DELETE/UPDATE and ORDER BY ... LIMIT - the choice is not cost-based

create table ten(a int);

insert into ten values (0),(1),(2),(3),(4),(5),(6),(7),(8),(9);

create table one_k(a int);

insert into one_k select A.a + B.a* 10 + C.a * 100 from ten A, ten B, ten C;

create table t20 (a int, b int, c int, filler char(100));

insert into t20 select a,a,a,a from one_k;

alter table t20 add key(a), add key(b);

mysql> explain delete from t20  where a  between 1 and 300 order by b limit 1;

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

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

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

|    1 | SIMPLE      | t20   | range | a             | a    | 5       | NULL |  300 | Using where; Using filesort |

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

mysql> explain delete from t20  where a+0  between 1 and 300 order by b limit 1;

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

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

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

|    1 | SIMPLE      | t20   | index | NULL          | b    | 5       | NULL |    1 | Using where |

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