
        } 

        /// 
        /// Build up the list of sortkeys. This list should comprise (in order):
        /// 
        ///  - Any prefix sort keys (these represent sort operations on the
        ///    driving table, that were logically above the nest) 
        ///  - The keys of the nest operation 
        ///  - The discriminator column for the nest operation
        ///  - the list of postfix sort keys (used to represent nested collections) 
        ///
        /// Note that we only add the first occurrance of a var to the list; further
        /// references to the same variable would be trumped by the first one.
        ///  
        /// 
        ///  
        private List BuildSortKeyList(SingleStreamNestOp ssnOp) { 
            VarVec sortVars = Command.CreateVarVec();
 
            // First add the prefix sort keys
            List sortKeyList = new List();
            foreach (InternalTrees.SortKey sk in ssnOp.PrefixSortKeys) {
                if (!sortVars.IsSet(sk.Var)) { 
                    sortVars.Set(sk.Var);
                    sortKeyList.Add(sk); 
                } 
            }
 
            // Then add the nestop keys
            foreach (Var v in ssnOp.Keys) {
                if (!sortVars.IsSet(v)) {
                    sortVars.Set(v); 
                    InternalTrees.SortKey sk = Command.CreateSortKey(v);
                    sortKeyList.Add(sk); 
                } 
            }
 
            // Then add the discriminator var
            PlanCompiler.Assert(!sortVars.IsSet(ssnOp.Discriminator), "prefix sort on discriminator?");
            sortKeyList.Add(Command.CreateSortKey(ssnOp.Discriminator));
 
            // Finally, add the postfix keys
            foreach (InternalTrees.SortKey sk in ssnOp.PostfixSortKeys) { 
                if (!sortVars.IsSet(sk.Var)) { 
                    sortVars.Set(sk.Var);
                    sortKeyList.Add(sk); 
                }
            }
            return sortKeyList;
        } 

 
        ///  
        /// convert MultiStreamNestOp to SingleStreamNestOp
        ///  
        /// 
        /// A MultiStreamNestOp is typically of the form M(D, N1, N2, ..., Nk)
        /// where D is the driver stream, and N1, N2 etc. represent the collections.
        /// 
        /// In general, this can be converted into a SingleStreamNestOp over:
        /// 
        ///    (D+ outerApply N1) AugmentedUnionAll (D+ outerApply N2) ... 
        ///
        /// Where: 
        ///
        ///     D+ is D with an extra discriminator column that helps to identify
        ///     the specific collection.
        /// 
        ///     AugmentedUnionAll is simply a unionAll where each branch of the
        ///     unionAll is augmented with nulls for the corresponding columns 
        ///     of other tables in the branch 
        ///
        /// The simple case where there is only a single nested collection is easier 
        /// to address, and can be represented by:
        ///
        ///     MultiStreamNest(D, N1) => SingleStreamNest(OuterApply(D, N1))
        /// 
        /// The more complex case, where there is more than one nested column, requires
        /// quite a bit more work: 
        /// 
        ///     MultiStreamNest(D, X, Y,...) => SingleStreamNest(UnionAll(Project{"1", D1...Dn, X1...Xn, nY1...nYn}(OuterApply(D, X)), Project{"2", D1...Dn, nX1...nXn, Y1...Yn}(OuterApply(D, Y)), ...))
        /// 
        /// Where:
        ///      D           is the driving collection
        ///      D1...Dn     are the columns from the driving collection
        ///      X           is the first nested collection 
        ///      X1...Xn     are the columns from the first nested collection
        ///      nX1...nXn   are null values for all columns from the first nested collection 
        ///      Y           is the second nested collection 
        ///      Y1...Yn     are the columns from the second nested collection
        ///      nY1...nYn   are null values for all columns from the second nested collection 
        /// 
        /// 
        /// 
        ///  
        /// 
        ///  
        private Node ConvertToSingleStreamNest(Node nestNode, Dictionary varRefReplacementMap, out VarList flattenedOutputVarList, out SimpleColumnMap[] parentKeyColumnMaps) { 
#if DEBUG
            string input = Dump.ToXml(Command, nestNode); 
#endif //DEBUG
            MultiStreamNestOp nestOp = (MultiStreamNestOp)nestNode.Op;

