Fix to #6647 - Left Join (GroupJoin) always materializes elements res…

…ulting in unnecessary data pulling

Problem was that for GroupJoin we would always force materialization on participating query sources.
We were doing this because we are not always able to correctly divide outer elements of the GroupJoin into correct groups.
However, if the GroupJoin clause is wrapped around SelectMany clause the groups don't matter because they are getting flattened by SelectMany anyway.

Fix is to recognize those scenarios and only force materialization when the correct grouping actually matters.
We can avoid materialization if the GroupJoin is followed by SelectMany clause (that references the grouping) and that the grouping itself is not present anywhere else in the query.
This addresses optional navigations, which is the 80% case. Manually created GroupJoins that are not modeling LeftOuterJoins still require additional materialization, but this can be addressed later as the priority is not nearly as high.

Other bugs fixed alongside the main change:

#7722 - Query : error during compilation for queries with navigation properties and First/Single/client method operators inside a subquery.

Problem here was that for some queries we don't know how to properly bind to a value buffer (when the result of binding is subquery, and not qsre).
Fix/mitigation is to recognize those scenarios and force materialization on the subqueries. This can be properly addressed in later commit (i.e. by improving the binding logic)

#7497 - Error calling Count() after GroupJoin()

(and removed the temporary fix to #7348 and implemented a proper one)
This turned out to be a chicken-and-egg problem - we had discrepancy between query sources that we marked for materialization in case of GroupJoin + result operator, and the actual client side operation that had to be performed.
We had to fix the group join materialization to allow for translation of those result operators, but without fixing this issue also we would get invalid queries in some cases.
Proper fix is to force client result operator only if the GroupJoin cannot be flattened. We also now do this for all result operators, not just a subset because it was possible to generate invalid queries with operaors like Count().
This is now consistent with the behavior of RequiresMaterializationExpressionVisitor.

Also fixed several minor issues that were encountered once no longer do extensive materialization.

src/EFCore.Relational/Query/ExpressionVisitors/RelationalEntityQueryableExpressionVisitor.cs

@@ -3,6 +3,7 @@
 
 using System;
 using System.Collections.Generic;
+using System.Diagnostics;
 using System.Linq;
 using System.Linq.Expressions;
 using System.Reflection;
@@ -11,12 +12,14 @@
 using Microsoft.EntityFrameworkCore.Metadata;
 using Microsoft.EntityFrameworkCore.Metadata.Internal;
 using Microsoft.EntityFrameworkCore.Query.Expressions;
+using Microsoft.EntityFrameworkCore.Query.Expressions.Internal;
 using Microsoft.EntityFrameworkCore.Query.ExpressionVisitors.Internal;
 using Microsoft.EntityFrameworkCore.Query.Internal;
 using Microsoft.EntityFrameworkCore.Query.ResultOperators.Internal;
 using Microsoft.EntityFrameworkCore.Query.Sql;
 using Microsoft.EntityFrameworkCore.Storage;
 using Microsoft.EntityFrameworkCore.Utilities;
+using Remotion.Linq;
 using Remotion.Linq.Clauses;
 using Remotion.Linq.Clauses.Expressions;
 
@@ -71,8 +74,13 @@ protected override Expression VisitSubQuery(SubQueryExpression expression)
 
             var queryModelVisitor = (RelationalQueryModelVisitor)CreateQueryModelVisitor();
 
+            var queryModelMapping = new Dictionary<QueryModel, QueryModel>();
+            expression.QueryModel.PopulateQueryModelMapping(queryModelMapping);
+
             queryModelVisitor.VisitQueryModel(expression.QueryModel);
 
+            expression.QueryModel.RecreateQueryModelFromMapping(queryModelMapping);
+
             if (_querySource != null)
             {
                 QueryModelVisitor.RegisterSubQueryVisitor(_querySource, queryModelVisitor);

src/EFCore.Relational/Query/Expressions/LeftOuterJoinExpression.cs

@@ -40,35 +40,6 @@ protected override Expression Accept(ExpressionVisitor visitor)
                 : base.Accept(visitor);
         }
 
-        /// <summary>
-        ///     Determines whether or not this LeftOuterJoinExpression handles the given query source.
-        /// </summary>
-        /// <param name="querySource"> The query source. </param>
-        /// <returns>
-        ///     true if the supplied query source is handled by this LeftOuterJoinExpression; otherwise false.
-        /// </returns>
-        public override bool HandlesQuerySource(IQuerySource querySource)
-        {
-            if (querySource is AdditionalFromClause additionalFromClause)
-            {
-                var subQueryModel = (additionalFromClause.FromExpression as SubQueryExpression)?.QueryModel;
-                if (subQueryModel != null
-                    && !subQueryModel.BodyClauses.Any()
-                    && subQueryModel.ResultOperators.Count == 1
-                    && subQueryModel.ResultOperators[0] is DefaultIfEmptyResultOperator
-                    && (subQueryModel.SelectClause.Selector as QuerySourceReferenceExpression)?.ReferencedQuerySource == subQueryModel.MainFromClause
-                    && (subQueryModel.MainFromClause.FromExpression as QuerySourceReferenceExpression)?.ReferencedQuerySource is GroupJoinClause targetGroupJoin)
-                {
-                    if (QuerySource == targetGroupJoin.JoinClause)
-                    {
-                        return true;
-                    }
-                }
-            }
-
-            return base.HandlesQuerySource(querySource);
-        }
-
         /// <summary>
         ///     Creates a <see cref="string" /> representation of the Expression.
         /// </summary>

