Query: Lift Subqueries when outer QM's select can be pushed down into…

… subqueryQM

Apply QueryOptimizer recursively on all QueryModels
Introduce SubQueryExpressionVisitor<IQueryModelVisitor> to apply QMV's recursively.

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

@@ -487,14 +487,14 @@ private static Expression UnfoldStructuralComparison(ExpressionType expressionTy
                 if (leftExpressions.Length == 1
                     && expressionType == ExpressionType.Equal)
                 {
-                    var translatedExpression = TransformNullComparison(leftExpressions[0], rightExpressions[0], binaryExpression.NodeType)
-                                               ?? Expression.MakeBinary(expressionType, leftExpressions[0], rightExpressions[0]);
-                    return Expression.AndAlso(translatedExpression, Expression.Constant(true, translatedExpression.Type));
+                    var translatedExpression = TransformNullComparison(leftExpressions[0], rightExpressions[0], expressionType)
+                                               ?? Expression.Equal(leftExpressions[0], rightExpressions[0]);
+                    return Expression.AndAlso(translatedExpression, Expression.Constant(true, typeof(bool)));
                 }
 
                 return leftExpressions
                     .Zip(rightExpressions, (l, r) =>
-                        TransformNullComparison(l, r, binaryExpression.NodeType)
+                        TransformNullComparison(l, r, expressionType)
                         ?? Expression.MakeBinary(expressionType, l, r))
                     .Aggregate((e1, e2) =>
                         expressionType == ExpressionType.Equal

src/EFCore/Query/ExpressionVisitors/Internal/TransformingQueryModelExpressionVisitor.cs

@@ -0,0 +1,47 @@
+// Copyright (c) .NET Foundation. All rights reserved.
+// Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
+
+using System.Linq.Expressions;
+using JetBrains.Annotations;
+using Microsoft.EntityFrameworkCore.Utilities;
+using Remotion.Linq;
+using Remotion.Linq.Clauses.Expressions;
+
+namespace Microsoft.EntityFrameworkCore.Query.ExpressionVisitors.Internal
+{
+    /// <summary>
+    ///     This API supports the Entity Framework Core infrastructure and is not intended to be used
+    ///     directly from your code. This API may change or be removed in future releases.
+    /// </summary>
+    public class TransformingQueryModelExpressionVisitor<TVisitor> : ExpressionVisitorBase
+        where TVisitor : IQueryModelVisitor
+    {
+        /// <summary>
+        ///     This API supports the Entity Framework Core infrastructure and is not intended to be used
+        ///     directly from your code. This API may change or be removed in future releases.
+        /// </summary>
+        protected virtual TVisitor TransformingQueryModelVisitor { get; }
+
+        /// <summary>
+        ///     This API supports the Entity Framework Core infrastructure and is not intended to be used
+        ///     directly from your code. This API may change or be removed in future releases.
+        /// </summary>
+        public TransformingQueryModelExpressionVisitor([NotNull] TVisitor transformingQueryModelVisitor)
+        {
+            Check.NotNull(transformingQueryModelVisitor, nameof(transformingQueryModelVisitor));
+
+            TransformingQueryModelVisitor = transformingQueryModelVisitor;
+        }
+
+        /// <summary>
+        ///     This API supports the Entity Framework Core infrastructure and is not intended to be used
+        ///     directly from your code. This API may change or be removed in future releases.
+        /// </summary>
+        protected override Expression VisitSubQuery(SubQueryExpression expression)
+        {
+            TransformingQueryModelVisitor.VisitQueryModel(expression.QueryModel);
+
+            return base.VisitSubQuery(expression);
+        }
+    }
+}

src/EFCore/Query/Internal/QueryOptimizer.cs

@@ -8,6 +8,7 @@
 using JetBrains.Annotations;
 using Microsoft.EntityFrameworkCore.Extensions.Internal;
 using Microsoft.EntityFrameworkCore.Metadata;
+using Microsoft.EntityFrameworkCore.Query.ExpressionVisitors.Internal;
 using Microsoft.EntityFrameworkCore.Query.ResultOperators;
 using Microsoft.EntityFrameworkCore.Utilities;
 using Remotion.Linq;
@@ -76,6 +77,7 @@ public virtual void Optimize(
             _queryAnnotations = queryAnnotations;
 
             VisitQueryModel(queryModel);
+            queryModel.TransformExpressions(e => new TransformingQueryModelExpressionVisitor<QueryOptimizer>(this).Visit(e));
         }
 
         /// <summary>
@@ -175,11 +177,13 @@ protected override void FlattenSubQuery(
 
             VisitQueryModel(subQueryModel);
 
-            if (subQueryModel.ResultOperators
-                    .All(ro => ro is CastResultOperator)
+            if ((subQueryModel.ResultOperators.All(ro => ro is CastResultOperator)
                 && !subQueryModel.BodyClauses.Any(bc => bc is OrderByClause)
                 || queryModel.IsIdentityQuery()
                 && !queryModel.ResultOperators.Any())
+                || (!queryModel.BodyClauses.Any()
+                && !queryModel.ResultOperators.Any()
+                && !subQueryModel.ResultOperators.Any(ro => ro is GroupResultOperator)))
             {
                 string itemName;
 
@@ -257,14 +261,14 @@ public override void VisitResultOperator(ResultOperatorBase resultOperator, Quer
                     {
                         var oldQuerySource = queryModel.MainFromClause;
 
-                        var entityQueryProvider 
+                        var entityQueryProvider
                             = ((oldQuerySource.FromExpression as ConstantExpression)?.Value as IQueryable)?.Provider as IAsyncQueryProvider;
 
                         if (entityQueryProvider != null)
                         {
                             queryModel.ResultOperators.RemoveAt(index);
 
-                            var newMainFromClause 
+                            var newMainFromClause
                                 = new MainFromClause(
                                     oldQuerySource.ItemName,
                                     entityType.ClrType,