            // We can't convert this node to a SingleStreamNest until all it's MultiStreamNest 
            // inputs are converted, so do that first.
            for (int i = 1; i < nestNode.Children.Count; i++) { 
                Node chi = nestNode.Children[i]; 

                if (chi.Op.OpType == OpType.MultiStreamNest) { 
                    CollectionInfo chiCi = nestOp.CollectionInfo[i - 1];

                    VarList childFlattenedOutputVars;
                    SimpleColumnMap[] childKeyColumnMaps; 

                    nestNode.Children[i] = ConvertToSingleStreamNest(chi, varRefReplacementMap, out childFlattenedOutputVars, out childKeyColumnMaps); 
 
                    // Now this may seem odd here, and it may look like we should have done this
                    // inside the recursive ConvertToSingleStreamNest call above, but that call 
                    // doesn't have access to the CollectionInfo for it's parent, which is what
                    // we need to manipulate before we enter the loop below where we try and fold
                    // THIS nestOp nodes into a singleStreamNestOp.
                    ColumnMap childColumnMap = ColumnMapTranslator.Translate(chiCi.ColumnMap, varRefReplacementMap); 

                    VarVec childKeys = Command.CreateVarVec(((SingleStreamNestOp)nestNode.Children[i].Op).Keys); 
 
                    nestOp.CollectionInfo[i - 1] = Command.CreateCollectionInfo(chiCi.CollectionVar,
                                                                                  childColumnMap, 
                                                                                  childFlattenedOutputVars,
                                                                                  childKeys,
                                                                                  chiCi.SortKeys,
                                                                                  null /*discriminatorValue*/ 
                                                                                  );
                } 
            } 

            // Make sure that the driving node has keys defined. Otherwise we're in 
            // trouble; we must be able to infer keys from the driving node; If it's
            // available, we'll use the NewGuid canonical function to put a key on the
            // driving node.
            Node drivingNode = nestNode.Child0; 
            KeyVec drivingNodeKeys = Command.PullupKeys(drivingNode);
 
            // Get a deterministic ordering of Vars from this node. 
            // NOTE: we're using the drivingNode's definitions, which is a VarVec so it
            //       won't match the order of the input's columns, but the key thing is 
            //       that we use the same order for all nested children, so it's OK.
            ExtendedNodeInfo drivingNodeInfo = Command.GetExtendedNodeInfo(drivingNode);
            VarVec drivingNodeVarVec = drivingNodeInfo.Definitions;
            VarList drivingNodeVars = Command.CreateVarList(drivingNodeVarVec); 

            if (drivingNodeKeys.NoKeys) { 
                // No Keys.  We'll Transform the statement to add a NewGuid as follows: 
                //
                //      MultiStreamNest(D, X, Y,...) => MultiStreamNest(Project{NewGuid},(D), X, Y,...) 

                md.EdmFunction newGuidFunction;

                if (!TryGetNewGuidFunction(out newGuidFunction)) { 
                    // If we can't get the NewGuid canonical function, we'll just throw
                    // an exception indicating that we need keys. 
                    throw EntityUtil.KeysRequiredForNesting(); 
                }
 
                // Construct the FunctionOp, the VarDef and The VarDefList to call the
                // NewGuid canonical function,
                Var newGuidVar;
                Node newVarDefListNode = Command.CreateVarDefListNode( 
                                            Command.CreateNode(
                                                Command.CreateFunctionOp(newGuidFunction)), out newGuidVar); 
 
                // We need to make a copy of the drivingNodeVarVec, because we don't want
                // to modify the one that's in the extendedNodeInfo.  Once we do, we set 
                // the var from the NewGuid in both it and the varlist.
                drivingNodeVarVec = Command.CreateVarVec(drivingNodeVarVec);
                drivingNodeVarVec.Set(newGuidVar);
                drivingNodeVars.Add(newGuidVar); 

