Implement optimizer_hints using OPTION clause

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.
+- [#865](https://github.com/rails-sqlserver/activerecord-sqlserver-adapter/pull/865) Implemented optimizer hints.
 
 ## v6.0.1
 

lib/active_record/connection_adapters/sqlserver_adapter.rb

@@ -152,6 +152,10 @@ def supports_savepoints?
         true
       end
 
+      def supports_optimizer_hints?
+        true
+      end
+
       def supports_lazy_transactions?
         true
       end

lib/arel/visitors/sqlserver.rb

@@ -80,6 +80,16 @@ def visit_Arel_Nodes_SelectStatement o, collector
         @select_statement = nil
       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_option_clause(v) }.join(", ")
+        collector << "OPTION (#{hints})"
+      end
+
       def visit_Arel_Table o, collector
         # Apparently, o.engine.connection can actually be a different adapter
         # than sqlserver. Can be removed if fixed in ActiveRecord. See:
@@ -144,6 +154,10 @@ def collect_in_clause(left, right, collector)
         super
       end
 
+      def collect_optimizer_hints(o, collector)
+        collector
+      end
+
       # SQLServer ToSql/Visitor (Additions)
 
       def visit_Arel_Nodes_SelectStatement_SQLServer_Lock collector, options = {}
@@ -247,6 +261,10 @@ def remove_invalid_ordering_from_select_statement(node)
 
         node.orders = [] unless node.offset || node.limit
       end
+
+      def sanitize_as_option_clause(value)
+        value.gsub(%r{OPTION \s* \( (.+) \)}xi, "\\1")
+      end
     end
   end
 end

test/cases/coerced_tests.rb

@@ -982,6 +982,21 @@ def test_reverse_arel_assoc_order_with_function_coerced
   end
 end
 
+module ActiveRecord
+  class RelationTest < ActiveRecord::TestCase
+    # Skipping this test. SQL Server doesn't support optimizer hint as comments
+    coerce_tests! :test_relation_with_optimizer_hints_filters_sql_comment_delimiters
+
+    coerce_tests! :test_does_not_duplicate_optimizer_hints_on_merge
+    def test_does_not_duplicate_optimizer_hints_on_merge_coerced
+      escaped_table = Post.connection.quote_table_name("posts")
+      expected = "SELECT #{escaped_table}.* FROM #{escaped_table} OPTION (OMGHINT)"
+      query = Post.optimizer_hints("OMGHINT").merge(Post.optimizer_hints("OMGHINT")).to_sql
+      assert_equal expected, query
+    end
+  end
+end
+
 require "models/post"
 class SanitizeTest < ActiveRecord::TestCase
   # Use nvarchar string (N'') in assert

test/cases/optimizer_hints_test_sqlserver.rb

@@ -0,0 +1,72 @@
+# frozen_string_literal: true
+
+require "cases/helper_sqlserver"
+require "models/company"
+
+class OptimizerHitsTestSQLServer < ActiveRecord::TestCase
+  fixtures :companies
+
+  it "apply optimizations" do
+    assert_sql(%r{\ASELECT .+ FROM .+ OPTION \(HASH GROUP\)\z}) do
+      companies = Company.optimizer_hints("HASH GROUP")
+      companies = companies.distinct.select("firm_id")
+      assert_includes companies.explain, "| Hash Match | Aggregate  |"
+    end
+
+    assert_sql(%r{\ASELECT .+ FROM .+ OPTION \(ORDER GROUP\)\z}) do
+      companies = Company.optimizer_hints("ORDER GROUP")
+      companies = companies.distinct.select("firm_id")
+      assert_includes companies.explain, "| Stream Aggregate | Aggregate  |"
+    end
+  end
+
+  it "apply multiple optimizations" do
+    assert_sql(%r{\ASELECT .+ FROM .+ OPTION \(HASH GROUP, FAST 1\)\z}) do
+      companies = Company.optimizer_hints("HASH GROUP", "FAST 1")
+      companies = companies.distinct.select("firm_id")
+      assert_includes companies.explain, "| Hash Match | Flow Distinct |"
+    end
+  end
+
+  it "support subqueries" do
+    assert_sql(%r{.*'SELECT COUNT\(count_column\) FROM \(SELECT .*\) subquery_for_count OPTION \(MAXDOP 2\)'.*}) do
+      companies = Company.optimizer_hints("MAXDOP 2")
+      companies = companies.select(:id).where(firm_id: [0, 1]).limit(3)
+      assert_equal 3, companies.count
+    end
+  end
+
+  it "sanitize values" do
+    assert_sql(%r{\ASELECT .+ FROM .+ OPTION \(HASH GROUP\)\z}) do
+      companies = Company.optimizer_hints("OPTION (HASH GROUP)")
+      companies = companies.distinct.select("firm_id")
+      companies.to_a
+    end
+
+    assert_sql(%r{\ASELECT .+ FROM .+ OPTION \(HASH GROUP\)\z}) do
+      companies = Company.optimizer_hints("OPTION(HASH GROUP)")
+      companies = companies.distinct.select("firm_id")
+      companies.to_a
+    end
+
+    assert_sql(%r{\ASELECT .+ FROM .+ OPTION \(TABLE HINT \(\[companies\], INDEX\(1\)\)\)\z}) do
+      companies = Company.optimizer_hints("OPTION(TABLE HINT ([companies], INDEX(1)))")
+      companies = companies.distinct.select("firm_id")
+      companies.to_a
+    end
+
+    assert_sql(%r{\ASELECT .+ FROM .+ OPTION \(HASH GROUP\)\z}) do
+      companies = Company.optimizer_hints("Option(HASH GROUP)")
+      companies = companies.distinct.select("firm_id")
+      companies.to_a
+    end
+  end
+
+  it "skip optimization after unscope" do
+    assert_sql("SELECT DISTINCT [companies].[firm_id] FROM [companies]") do
+      companies = Company.optimizer_hints("HASH GROUP")
+      companies = companies.distinct.select("firm_id")
+      companies.unscope(:optimizer_hints).load
+    end
+  end
+end