text: use product
  ALTER TABLE subscriptions
    ADD COLUMN subscription_id int(10) unsigned NOT NULL AUTO_INCREMENT FIRST,
    ADD KEY sub_id_user (subscription_id, user_id),
    AUTO_INCREMENT=456
    PARTITION BY RANGE (user_id)
    SUBPARTITION BY HASH(post_id)
    SUBPARTITIONS 2 (
      PARTITION p0 VALUES LESS THAN (123),
      PARTITION p1 VALUES LESS THAN MAXVALUE
    )
  ;