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.

1 files changed, 12 insertions, 2 deletions

diff --git a/activerecord/lib/active_record/connection_adapters/postgresql_adapter.rb b/activerecord/lib/active_record/connection_adapters/postgresql_adapter.rb
index 672f11cfcb..2905660499 100644
--- a/activerecord/lib/active_record/connection_adapters/postgresql_adapter.rb
+++ b/activerecord/lib/active_record/connection_adapters/postgresql_adapter.rb
@@ -351,6 +351,13 @@ module ActiveRecord
         postgresql_version >= 90400
       end
 
+      def supports_optimizer_hints?
+        unless defined?(@has_pg_hint_plan)
+          @has_pg_hint_plan = extension_available?("pg_hint_plan")
+        end
+        @has_pg_hint_plan
+      end
+
       def supports_lazy_transactions?
         true
       end
@@ -381,9 +388,12 @@ module ActiveRecord
         }
       end
 
+      def extension_available?(name)
+        query_value("SELECT true FROM pg_available_extensions WHERE name = #{quote(name)}", "SCHEMA")
+      end
+
       def extension_enabled?(name)
-        res = exec_query("SELECT EXISTS(SELECT * FROM pg_available_extensions WHERE name = '#{name}' AND installed_version IS NOT NULL) as enabled", "SCHEMA")
-        res.cast_values.first
+        query_value("SELECT installed_version IS NOT NULL FROM pg_available_extensions WHERE name = #{quote(name)}", "SCHEMA")
       end
 
       def extensions