                // While we're at it, make sure we update the KeyVars to include the var from 
                // the NewGuid, effectively solveing our problem. 
                drivingNodeKeys = new KeyVec(Command);
                drivingNodeKeys.KeyVars.Set(newGuidVar); 

                // OK, construct a new projectOp and node to go above the driving node, and
                // hook it into the driving node of the nest op.
                drivingNode = Command.CreateNode(Command.CreateProjectOp(drivingNodeVarVec), drivingNode, newVarDefListNode); 
                nestNode.Child0 = drivingNode;
 
                // Finally, we have to create a new MultiStreamNestOp that includes the key 
                // value we just added in its outputs.
                VarVec newNestopOutputs = Command.CreateVarVec(nestOp.Outputs); 
                newNestopOutputs.Set(newGuidVar);
                nestOp = Command.CreateMultiStreamNestOp(nestOp.PrefixSortKeys, newNestopOutputs, nestOp.CollectionInfo);
                nestNode.Op = nestOp;
                Command.RecomputeNodeInfo(nestNode); 
            }
 
            // Normalize all collection inputs to the nestOp. Specifically, remove any 
            // SortOps (adding the sort keys to the postfix sortkey list). Additionally,
            // add a discriminatorVar to each collection child 
            VarList discriminatorVarList;
            List> postfixSortKeyList;
            NormalizeNestOpInputs(nestOp, nestNode, out discriminatorVarList, out postfixSortKeyList);
 
            // Now build up the union-all subquery
            List> varMapList; 
            Var outputDiscriminatorVar; 
            Node unionAllNode = BuildUnionAllSubqueryForNestOp(nestOp, nestNode, drivingNodeVars, discriminatorVarList, out outputDiscriminatorVar, out varMapList);
            Dictionary drivingNodeVarMap = varMapList[0]; 

            // OK.  We've finally created the UnionAll over each of the project/outerApply
            // combinations.  We know that the output columns will be:
            // 
            //      Discriminator, DrivingColumns, Collection1Columns, Collection2Columns, ...
            // 
            // Now, rebuild the columnMaps, since all of the columns in the original column 
            // maps are now referencing newer variables.  To do that, we'll walk the list of
            // outputs from the unionAll, and construct new VarRefColumnMaps for each one, 
            // and adding it to a ColumnMapPatcher, which we'll use to actually fix everything
            // up.
            //
            // While we're at it, we'll build a new list of top-level output columns, which 
            // should include only the Discriminator, the columns from the driving collection,
            // and and one column for each of the nested collections. 
 
            // Start building the flattenedOutputVarList that the top level PhysicalProjectOp
            // is to output. 
            flattenedOutputVarList = Command.CreateVarList(RemapVars(drivingNodeVars, drivingNodeVarMap));
            VarVec flattenedOutputVarVec = Command.CreateVarVec(flattenedOutputVarList);
            VarVec nestOpOutputs = Command.CreateVarVec(flattenedOutputVarVec);
 
            // Add any adjustments to the driving nodes vars to the column map patcher
            foreach (KeyValuePair kv in drivingNodeVarMap) { 
                if (kv.Key != kv.Value) { 
                    varRefReplacementMap[kv.Key] = new VarRefColumnMap(kv.Value);
                } 
            }

            RemapSortKeys(nestOp.PrefixSortKeys, drivingNodeVarMap);
 
            List newPostfixSortKeys = new List();
            List newCollectionInfoList = new List(); 
 
            // Build the discriminator column map, and ensure it's in the outputs
            VarRefColumnMap discriminatorColumnMap = new VarRefColumnMap(outputDiscriminatorVar); 
            nestOpOutputs.Set(outputDiscriminatorVar);

            if (!flattenedOutputVarVec.IsSet(outputDiscriminatorVar)) {
                flattenedOutputVarList.Add(outputDiscriminatorVar); 
                flattenedOutputVarVec.Set(outputDiscriminatorVar);
            } 
 
