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, 13 insertions, 0 deletions

diff --git a/activerecord/lib/arel/visitors/to_sql.rb b/activerecord/lib/arel/visitors/to_sql.rb
index d0dec63860..7e3e265208 100644
--- a/activerecord/lib/arel/visitors/to_sql.rb
+++ b/activerecord/lib/arel/visitors/to_sql.rb
@@ -219,6 +219,7 @@ module Arel # :nodoc: all
         def visit_Arel_Nodes_SelectCore(o, collector)
           collector << "SELECT"
 
+          collector = collect_optimizer_hints(o, collector)
           collector = maybe_visit o.set_quantifier, collector
 
           collect_nodes_for o.projections, collector, SPACE
@@ -236,6 +237,10 @@ module Arel # :nodoc: all
           collector
         end
 
+        def visit_Arel_Nodes_OptimizerHints(o, collector)
+          collector << "/*+ #{sanitize_as_sql_comment(o).join(" ")} */"
+        end
+
         def collect_nodes_for(nodes, collector, spacer, connector = COMMA)
           unless nodes.empty?
             collector << spacer
@@ -799,6 +804,14 @@ module Arel # :nodoc: all
           @connection.quote_column_name(name)
         end
 
+        def sanitize_as_sql_comment(o)
+          o.expr.map { |v| v.gsub(%r{ /\*\+?\s* | \s*\*/ }x, "") }
+        end
+
+        def collect_optimizer_hints(o, collector)
+          maybe_visit o.optimizer_hints, collector
+        end
+
         def maybe_visit(thing, collector)
           return collector unless thing
           collector << " "