create table t2(

  d int, e int, key idx1(d), key idx2(e), f varchar(32)

) engine=myisam;

insert into t2 values

 (4,5,'a'), (7,8,'b'), (4,3,'c'), (1,2,'d'), (2,1,'e'), (5,5,'f'),

 (2,3,'g'), (7,7,'h'), (3,3,'i'), (3,2,'j'), (2,2,'k'), (5,5,'l'),

 (4,5,'aa'), (7,8,'bb'), (4,3,'cc'), (1,2,'dd'), (2,1,'ee'), (5,5,'ff'),

 (2,3,'gg'), (7,7,'hh'), (3,3,'ii'), (3,2,'jj'), (2,2,'kk'), (5,5,'ll'),

 (4,5,'aaa'), (7,8,'bbb'), (4,3,'ccc'), (1,2,'ddd'), (2,1,'eee'), (5,5,'fff'),

 (2,3,'ggg'), (7,7,'hhh'), (3,3,'iii'), (3,2,'jjj'), (2,2,'kkk'), (5,5,'lll'),

 (14,15,'a'), (17,18,'b'), (14,13,'c'), (11,12,'d'), (12,11,'e'), (15,15,'f'),

 (12,13,'g'), (17,17,'h'), (13,13,'i'), (13,12,'j'), (12,12,'k'), (15,15,'l'),

 (24,25,'a'), (27,28,'b'), (24,23,'c'), (21,22,'d'), (22,21,'e'), (25,25,'f'),

 (22,23,'g'), (27,27,'h'), (23,23,'i'), (23,22,'j'), (22,22,'k'), (25,25,'l'),

 (34,35,'a'), (37,38,'b'), (34,33,'c'), (31,32,'d'), (32,31,'e'), (35,35,'f'),

 (32,33,'g'), (37,37,'h'), (33,33,'i'), (33,32,'j'), (32,32,'k'), (35,35,'l'),

 (44,45,'a'), (47,48,'b'), (44,43,'c'), (41,42,'d'), (42,41,'e'), (45,45,'f'),

 (42,43,'g'), (47,47,'h'), (43,43,'i'), (43,42,'j'), (42,42,'k'), (45,45,'l');

select * from t2 where d=3 and e=3  or d=2 and e=2;

Here index merge uses index idx1 twice. This does not make any sense.

The result set is correct though:

MariaDB [test]> select * from t2 where d=3 and e=3  or d=2 and e=2;

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

| d    | e    | f    |

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

|    3 |    3 | i    |

|    2 |    2 | k    |

|    3 |    3 | ii   |

|    2 |    2 | kk   |

|    3 |    3 | iii  |

|    2 |    2 | kkk  |

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

MariaDB [test]> set optimizer_switch='index_merge=off';

MariaDB [test]> explain select * from t2 where d=3 and e=3  or d=2 and e=2;

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

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

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

|    1 | SIMPLE      | t2    | range | idx1,idx2     | idx1 | 5       | NULL |   15 | Using index condition; Using where |

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