Use PK for full-data-copy paging.

If there is multiple unique key (including PK), a non-PK key might be used. It is not the clustering key and it will be slow.

CREATE TABLE a.a (
  `id` bigint(20) unsigned NOT NULL AUTO_INCREMENT,
  `mobile` varchar(11) NOT NULL,
  PRIMARY KEY (`id`),
  UNIQUE KEY `mobile_unique` (`mobile`) USING BTREE
);