src/EFCore.Relational/Query/Expressions/SelectExpression.cs

@@ -251,7 +251,10 @@ public override bool HandlesQuerySource(IQuerySource querySource)
         {
             Check.NotNull(querySource, nameof(querySource));
 
-            return _tables.Any(te => te.QuerySource == querySource || te.HandlesQuerySource(querySource));
+            var processedQuerySource = PreProcessQuerySource(querySource);
+
+            return _tables.Any(te => te.QuerySource == processedQuerySource || te.HandlesQuerySource(processedQuerySource))
+                || base.HandlesQuerySource(querySource);
         }
 
         /// <summary>

src/EFCore.Relational/Query/Expressions/TableExpressionBase.cs

@@ -2,10 +2,14 @@
 // Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
 
 using System;
+using System.Linq;
 using System.Linq.Expressions;
 using JetBrains.Annotations;
 using Microsoft.EntityFrameworkCore.Utilities;
 using Remotion.Linq.Clauses;
+using Remotion.Linq.Clauses.Expressions;
+using Remotion.Linq.Clauses.ResultOperators;
+using Microsoft.EntityFrameworkCore.Query.Internal;
 
 namespace Microsoft.EntityFrameworkCore.Query.Expressions
 {
@@ -88,7 +92,16 @@ public virtual bool HandlesQuerySource([NotNull] IQuerySource querySource)
         {
             Check.NotNull(querySource, nameof(querySource));
 
-            return _querySource == querySource;
+            return _querySource == PreProcessQuerySource(querySource);
+        }
+
+        protected virtual IQuerySource PreProcessQuerySource([NotNull] IQuerySource querySource)
+        {
+            Check.NotNull(querySource, nameof(querySource));
+
+            var newQuerySource = (querySource as AdditionalFromClause)?.TryGetFlattenedGroupJoinClause() ?? querySource;
+
+            return (newQuerySource as GroupJoinClause)?.JoinClause ?? newQuerySource;
         }
 
         /// <summary>

src/EFCore.Relational/Query/Internal/RelationalResultOperatorHandler.cs

@@ -155,19 +155,6 @@ var handlerContext
                 return handlerContext.EvalOnClient();
             }
 
-            if (relationalQueryModelVisitor.RequiresClientSingleColumnResultOperator
-                && !(resultOperator is SkipResultOperator
-                    || resultOperator is TakeResultOperator
-                    || resultOperator is FirstResultOperator
-                    || resultOperator is SingleResultOperator
-                    || resultOperator is CountResultOperator
-                    || resultOperator is AllResultOperator
-                    || resultOperator is AnyResultOperator
-                    || resultOperator is GroupResultOperator))
-            {
-                return handlerContext.EvalOnClient();
-            }
-
             return resultHandler(handlerContext);
         }
 
@@ -241,9 +228,8 @@ private static Expression HandleAverage(HandlerContext handlerContext)
 
                 if (!(expression.RemoveConvert() is SelectExpression))
                 {
-
-                    var inputType = expression.Type;
-                    var outputType = expression.Type;
+                    var inputType = handlerContext.QueryModel.SelectClause.Selector.Type;
+                    var outputType = inputType;
 
                     var nonNullableInputType = inputType.UnwrapNullableType();
                     if (nonNullableInputType == typeof(int)
@@ -253,7 +239,7 @@ private static Expression HandleAverage(HandlerContext handlerContext)
                     }
 
                     expression = new ExplicitCastExpression(expression, outputType);
-                    var averageExpression = new SqlFunctionExpression("AVG", expression.Type, new [] { expression });
+                    var averageExpression = new SqlFunctionExpression("AVG", outputType, new [] { expression });
 
                     handlerContext.SelectExpression.SetProjectionExpression(averageExpression);
 
@@ -589,7 +575,7 @@ private static Expression HandleMin(HandlerContext handlerContext)
 
                 if (!(expression.RemoveConvert() is SelectExpression))
                 {
-                    var minExpression = new SqlFunctionExpression("MIN", expression.Type, new [] { expression });
+                    var minExpression = new SqlFunctionExpression("MIN", handlerContext.QueryModel.SelectClause.Selector.Type, new [] { expression });
 
                     handlerContext.SelectExpression.SetProjectionExpression(minExpression);
 