            // Build the key column maps, and ensure they're in the outputs as well.
            VarVec parentKeys = RemapVarVec(drivingNodeKeys.KeyVars, drivingNodeVarMap); 
            parentKeyColumnMaps = new SimpleColumnMap[parentKeys.Count];

            int index = 0;
            foreach (Var keyVar in parentKeys) { 
                parentKeyColumnMaps[index] = new VarRefColumnMap(keyVar);
                index++; 
 
                if (!flattenedOutputVarVec.IsSet(keyVar)) {
                    flattenedOutputVarList.Add(keyVar); 
                    flattenedOutputVarVec.Set(keyVar);
                }
            }
 
            // Now that we've handled the driving node, deal with each of the
            // nested inputs, in sequence. 
            for (int i = 1; i < nestNode.Children.Count; i++) { 
                CollectionInfo ci = nestOp.CollectionInfo[i - 1];
                List postfixSortKeys = postfixSortKeyList[i]; 

                RemapSortKeys(postfixSortKeys, varMapList[i]);
                newPostfixSortKeys.AddRange(postfixSortKeys);
 
                ColumnMap newColumnMap = ColumnMapTranslator.Translate(ci.ColumnMap, varMapList[i]);
                VarList newFlattenedElementVars = RemapVarList(ci.FlattenedElementVars, varMapList[i]); 
                VarVec newCollectionKeys = RemapVarVec(ci.Keys, varMapList[i]); 

                RemapSortKeys(ci.SortKeys, varMapList[i]); 

                CollectionInfo newCollectionInfo = Command.CreateCollectionInfo(
                                                                                ci.CollectionVar,
                                                                                newColumnMap, 
                                                                                newFlattenedElementVars,
                                                                                newCollectionKeys, 
                                                                                ci.SortKeys, 
                                                                                i);
                newCollectionInfoList.Add(newCollectionInfo); 

                // For a collection Var, we add the flattened elementVars for the
                // collection in place of the collection Var itself, and we create
                // a new column map to represent all the stuff we've done. 

                foreach (Var v in newFlattenedElementVars) { 
                    if (!flattenedOutputVarVec.IsSet(v)) { 
                        flattenedOutputVarList.Add(v);
                        flattenedOutputVarVec.Set(v); 
                    }
                }

                nestOpOutputs.Set(ci.CollectionVar); 

                int keyColumnMapIndex = 0; 
                SimpleColumnMap[] keyColumnMaps = new SimpleColumnMap[newCollectionInfo.Keys.Count]; 
                foreach (Var keyVar in newCollectionInfo.Keys) {
                    keyColumnMaps[keyColumnMapIndex] = new VarRefColumnMap(keyVar); 
                    keyColumnMapIndex++;
                }

                SortKeyInfo[] sortKeyColumnMaps = new SortKeyInfo[newCollectionInfo.SortKeys.Count]; 
                for(int k = 0; k < sortKeyColumnMaps.Length; ++k) {
                    InternalTrees.SortKey sortKey = newCollectionInfo.SortKeys[k]; 
                    VarRefColumnMap varRefColumnMap = new VarRefColumnMap(sortKey.Var); 
                    sortKeyColumnMaps[k] = new SortKeyInfo(varRefColumnMap, sortKey.AscendingSort, sortKey.Collation);
                } 

                DiscriminatedCollectionColumnMap collectionColumnMap = new DiscriminatedCollectionColumnMap(
                                                                            TypeUtils.CreateCollectionType(newCollectionInfo.ColumnMap.Type),
                                                                            newCollectionInfo.ColumnMap.Name, 
                                                                            newCollectionInfo.ColumnMap,
                                                                            keyColumnMaps, 
                                                                            parentKeyColumnMaps, 
                                                                            sortKeyColumnMaps,
                                                                            discriminatorColumnMap, 
                                                                            newCollectionInfo.DiscriminatorValue
                                                                            );
                varRefReplacementMap[ci.CollectionVar] = collectionColumnMap;
            } 

