Add index hint for releasing blocked executions

Use Rails' `optimizer_hints` to hint MySQL to use the
`index_solid_queue_blocked_executions_for_release` composite index
when releasing blocked executions, avoiding query planner misses
and unnecessary row locks.

The hint is rendered as a `/*+ ... */` SQL comment, which MySQL's
optimizer reads and SQLite and PostgreSQL ignore as a regular
comment.

Fixes #694

Author: rosa

app/models/solid_queue/blocked_execution.rb

@@ -26,7 +26,9 @@ def release_many(concurrency_keys)
 
       def release_one(concurrency_key)
         transaction do
-          if execution = ordered.where(concurrency_key: concurrency_key).limit(1).non_blocking_lock.first
+          if execution = ordered.where(concurrency_key: concurrency_key).limit(1)
+              .use_index(:index_solid_queue_blocked_executions_for_release)
+              .non_blocking_lock.first
             execution.release
           end
         end

app/models/solid_queue/record.rb

@@ -20,6 +20,13 @@ def supports_insert_conflict_target?
           connection.supports_insert_conflict_target?
         end
       end
+
+      # Pass index hints to the query optimizer using SQL comment hints.
+      # Uses MySQL 8 optimizer hint query comments, which SQLite and
+      # PostgreSQL ignore.
+      def use_index(*indexes)
+        optimizer_hints "INDEX(#{quoted_table_name} #{indexes.join(', ')})"
+      end
     end
   end
 end