Add hook for IQueryOptimizer

Creates Npgsql implementations of `IExpressionFragmentTranslator`
and `IQueryOptimizer` to support more complex translations.

These classes make it easier to inject new fragment translations
and expression rewrites into the EF Core translation process.

This is split out from npgsql#431. Working examples can be found there.

src/EFCore.PG/Extensions/NpgsqlServiceCollectionExtensions.cs

@@ -30,6 +30,7 @@
 using Microsoft.EntityFrameworkCore.Query;
 using Microsoft.EntityFrameworkCore.Query.ExpressionTranslators;
 using Microsoft.EntityFrameworkCore.Query.ExpressionVisitors;
+using Microsoft.EntityFrameworkCore.Query.Internal;
 using Microsoft.EntityFrameworkCore.Query.Sql;
 using Microsoft.EntityFrameworkCore.Storage;
 using Microsoft.EntityFrameworkCore.Update;
@@ -113,6 +114,7 @@ public static IServiceCollection AddEntityFrameworkNpgsql([NotNull] this IServic
                     .TryAdd<IMemberTranslator, NpgsqlCompositeMemberTranslator>()
                     .TryAdd<ICompositeMethodCallTranslator, NpgsqlCompositeMethodCallTranslator>()
                     .TryAdd<IExpressionFragmentTranslator, NpgsqlCompositeExpressionFragmentTranslator>()
+                    .TryAdd<IQueryOptimizer, NpgsqlQueryOptimizer>()
                     .TryAdd<IQuerySqlGeneratorFactory, NpgsqlQuerySqlGeneratorFactory>()
                     .TryAdd<ISqlTranslatingExpressionVisitorFactory, NpgsqlSqlTranslatingExpressionVisitorFactory>()
                     .TryAdd<ISingletonOptions, INpgsqlOptions>(p => p.GetService<INpgsqlOptions>())

src/EFCore.PG/Query/Internal/NpgsqlQueryOptimizer.cs

@@ -0,0 +1,55 @@
+#region License
+
+// The PostgreSQL License
+//
+// Copyright (C) 2016 The Npgsql Development Team
+//
+// Permission to use, copy, modify, and distribute this software and its
+// documentation for any purpose, without fee, and without a written
+// agreement is hereby granted, provided that the above copyright notice
+// and this paragraph and the following two paragraphs appear in all copies.
+//
+// IN NO EVENT SHALL THE NPGSQL DEVELOPMENT TEAM BE LIABLE TO ANY PARTY
+// FOR DIRECT, INDIRECT, SPECIAL, INCIDENTAL, OR CONSEQUENTIAL DAMAGES,
+// INCLUDING LOST PROFITS, ARISING OUT OF THE USE OF THIS SOFTWARE AND ITS
+// DOCUMENTATION, EVEN IF THE NPGSQL DEVELOPMENT TEAM HAS BEEN ADVISED OF
+// THE POSSIBILITY OF SUCH DAMAGE.
+//
+// THE NPGSQL DEVELOPMENT TEAM SPECIFICALLY DISCLAIMS ANY WARRANTIES,
+// INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY
+// AND FITNESS FOR A PARTICULAR PURPOSE. THE SOFTWARE PROVIDED HEREUNDER IS
+// ON AN "AS IS" BASIS, AND THE NPGSQL DEVELOPMENT TEAM HAS NO OBLIGATIONS
+// TO PROVIDE MAINTENANCE, SUPPORT, UPDATES, ENHANCEMENTS, OR MODIFICATIONS.
+
+#endregion
+
+using System.Linq.Expressions;
+using JetBrains.Annotations;
+using Microsoft.EntityFrameworkCore.Query;
+using Microsoft.EntityFrameworkCore.Query.Internal;
+using Remotion.Linq;
+
+namespace Npgsql.EntityFrameworkCore.PostgreSQL.Query.Internal
+{
+    /// <summary>
+    /// The default relational LINQ query optimizer for Npgsql.
+    /// </summary>
+    public class NpgsqlQueryOptimizer : QueryOptimizer
+    {
+        /// <summary>
+        /// The default expression visitors registered by the Npgsql provider.
+        /// </summary>
+        [NotNull] [ItemNotNull] static readonly ExpressionVisitor[] ExpressionVisitors = {};
+
+        /// <inheritdoc />
+        public override void Optimize(QueryCompilationContext queryCompilationContext, QueryModel queryModel)
+        {
+            base.Optimize(queryCompilationContext, queryModel);
+
+            for (int i = 0; i < ExpressionVisitors.Length; i++)
+            {
+                queryModel.TransformExpressions(ExpressionVisitors[i].Visit);
+            }
+        }
+    }
+}