            // Finally, build up the SingleStreamNest Node 
            SingleStreamNestOp newSsnOp = Command.CreateSingleStreamNestOp( 
                                                            parentKeys,
                                                            nestOp.PrefixSortKeys, 
                                                            newPostfixSortKeys,
                                                            nestOpOutputs,
                                                            newCollectionInfoList,
                                                            outputDiscriminatorVar); 
            Node newNestNode = Command.CreateNode(newSsnOp, unionAllNode);
 
#if DEBUG 
            int size = input.Length;// GC.KeepAlive makes FxCop Grumpy.
            string output = Dump.ToXml(Command, newNestNode); 
#endif //DEBUG

            return newNestNode;
        } 

        ///  
        /// "Normalize" each input to the NestOp. 
        /// We're now in the context of a MultiStreamNestOp, and we're trying to convert this
        /// into a SingleStreamNestOp. 
        ///
        /// Normalization specifically refers to
        ///   - augmenting each input with a discriminator value (that describes the collection)
        ///   - removing the sort node at the root (and capturing this information as part of the sortkeys) 
        /// 
        /// the nestOp 
        /// the nestOp subtree 
        /// Discriminator Vars for each Collection input
        /// SortKeys (postfix) for each Collection input 
        ///
        ///
        private void NormalizeNestOpInputs(NestBaseOp nestOp, Node nestNode, out VarList discriminatorVarList, out List> sortKeys) {
            discriminatorVarList = Command.CreateVarList(); 

            // We insert a dummy var and value at poistion 0 for the deriving node, which 
            // we should never reference; 
            discriminatorVarList.Add(null);
 
            sortKeys = new List>();
            sortKeys.Add(nestOp.PrefixSortKeys);

            for (int i = 1; i < nestNode.Children.Count; i++) { 
                Node inputNode = nestNode.Children[i];
                // Since we're called from ConvertToSingleStreamNest, it is possible that we have a 
                // SingleStreamNest here, because the input to the MultiStreamNest we're converting 
                // may have been a MultiStreamNest that was converted to a SingleStreamNest.
                SingleStreamNestOp ssnOp = inputNode.Op as SingleStreamNestOp; 

                // If this collection is a SingleStreamNest, we pull up the key information
                // in it, and pullup the input;
                if (null != ssnOp) { 
                    // Note that the sortKeys argument is 1:1 with the nestOp inputs, that is
                    // each input may have exactly one entry in the list, so we have to combine 
                    // all of the sort key components (Prefix+Keys+Discriminator+PostFix) into 
                    // one list.
                    List mySortKeys = BuildSortKeyList(ssnOp); 
                    sortKeys.Add(mySortKeys);

                    inputNode = inputNode.Child0;
                } 
                else {
                    // If the current collection has a SortNode specified, then pull that 
                    // out, and add the information to the list of postfix SortColumns 
                    SortOp sortOp = inputNode.Op as SortOp;
                    if (null != sortOp) { 
                        inputNode = inputNode.Child0; // bypass the sort node
                        // Add the sort keys to the list of postfix sort keys
                        sortKeys.Add(sortOp.Keys);
                    } 
                    else {
                        // No postfix sort keys for this case 
                        sortKeys.Add(new List()); 
                    }
                } 

                // #447304: Ensure that any SortKey Vars will be projected from the input in addition to showing up in the postfix sort keys
                // by adding them to the FlattenedElementVars for this NestOp input's CollectionInfo.
                VarList flattenedElementVars = nestOp.CollectionInfo[i - 1].FlattenedElementVars; 
                foreach (InternalTrees.SortKey sortKey in sortKeys[i]) {
                    if (!flattenedElementVars.Contains(sortKey.Var)) { 
                        flattenedElementVars.Add(sortKey.Var); 
                    }
                } 

