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, 24 insertions, 0 deletions

diff --git a/activerecord/lib/active_record/relation/query_methods.rb b/activerecord/lib/active_record/relation/query_methods.rb
index 0f2cff4c9c..2603fac8b2 100644
--- a/activerecord/lib/active_record/relation/query_methods.rb
+++ b/activerecord/lib/active_record/relation/query_methods.rb
@@ -901,6 +901,29 @@ module ActiveRecord
       self
     end
 
+    # Specify optimizer hints to be used in the SELECT statement.
+    #
+    # Example (for MySQL):
+    #
+    #   Topic.optimizer_hints("MAX_EXECUTION_TIME(50000)", "NO_INDEX_MERGE(topics)")
+    #   # SELECT /*+ MAX_EXECUTION_TIME(50000) NO_INDEX_MERGE(topics) */ `topics`.* FROM `topics`
+    #
+    # Example (for PostgreSQL with pg_hint_plan):
+    #
+    #   Topic.optimizer_hints("SeqScan(topics)", "Parallel(topics 8)")
+    #   # SELECT /*+ SeqScan(topics) Parallel(topics 8) */ "topics".* FROM "topics"
+    def optimizer_hints(*args)
+      check_if_method_has_arguments!(:optimizer_hints, args)
+      spawn.optimizer_hints!(*args)
+    end
+
+    def optimizer_hints!(*args) # :nodoc:
+      args.flatten!
+
+      self.optimizer_hints_values += args
+      self
+    end
+
     # Reverse the existing order clause on the relation.
     #
     #   User.order('name ASC').reverse_order # generated SQL has 'ORDER BY name DESC'
@@ -977,6 +1000,7 @@ module ActiveRecord
 
         build_select(arel)
 
+        arel.optimizer_hints(*optimizer_hints_values) unless optimizer_hints_values.empty?
         arel.distinct(distinct_value)
         arel.from(build_from) unless from_clause.empty?
         arel.lock(lock_value) if lock_value