Add index on builds(id DESC, repository_id, event_type)
user
```ruby
class CreateIndexOnBuildsIdDescAndRepositoryIdAndEventType < ActiveRecord::Migration
  def up
    execute "CREATE INDEX CONCURRENTLY index_builds_on_id_repository_id_and_event_type_desc ON builds (id DESC, repository_id, event_type);"
  end

  def down
    execute "DROP INDEX index_builds_on_id_repository_id_and_event_type_desc;"
  end
end
```
assistant