Implemented optimizer hints

Author: aidanharan

CHANGELOG.md

@@ -11,7 +11,8 @@
 #### Added
 
 - [#855](https://github.com/rails-sqlserver/activerecord-sqlserver-adapter/pull/855) Add helpers to create/change/drop a schema.
-- [#857](https://github.com/rails-sqlserver/activerecord-sqlserver-adapter/pull/857) Included WAITFOR as read query type. 
+- [#857](https://github.com/rails-sqlserver/activerecord-sqlserver-adapter/pull/857) Included WAITFOR as read query type.
+- [#864](https://github.com/rails-sqlserver/activerecord-sqlserver-adapter/pull/864) Implemented optimizer hints.
 
 ## v6.0.1
 

lib/active_record/connection_adapters/sqlserver_adapter.rb

@@ -176,6 +176,10 @@ def supports_insert_conflict_target?
         false
       end
 
+      def supports_optimizer_hints?
+        true
+      end
+
       def disable_referential_integrity
         tables = tables_with_referential_integrity
         tables.each { |t| do_execute "ALTER TABLE #{quote_table_name(t)} NOCHECK CONSTRAINT ALL" }

lib/arel/visitors/sqlserver.rb

@@ -144,6 +144,20 @@ def collect_in_clause(left, right, collector)
         super
       end
 
+      def visit_Arel_Nodes_SelectCore(o, collector)
+        collector = super
+        maybe_visit o.optimizer_hints, collector
+      end
+
+      def visit_Arel_Nodes_OptimizerHints(o, collector)
+        hints = o.expr.map { |v| sanitize_as_sql_comment(v) }.join(", ")
+        collector << "OPTION (#{hints})"
+      end
+
+      def collect_optimizer_hints(o, collector)
+        collector
+      end
+
       # SQLServer ToSql/Visitor (Additions)
 
       def visit_Arel_Nodes_SelectStatement_SQLServer_Lock collector, options = {}

test/cases/optimizer_hints_test_sqlserver.rb

@@ -0,0 +1,34 @@
+# frozen_string_literal: true
+
+require "cases/helper_sqlserver"
+require "models/post"
+
+class OptimizerHintsTestSQLServer < ActiveRecord::TestCase
+  fixtures :posts
+
+  def test_optimizer_hints
+    expected_sql = "SELECT [posts].* FROM [posts] OPTION (MAXDOP 2)"
+    current_sql = Post.optimizer_hints("MAXDOP 2").to_sql
+    assert_equal expected_sql, current_sql
+  end
+
+  def test_multiple_optimizer_hints
+    expected_sql = "SELECT [posts].* FROM [posts] OPTION (MAXDOP 2, KEEPFIXED PLAN)"
+    current_sql = Post.optimizer_hints("MAXDOP 2").optimizer_hints("KEEPFIXED PLAN").to_sql
+    assert_equal expected_sql, current_sql
+  end
+
+  def test_optimizer_hints_with_count_subquery
+    assert_sql(%r{.*'SELECT COUNT\(count_column\) FROM \(SELECT .*\) subquery_for_count OPTION \(MAXDOP 2\)'.*}) do
+      posts = Post.optimizer_hints("MAXDOP 2")
+      posts = posts.select(:id).where(author_id: [0, 1]).limit(5)
+      assert_equal 5, posts.count
+    end
+  end
+
+  def test_optimizer_hints_with_unscope
+    expected_sql = "SELECT [posts].* FROM [posts]"
+    current_sql = Post.optimizer_hints("MAXDOP 2").unscope(:optimizer_hints).to_sql
+    assert_equal expected_sql, current_sql
+  end
+end