Support Optimizer Hints

We as Arm Treasure Data are using Optimizer Hints with a monkey patch
(https://gist.github.com/kamipo/4c8539f0ce4acf85075cf5a6b0d9712e),
especially in order to use `MAX_EXECUTION_TIME` (refer #31129).

Example:

```ruby
class Job < ApplicationRecord
  default_scope { optimizer_hints("MAX_EXECUTION_TIME(50000) NO_INDEX_MERGE(jobs)") }
end
```

Optimizer Hints is supported not only for MySQL but also for most
databases (PostgreSQL on RDS, Oracle, SQL Server, etc), it is really
helpful to turn heavy queries for large scale applications.

2 files changed, 52 insertions, 0 deletions

diff --git a/activerecord/test/cases/adapters/mysql2/optimizer_hints_test.rb b/activerecord/test/cases/adapters/mysql2/optimizer_hints_test.rb
new file mode 100644
index 0000000000..349de49b36
--- /dev/null
+++ b/activerecord/test/cases/adapters/mysql2/optimizer_hints_test.rb
@@ -0,0 +1,24 @@
+# frozen_string_literal: true
+
+require "cases/helper"
+require "models/post"
+
+if supports_optimizer_hints?
+  class Mysql2OptimzerHintsTest < ActiveRecord::Mysql2TestCase
+    fixtures :posts
+
+    def test_optimizer_hints
+      assert_sql(%r{\ASELECT /\*\+ NO_RANGE_OPTIMIZATION\(posts index_posts_on_author_id\) \*/}) do
+        posts = Post.optimizer_hints("NO_RANGE_OPTIMIZATION(posts index_posts_on_author_id)")
+        posts = posts.select(:id).where(author_id: [0, 1])
+        assert_includes posts.explain, "| index | index_posts_on_author_id | index_posts_on_author_id |"
+      end
+
+      assert_sql(%r{\ASELECT /\*\+ NO_RANGE_OPTIMIZATION\(posts index_posts_on_author_id\) \*/}) do
+        posts = Post.optimizer_hints("/*+ NO_RANGE_OPTIMIZATION(posts index_posts_on_author_id) */")
+        posts = posts.select(:id).where(author_id: [0, 1])
+        assert_includes posts.explain, "| index | index_posts_on_author_id | index_posts_on_author_id |"
+      end
+    end
+  end
+end
diff --git a/activerecord/test/cases/adapters/postgresql/optimizer_hints_test.rb b/activerecord/test/cases/adapters/postgresql/optimizer_hints_test.rb
new file mode 100644
index 0000000000..1bfa815cac
--- /dev/null
+++ b/activerecord/test/cases/adapters/postgresql/optimizer_hints_test.rb
@@ -0,0 +1,28 @@
+# frozen_string_literal: true
+
+require "cases/helper"
+require "models/post"
+
+if supports_optimizer_hints?
+  class PostgresqlOptimzerHintsTest < ActiveRecord::PostgreSQLTestCase
+    fixtures :posts
+
+    def setup
+      enable_extension!("pg_hint_plan", ActiveRecord::Base.connection)
+    end
+
+    def test_optimizer_hints
+      assert_sql(%r{\ASELECT /\*\+ SeqScan\(posts\) \*/}) do
+        posts = Post.optimizer_hints("SeqScan(posts)")
+        posts = posts.select(:id).where(author_id: [0, 1])
+        assert_includes posts.explain, "Seq Scan on posts"
+      end
+
+      assert_sql(%r{\ASELECT /\*\+ SeqScan\(posts\) \*/}) do
+        posts = Post.optimizer_hints("/*+ SeqScan(posts) */")
+        posts = posts.select(:id).where(author_id: [0, 1])
+        assert_includes posts.explain, "Seq Scan on posts"
+      end
+    end
+  end
+end