MariaDB [test]> CREATE TABLE t1 (pk INT NOT NULL AUTO_INCREMENT PRIMARY KEY, i INT, KEY(i)) ENGINE=MyISAM;

Query OK, 0 rows affected (0.22 sec)

MariaDB [test]> INSERT INTO t1 (i) VALUES (100),(200);

Query OK, 2 rows affected (0.01 sec)

Records: 2  Duplicates: 0  Warnings: 0

MariaDB [test]> 

MariaDB [test]> CREATE TABLE t2 (j INT) ENGINE=MyISAM;

Query OK, 0 rows affected (0.26 sec)

MariaDB [test]> INSERT INTO t2 VALUES (10),(20);

Query OK, 2 rows affected (0.00 sec)

Records: 2  Duplicates: 0  Warnings: 0

MariaDB [test]> EXPLAIN INSERT INTO t1 (i) SELECT j FROM t2;

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

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

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

|    1 | SIMPLE      | t2    | ALL  | NULL          | NULL | NULL    | NULL |    2 |       |

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

1 row in set (0.00 sec)

MariaDB [test]> INSERT INTO t1 (i) VALUES (300);

Query OK, 1 row affected (0.00 sec)

MariaDB [test]> CHECK TABLE t1;

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

| Table   | Op    | Msg_type | Msg_text          |

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

| test.t1 | check | error    | Found 2 keys of 3 |

| test.t1 | check | error    | Corrupt           |

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

2 rows in set (0.00 sec)

DROP TABLE IF EXISTS t1, t2;

CREATE TABLE t1 (pk INT NOT NULL AUTO_INCREMENT PRIMARY KEY, i INT, KEY(i)) ENGINE=MyISAM;

INSERT INTO t1 (i) VALUES (100),(200);

CREATE TABLE t2 (j INT) ENGINE=MyISAM;

INSERT INTO t2 VALUES (10),(20);

EXPLAIN INSERT INTO t1 (i) SELECT j FROM t2;

INSERT INTO t1 (i) VALUES (300);

CHECK TABLE t1;

DROP TABLE t1, t2;