MariaDB [test]> SHOW CREATE TABLE myisam1;

+---------+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+

| Table   | Create Table                                                                                                                                                                           |

+---------+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+

| myisam1 | CREATE TABLE `myisam1` (

  `a` int(11) NOT NULL,

  `b2` int(11) DEFAULT NULL,

  PRIMARY KEY (`a`)

) ENGINE=MyISAM DEFAULT CHARSET=latin1

/*!50100 PARTITION BY KEY (a)

PARTITIONS 3 */ |

+---------+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+

1 row in set (0.00 sec)

MariaDB [test]> START TRANSACTION;

Query OK, 0 rows affected (0.01 sec)

MariaDB [test]> EXPLAIN SELECT * FROM myisam1 WHERE 0;

+------+-------------+-------+------+---------------+------+---------+------+------+------------------+

| id   | select_type | table | type | possible_keys | key  | key_len | ref  | rows | Extra            |

+------+-------------+-------+------+---------------+------+---------+------+------+------------------+

|    1 | SIMPLE      | NULL  | NULL | NULL          | NULL | NULL    | NULL | NULL | Impossible WHERE |

+------+-------------+-------+------+---------------+------+---------+------+------+------------------+

1 row in set (0.00 sec)

MariaDB [test]> ALTER TABLE myisam1 CHANGE b2  b INT;

Stage: 2 of 2 'enabling keys'      0% of stage done

MariaDB [(none)]> SELECT * FROM information_schema.METADATA_LOCK_INFO;

+-----------+-------------------------+-----------------+----------------------+--------------+------------+

| THREAD_ID | LOCK_MODE               | LOCK_DURATION   | LOCK_TYPE            | TABLE_SCHEMA | TABLE_NAME |

+-----------+-------------------------+-----------------+----------------------+--------------+------------+

|         5 | MDL_INTENTION_EXCLUSIVE | MDL_STATEMENT   | Global read lock     |              |            |

|         4 | MDL_SHARED_READ         | MDL_TRANSACTION | Table metadata lock  | test         | myisam1    |

|         5 | MDL_SHARED_NO_WRITE     | MDL_TRANSACTION | Table metadata lock  | test         | myisam1    |

|         5 | MDL_INTENTION_EXCLUSIVE | MDL_TRANSACTION | Schema metadata lock | test         |            |

+-----------+-------------------------+-----------------+----------------------+--------------+------------+