create table t1 (a int, b int, key(a));

insert into t1 select seq,seq from seq_1_to_10000;

explain with T as (select a, b from t1 order by a limit 10) select * from T;

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

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

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

|    1 | PRIMARY     | <derived2> | ALL   | NULL          | NULL | NULL    | NULL | 10    |       |

|    2 | DERIVED     | t1         | index | NULL          | a    | 5       | NULL | 10157 |       |

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

explain select a, b from t1 order by a limit 10;

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

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

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

|    1 | SIMPLE      | t1    | index | NULL          | a    | 5       | NULL | 10   |       |

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