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 t2 (key1 int, col1 int, key(key1));  

insert into t2 select a,a from ten;

insert into t2 select 15,15 from one_k;

alter table t2 add key2 int, add key(key2);

explain 

select * from t2 ignore index for order by (key1) where col1<0 order by key1 limit 10;

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

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

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

|    1 | SIMPLE      | t2    | index | NULL          | key1 | 5       | NULL |   10 | Using where |

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

MariaDB [j2]> flush status;

Query OK, 0 rows affected (0.00 sec)

MariaDB [j2]> select * from t2 ignore index for order by (key1)  where col1<0 order by key1 limit 10;

Empty set (0.03 sec)

MariaDB [j2]> show status like '%sort%';

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

| Variable_name             | Value |

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

| Sort_merge_passes         | 0     |

| Sort_priority_queue_sorts | 1     |

| Sort_range                | 0     |

| Sort_rows                 | 0     |

| Sort_scan                 | 1     |

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

5 rows in set (0.01 sec)