@@ -611,7 +597,7 @@ private static Expression HandleMax(HandlerContext handlerContext)
 
                 if (!(expression.RemoveConvert() is SelectExpression))
                 {
-                    var maxExpression = new SqlFunctionExpression("MAX", expression.Type, new [] { expression });
+                    var maxExpression = new SqlFunctionExpression("MAX", handlerContext.QueryModel.SelectClause.Selector.Type, new [] { expression });
 
                     handlerContext.SelectExpression.SetProjectionExpression(maxExpression);
 
@@ -666,7 +652,7 @@ private static Expression HandleSum(HandlerContext handlerContext)
 
                 if (!(expression.RemoveConvert() is SelectExpression))
                 {
-                    var sumExpression = new SqlFunctionExpression("SUM", expression.Type, new [] { expression });
+                    var sumExpression = new SqlFunctionExpression("SUM", handlerContext.QueryModel.SelectClause.Selector.Type, new [] { expression });
 
                     handlerContext.SelectExpression.SetProjectionExpression(sumExpression);
 

src/EFCore.Relational/Query/RelationalQueryModelVisitor.cs

@@ -58,9 +58,9 @@ private readonly Dictionary<IQuerySource, RelationalQueryModelVisitor> _subQuery
         private bool _requiresClientProjection;
         private bool _requiresClientOrderBy;
         private bool _requiresClientResultOperator;
-        private bool _requiresClientSingleColumnResultOperator;
 
         private Dictionary<IncludeSpecification, List<int>> _navigationIndexMap = new Dictionary<IncludeSpecification, List<int>>();
+        private List<GroupJoinClause> _unflattenedGroupJoinClauses = new List<GroupJoinClause>();
 
         /// <summary>
         ///     This API supports the Entity Framework Core infrastructure and is not intended to be used
@@ -169,23 +169,10 @@ public virtual bool RequiresClientProjection
         /// </value>
         public virtual bool RequiresClientResultOperator
         {
-            get { return _requiresClientResultOperator || RequiresClientEval; }
+            get { return _unflattenedGroupJoinClauses.Any() || _requiresClientResultOperator || RequiresClientEval; }
             set { _requiresClientResultOperator = value; }
         }
 
-        /// <summary>
-        ///     Gets or sets a value indicating whether the query requires client evaluation for result operators potentially apply to a subset of
-        ///     columns rather than entire row.
-        /// </summary>
-        /// <value>
-        ///     true if the query requires client single column result operator, false if not.
-        /// </value>
-        internal virtual bool RequiresClientSingleColumnResultOperator
-        {
-            get { return _requiresClientSingleColumnResultOperator || _requiresClientResultOperator || RequiresClientEval; }
-            set { _requiresClientSingleColumnResultOperator = value; }
-        }
-
         /// <summary>
         ///     Gets or sets a value indicating whether this query model visitor will be
         ///     able to bind directly to properties from its parent query without requiring
@@ -272,9 +259,6 @@ public virtual SelectExpression TryGetQuery([NotNull] IQuerySource querySource)
         {
             Check.NotNull(querySource, nameof(querySource));
 
-            querySource
-                = (querySource as GroupJoinClause)?.JoinClause ?? querySource;
-
             SelectExpression selectExpression;
             return QueriesBySource.TryGetValue(querySource, out selectExpression)
                 ? selectExpression
@@ -559,6 +543,8 @@ public override void VisitGroupJoinClause(
 
             base.VisitGroupJoinClause(groupJoinClause, queryModel, index);
 
+            _unflattenedGroupJoinClauses.Add(groupJoinClause);
+
             if (!TryFlattenGroupJoin(
                 groupJoinClause,
                 queryModel,
@@ -574,12 +560,6 @@ public override void VisitGroupJoinClause(
             {
                 WarnClientEval(groupJoinClause.JoinClause);
             }
-
-            // Workaround until #6647 is addressed - GroupJoin requires materialization of entire entity which results in all columns of that entity being projected
-            // this in turn causes result operators to be applied on all of those columns, even if the query specifies a subset of columns to perform the operation on
-            // this could lead to incorrect results (e.g. for Distinct)
-            // This however is safe to do for some operators, e.g. FirstOrDefault, Count(), Take() because their result is the same whether they are applied on single column or entire row
-            RequiresClientSingleColumnResultOperator = true;
         }
 
         private Dictionary<IQuerySource, Expression> SnapshotQuerySourceMapping(QueryModel queryModel)
@@ -1603,6 +1583,8 @@ var referencedQuerySource
             queryModel.BodyClauses.Remove(groupJoinClause);
             queryModel.BodyClauses.Remove(additionalFromClause);
 
+            _unflattenedGroupJoinClauses.Remove(groupJoinClause);
+
             var querySourceMapping = new QuerySourceMapping();
 
             querySourceMapping.AddMapping(