                // Add a discriminator column to the collection-side - this must
                // happen before the outer-apply is added on; we need to use the value of
                // the discriminator to distinguish between null and empty collections 
                Var discriminatorVar;
                Node augmentedInput = AugmentNodeWithInternalConstant(inputNode, i, out discriminatorVar); 
                nestNode.Children[i] = augmentedInput; 
                discriminatorVarList.Add(discriminatorVar);
            } 
        }

        /// 
        /// Add an internal integer constant to a node. Specifically: 
        ///
        ///     N ==> Project(N,{definitions-from-N, internalConstant}) 
        ///  
        /// the input node to augment
        /// the value for the itnernal constant  
        /// the computed Var for the internal constant
        /// the augmented node
        private Node AugmentNodeWithInternalConstant(Node input, int internalConstantValue, out Var internalConstantVar) {
            // Construct the internal constant value, a ConstantOp node, and 
            // a VarDef node that that defines it.
            InternalConstantOp discriminatorOp = Command.CreateInternalConstantOp(Command.IntegerType, internalConstantValue); 
            Node discriminatorNode = Command.CreateNode(discriminatorOp); 
            Node varDefListNode = Command.CreateVarDefListNode(discriminatorNode, out internalConstantVar);
 
            // Now identify the list of definitions from the input, and project out
            // every one of them and include the internalConstantVar
            ExtendedNodeInfo inputNodeInfo = Command.GetExtendedNodeInfo(input);
            VarVec projectOutputs = Command.CreateVarVec(inputNodeInfo.Definitions); 
            projectOutputs.Set(internalConstantVar);
 
            ProjectOp projectOp = Command.CreateProjectOp(projectOutputs); 
            Node projectNode = Command.CreateNode(projectOp, input, varDefListNode);
 
            return projectNode;
        }

        ///  
        /// Convert a SingleStreamNestOp into a massive UnionAllOp
        ///  
        ///  
        /// 
        ///  
        /// 
        ///
        /// 
        ///  
        /// 
        private Node BuildUnionAllSubqueryForNestOp(NestBaseOp nestOp, Node nestNode, VarList drivingNodeVars, VarList discriminatorVarList, out Var discriminatorVar, out List> varMapList) { 
            Node drivingNode = nestNode.Child0; 

            // For each of the NESTED collections... 
            Node unionAllNode = null;
            VarList unionAllOutputs = null;
            for (int i = 1; i < nestNode.Children.Count; i++) {
                // Ensure we only use the driving collection tree once, so other 
                // transformations do not unintentionally change more than one path.
                // To prevent nodes in the tree from being used in multiple paths, 
                // we copy the driving input on successive nodes. 
                VarList newDrivingNodeVars;
                Node newDrivingNode; 
                VarList newFlattenedElementVars;
                Op op;

                if (i > 1) { 
                    newDrivingNode = OpCopier.Copy(Command, drivingNode, drivingNodeVars, out newDrivingNodeVars);
                    // 
                    // Bug 450245: If we copied the driver node, then references to driver node vars 
                    // from the collection subquery must be patched up
                    // 
                    VarRemapper varRemapper = new VarRemapper(this.Command);
                    for (int j = 0; j < drivingNodeVars.Count; j++) {
                        varRemapper.AddMapping(drivingNodeVars[j], newDrivingNodeVars[j]);
                    } 
                    // Remap all references in the current subquery
                    varRemapper.RemapSubtree(nestNode.Children[i]); 
 
                    // Bug 479183: Remap the flattened element vars
                    newFlattenedElementVars = varRemapper.RemapVarList(nestOp.CollectionInfo[i - 1].FlattenedElementVars); 

                    // Create a cross apply for all but the first collection
                    op = Command.CreateCrossApplyOp();
                } 
                else {
                    newDrivingNode = drivingNode; 
                    newDrivingNodeVars = drivingNodeVars; 
                    newFlattenedElementVars = nestOp.CollectionInfo[i - 1].FlattenedElementVars;
 
                    // Create an outer apply for the first collection,
                    // that way we ensure at least one row for each row in the driver node.
                    op = Command.CreateOuterApplyOp();
                } 

