Fix to #9395 - Query: Apply CompositePredicateExpressionVisitor to jo…

…in predicates

Some optimizations that are applied to predicates in SelectExpression can be safely applied to join predicates as well. Fix modifies CompositePredicateExpressionVisitor to also look into JoinExpression predicates and not only SelectExpression predicates.
Also changes the way we build join predicates themselves, when inheritance is concerned - now the discriminator predicate will be extracted out of the conditional expression, so that the comparisons are sargable.

Before:

((a is Derived) ? a.DerivedFK : null) == b.PK - which translated to CASE block (not sargable)

After:

a is Derived && a.DerivedFK == b.PK

src/EFCore.Relational/Query/ExpressionVisitors/Internal/CompositePredicateExpressionVisitor.cs

@@ -31,7 +31,6 @@ public CompositePredicateExpressionVisitor(bool useRelationalNulls)
         protected override Expression VisitExtension(Expression expression)
         {
             var selectExpression = expression as SelectExpression;
-
             if (selectExpression?.Predicate != null)
             {
                 var predicate = new EqualityPredicateInExpressionOptimizer().Visit(selectExpression.Predicate);
@@ -53,6 +52,10 @@ protected override Expression VisitExtension(Expression expression)
 
                 selectExpression.Predicate = predicate;
             }
+            else if (expression is PredicateJoinExpressionBase joinExpression)
+            {
+                joinExpression.Predicate = new EqualityPredicateInExpressionOptimizer().Visit(joinExpression.Predicate);
+            }
 
             return base.VisitExtension(expression);
         }

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

@@ -1104,6 +1104,12 @@ var equalityExpression
                         ? new NullCompensatedExpression(newOperand)
                         : nullCompensatedExpression;
                 }
+                case DiscriminatorPredicateExpression discriminatorPredicateExpression:
+                    {
+                        var newPredicate = base.VisitExtension(expression);
+
+                        return new DiscriminatorPredicateExpression(newPredicate, discriminatorPredicateExpression.QuerySource);
+                    }
                 default:
                     return base.VisitExtension(expression);
             }

src/EFCore.Relational/Query/RelationalQueryModelVisitor.cs

@@ -25,6 +25,7 @@
 using Remotion.Linq.Clauses;
 using Remotion.Linq.Clauses.Expressions;
 using Remotion.Linq.Clauses.ResultOperators;
+using Remotion.Linq.Parsing;
 
 namespace Microsoft.EntityFrameworkCore.Query
 {
@@ -1363,7 +1364,7 @@ var discriminatorPredicate
                                         Expression.Constant(concreteEntityType.Relational().DiscriminatorValue, discriminatorPropertyExpression.Type)))
                             .Aggregate((current, next) => Expression.OrElse(next, current));
 
-                    return discriminatorPredicate;
+                    return new DiscriminatorPredicateExpression(discriminatorPredicate, typeBinaryExpression.Expression.TryGetReferencedQuerySource());
                 }
 
                 return Expression.Constant(true, typeof(bool));
@@ -1568,6 +1569,9 @@ var predicate
                 return false;
             }
 
+            var discriminatorPredicateOptimizer = new DiscriminatorPredicateOptimizingVisitor();
+            predicate = discriminatorPredicateOptimizer.Visit(predicate);
+
             QueriesBySource.Remove(joinClause);
 
             outerSelectExpression.RemoveRangeFromProjection(previousProjectionCount);
@@ -1700,6 +1704,9 @@ var predicate
                         Expression.Equal(joinClause.OuterKeySelector, joinClause.InnerKeySelector));
             }
 
+            var discriminatorPredicateOptimizer = new DiscriminatorPredicateOptimizingVisitor();
+            predicate = discriminatorPredicateOptimizer.Visit(predicate);
+
             QueriesBySource.Remove(joinClause);
 
             outerSelectExpression.RemoveRangeFromProjection(previousProjectionCount);
@@ -1778,6 +1785,28 @@ var newShapedQueryMethod
             return true;
         }
 
+        private class DiscriminatorPredicateOptimizingVisitor : RelinqExpressionVisitor
+        {
+            protected override Expression VisitBinary(BinaryExpression binaryExpression)
+            {
+                if (binaryExpression.NodeType == ExpressionType.Equal
+                    && binaryExpression.Left.RemoveConvert() is ConditionalExpression conditionalExpression
+                    && conditionalExpression.Test is DiscriminatorPredicateExpression discriminatorPredicateExpression
+                    && conditionalExpression.IfFalse.IsNullConstantExpression())
+                {
+                    return Expression.AndAlso(
+                        discriminatorPredicateExpression.Reduce(),
+                        Expression.Equal(
+                            conditionalExpression.IfTrue.Type == binaryExpression.Right.Type
+                                ? conditionalExpression.IfTrue
+                                : Expression.Convert(conditionalExpression.IfTrue, binaryExpression.Right.Type),
+                            binaryExpression.Right));
+                }
+
+                return base.VisitBinary(binaryExpression);
+            }
+        }
+
         private bool IsFlattenableGroupJoinDefaultIfEmpty(
             [NotNull] GroupJoinClause groupJoinClause,
             QueryModel queryModel,