Enable buffering for split queries

Otherwise MARS is assumed by EF Core, which we don't support.

See dotnet/efcore#21420

(cherry picked from commit ab5d62a)

src/EFCore.PG/Extensions/NpgsqlServiceCollectionExtensions.cs

@@ -86,6 +86,7 @@ public static IServiceCollection AddEntityFrameworkNpgsql([NotNull] this IServic
                     .TryAdd<ISqlExpressionFactory, NpgsqlSqlExpressionFactory>()
                     .TryAdd<ISingletonOptions, INpgsqlOptions>(p => p.GetService<INpgsqlOptions>())
                     .TryAdd<IValueConverterSelector, NpgsqlValueConverterSelector>()
+                    .TryAdd<IQueryCompilationContextFactory, NpgsqlQueryCompilationContextFactory>()
                     .TryAddProviderSpecificServices(
                         b => b
                              .TryAddSingleton<INpgsqlValueGeneratorCache, NpgsqlValueGeneratorCache>()

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

@@ -0,0 +1,17 @@
+using JetBrains.Annotations;
+using Microsoft.EntityFrameworkCore.Query;
+
+namespace Npgsql.EntityFrameworkCore.PostgreSQL.Query.Internal
+{
+    public class NpgsqlQueryCompilationContext : RelationalQueryCompilationContext
+    {
+        public NpgsqlQueryCompilationContext(
+            [NotNull] QueryCompilationContextDependencies dependencies,
+            [NotNull] RelationalQueryCompilationContextDependencies relationalDependencies, bool async)
+            : base(dependencies, relationalDependencies, async)
+        {
+        }
+
+        public override bool IsBuffering => base.IsBuffering || IsSplitQuery;
+    }
+}

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

@@ -0,0 +1,26 @@
+using JetBrains.Annotations;
+using Microsoft.EntityFrameworkCore.Query;
+using Npgsql.EntityFrameworkCore.PostgreSQL.Utilities;
+
+namespace Npgsql.EntityFrameworkCore.PostgreSQL.Query.Internal
+{
+    public class NpgsqlQueryCompilationContextFactory : IQueryCompilationContextFactory
+    {
+        readonly QueryCompilationContextDependencies _dependencies;
+        readonly RelationalQueryCompilationContextDependencies _relationalDependencies;
+
+        public NpgsqlQueryCompilationContextFactory(
+            [NotNull] QueryCompilationContextDependencies dependencies,
+            [NotNull] RelationalQueryCompilationContextDependencies relationalDependencies)
+        {
+            Check.NotNull(dependencies, nameof(dependencies));
+            Check.NotNull(relationalDependencies, nameof(relationalDependencies));
+
+            _dependencies = dependencies;
+            _relationalDependencies = relationalDependencies;
+        }
+
+        public virtual QueryCompilationContext Create(bool async)
+            => new NpgsqlQueryCompilationContext(_dependencies, _relationalDependencies, async);
+    }
+}