                // Create an outer apply with the driver node and the nested collection. 
                Node applyNode = Command.CreateNode(op, newDrivingNode, nestNode.Children[i]); 

                // Now create a ProjectOp that augments the output from the OuterApplyOp 
                // with nulls for each column from other collections

                // Build the VarDefList (the list of vars) for the Project, starting
                // with the collection discriminator var 
                List varDefListChildren = new List();
                VarList projectOutputs = Command.CreateVarList(); 
 
                // Add the collection discriminator var to the output.
                projectOutputs.Add(discriminatorVarList[i]); 

                // Add all columns from the driving node
                projectOutputs.AddRange(newDrivingNodeVars);
 
                // Add all the vars from all the nested collections;
                for (int j = 1; j < nestNode.Children.Count; j++) { 
                    CollectionInfo otherCollectionInfo = nestOp.CollectionInfo[j - 1]; 
                    // For the current nested collection, we just pick the var that's
                    // coming from there and don't need have a new var defined, but for 
                    // the rest we construct null values.
                    if (i == j) {
                        projectOutputs.AddRange(newFlattenedElementVars);
                    } 
                    else {
                        foreach (Var v in otherCollectionInfo.FlattenedElementVars) { 
                            NullOp nullOp = Command.CreateNullOp(v.Type); 
                            Node nullOpNode = Command.CreateNode(nullOp);
                            Var nullOpVar; 
                            Node nullOpVarDefNode = Command.CreateVarDefNode(nullOpNode, out nullOpVar);
                            varDefListChildren.Add(nullOpVarDefNode);
                            projectOutputs.Add(nullOpVar);
                        } 
                    }
                } 
 
                Node varDefListNode = Command.CreateNode(Command.CreateVarDefListOp(), varDefListChildren);
 
                // Now, build up the projectOp
                VarVec projectOutputsVarSet = Command.CreateVarVec(projectOutputs);
                ProjectOp projectOp = Command.CreateProjectOp(projectOutputsVarSet);
                Node projectNode = Command.CreateNode(projectOp, applyNode, varDefListNode); 

                // finally, build the union all 
                if (unionAllNode == null) { 
                    unionAllNode = projectNode;
                    unionAllOutputs = projectOutputs; 
                }
                else {
                    VarMap unionAllMap = new VarMap();
                    VarMap projectMap = new VarMap(); 
                    for (int idx = 0; idx < unionAllOutputs.Count; idx++) {
                        Var outputVar = Command.CreateSetOpVar(unionAllOutputs[idx].Type); 
                        unionAllMap.Add(outputVar, unionAllOutputs[idx]); 
                        projectMap.Add(outputVar, projectOutputs[idx]);
                    } 
                    UnionAllOp unionAllOp = Command.CreateUnionAllOp(unionAllMap, projectMap);
                    unionAllNode = Command.CreateNode(unionAllOp, unionAllNode, projectNode);

                    // Get the output vars from the union-op. This must be in the same order 
                    // as the original list of Vars
                    unionAllOutputs = GetUnionOutputs(unionAllOp, unionAllOutputs); 
                } 
            }
 
            // We're done building the node, but now we have to build a mapping from
            // the before-Vars to the after-Vars
            varMapList = new List>();
            IEnumerator outputVarsEnumerator = unionAllOutputs.GetEnumerator(); 
            if (!outputVarsEnumerator.MoveNext()) {
                throw EntityUtil.InternalError(EntityUtil.InternalErrorCode.ColumnCountMismatch, 4); // more columns from children than are on the unionAll? 
            } 
            // The discriminator var is always first
            discriminatorVar = outputVarsEnumerator.Current; 

