      # Renames a column without requiring downtime.
      #
      # Concurrent renames work by using database triggers to ensure both the
      # old and new column are in sync. However, this method will _not_ remove
      # the triggers or the old column automatically; this needs to be done
      # manually in a post-deployment migration. This can be done using the
      # method `cleanup_concurrent_column_rename`.
…
      def rename_column_concurrently(table, old, new, type: nil, type_cast_function: nil, batch_column_name: :id)
        unless column_exists?(table, batch_column_name)
          raise "Column #{batch_column_name} does not exist on #{table}"
        end

        if transaction_open?
          raise 'rename_column_concurrently can not be run inside a transaction'
        end

        check_trigger_permissions!(table)

        create_column_from(table, old, new, type: type, batch_column_name: batch_column_name, type_cast_function: type_cast_function)

        install_rename_triggers(table, old, new)
      end
…
      def cleanup_concurrent_column_rename(table, old, new)
        trigger_name = rename_trigger_name(table, old, new)

        check_trigger_permissions!(table)

        remove_rename_triggers(table, trigger_name)

        remove_column(table, old)
      end

Warm-up