'Modify column' doesn't support 'set auto_increment'?

[maxshuang]

General Question

I scan the official web page about the 'Modify Column' and find no compatible statement about not supporting 'set auto_increment' of 'Modify column'

But the following behavior of 'modify column' SQL is not consistent for mysql and tidb:

• mysql:

#Server version: 5.7.38-log Source distribution

mysql> create table t2(id int key);
Query OK, 0 rows affected (0.29 sec)

mysql> alter table t2 modify column `id` bigint(20) not null auto_increment;
Query OK, 0 rows affected, 1 warning (0.29 sec)
Records: 0  Duplicates: 0  Warnings: 1

mysql> show create table t2;
+-------+---------------------------------------------------------------------------------------------------------------------------------------------+
| Table | Create Table                                                                                                                                |
+-------+---------------------------------------------------------------------------------------------------------------------------------------------+
| t2    | CREATE TABLE `t2` (
  `id` bigint NOT NULL AUTO_INCREMENT,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_bin |
+-------+---------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.23 sec)

*TIDB:

#Server version: 5.7.25-TiDB-v6.3.0-alpha-1105-g20bb0464e TiDB Server (Apache License 2.0) Community Edition, MySQL 5.7 compatible

mysql> create table t2(id int key);
Query OK, 0 rows affected (0.17 sec)

mysql> alter table t2 modify column `id` bigint(20) not null auto_increment;
ERROR 8200 (HY000): Unsupported modify column: can't set auto_increment

[maxshuang]

limited by:

[dveeden]

See also: #39143