SET note_verbosity=unusable_keys;

CREATE OR REPLACE TABLE t1 (a INT, i CHAR(2), KEY(i));

DELIMITER $$

FOR i IN 1..31

DO

  INSERT INTO t1 VALUES (a, 10+i);

END FOR;

$$

DELIMITER ;

EXPLAIN SELECT * FROM t1 WHERE i>30 ORDER BY i LIMIT 5;

SHOW WARNINGS;

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

| Level | Code | Message                                                                                     |

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

|  Note | 1105 | Cannot use key `i` part[0] for lookup: `test`.`t1`.`i` of type `char` >= "30" of type `int` |

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

EXPLAIN SELECT * FROM t1 WHERE i>=30 ORDER BY i LIMIT 5;

SHOW WARNINGS;

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

| Level | Code | Message                                                                                    |

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

|  Note | 1105 | Cannot use key `i` part[0] for lookup: `test`.`t1`.`i` of type `char` > "30" of type `int` |

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