            // Build a map for each input
            for (int i = 0; i < nestNode.Children.Count; i++) {
                Dictionary varMap = new Dictionary(); 
                VarList varList = (i == 0) ? drivingNodeVars : nestOp.CollectionInfo[i - 1].FlattenedElementVars;
                foreach (Var v in varList) { 
                    if (!outputVarsEnumerator.MoveNext()) { 
                        throw EntityUtil.InternalError(EntityUtil.InternalErrorCode.ColumnCountMismatch, 5); // more columns from children than are on the unionAll?
                    } 
                    varMap[v] = outputVarsEnumerator.Current;
                }
                varMapList.Add(varMap);
            } 
            if (outputVarsEnumerator.MoveNext()) {
                throw EntityUtil.InternalError(EntityUtil.InternalErrorCode.ColumnCountMismatch, 6); // at this point, we better be done with both lists... 
            } 

            return unionAllNode; 
        }

        /// 
        /// Get back an ordered list of outputs from a union-all op. The ordering should 
        /// be identical to the ordered list "leftVars" which describes the left input of
        /// the unionAllOp 
        ///  
        /// the unionall Op
        /// vars of the left input 
        /// output vars ordered in the same way as the left input
        private static VarList GetUnionOutputs(UnionAllOp unionOp, VarList leftVars) {
            VarMap varMap = unionOp.VarMap[0];
            Dictionary reverseVarMap = varMap.GetReverseMap(); 

            VarList unionAllVars = Command.CreateVarList(); 
            foreach (Var v in leftVars) { 
                Var newVar = reverseVarMap[v];
                unionAllVars.Add(newVar); 
            }

            return unionAllVars;
        } 

        ///  
        /// Tries to get NewGuid canonical function. 
        /// 
        /// out EdmFunction for NewGuid canonical function 
        /// true if a unique Edm.NewGuid canonical function was found, false otherwise
        private bool TryGetNewGuidFunction(out md.EdmFunction newGuidFunction) {
            newGuidFunction = null;
            System.Collections.ObjectModel.ReadOnlyCollection functions = m_compilerState.MetadataWorkspace.GetFunctions("NewGuid", "Edm", System.Data.Metadata.Edm.DataSpace.CSpace); 
            if (null != functions && 1 == functions.Count) {
                newGuidFunction = functions[0]; 
            } 
            return (null != newGuidFunction);
        } 

        #endregion

        #endregion 
    }
 
    #region Class OpCopierTrackingCollectionVars 
    /// 
    /// Wrapper around OpCopier to keep track of the defining subtrees 
    /// of collection vars defined in the subtree being returned as a copy.
    /// 
    internal class OpCopierTrackingCollectionVars : OpCopier {
        #region Private State 
        private Dictionary m_newCollectionVarDefinitions = new Dictionary();
        #endregion 
 
        #region Private Constructor
        private OpCopierTrackingCollectionVars(Command cmd) : base(cmd) { 
        }
        #endregion

        #region Public Surface 
        /// 
        /// Equivalent to OpCopier.Copy, only in addition it keeps track of the defining subtrees 
        /// of collection vars defined in the subtree rooted at the copy of the input node n. 
        /// 
        ///  
        /// 
        /// 
        /// 
        ///  
        internal static Node Copy(Command cmd, Node n, out VarMap varMap, out Dictionary newCollectionVarDefinitions) {
            OpCopierTrackingCollectionVars oc = new OpCopierTrackingCollectionVars(cmd); 
            Node newNode = oc.CopyNode(n); 
            varMap = oc.m_varMap;
            newCollectionVarDefinitions = oc.m_newCollectionVarDefinitions; 
            return newNode;
        }
        #endregion
 
        #region Visitor Members
        ///  
        /// Tracks the collection vars after calling the base implementation 
        /// 
        ///  
        /// 
        /// 
        public override Node Visit(MultiStreamNestOp op, Node n) {
            Node result = base.Visit(op, n); 
            MultiStreamNestOp newOp = (MultiStreamNestOp)result.Op;
 
            for (int i = 0; i < newOp.CollectionInfo.Count; i++) { 
                m_newCollectionVarDefinitions.Add(newOp.CollectionInfo[i].CollectionVar, result.Children[i + 1]);
            } 
            return result;
        }
        #endregion
    } 
    #endregion
} 

// File provided for Reference Use Only by Microsoft Corporation (c) 2007.

                        

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