| problem_tag | CREATE TABLE `problem_tag` (
  `problemtagid` bigint(20) unsigned NOT NULL,
  `eventid` bigint(20) unsigned NOT NULL,
  `tag` varchar(255) COLLATE utf8mb4_bin NOT NULL DEFAULT '',
  `value` varchar(255) COLLATE utf8_bin NOT NULL DEFAULT '',
  PRIMARY KEY (`problemtagid`),
  KEY `problem_tag_1` (`eventid`),
  KEY `problem_tag_2` (`tag`,`value`),
  CONSTRAINT `c_problem_tag_1` FOREIGN KEY (`eventid`) REFERENCES `problem` (`eventid`) ON DELETE CASCADE
) ENGINE=InnoDB DEFAULT CHARACTER SET utf8mb4 COLLATE utf8mb4_bin |

explain select eventid,tag,value from problem_tag where eventid in (2,3,6,7,10,12,13,15,18,20,22,24,26,28,30,32,34,36,38,40,42,44,46,48,50) order by eventid;
+------+-------------+-------------+------+---------------+------+---------+------+------+-----------------------------+
| id   | select_type | table       | type | possible_keys | key  | key_len | ref  | rows | Extra                       |
+------+-------------+-------------+------+---------------+------+---------+------+------+-----------------------------+
|    1 | SIMPLE      | problem_tag | ALL  | problem_tag_1 | NULL | NULL    | NULL |   19 | Using where; Using filesort |
+------+-------------+-------------+------+---------------+------+---------+------+------+-----------------------------+

CREATE INDEX problem_tag_3 ON problem_tag (eventid,tag,value);
DROP INDEX problem_tag_1 ON problem_tag;
DROP INDEX problem_tag_2 ON problem_tag;

explain select eventid,tag,value from problem_tag where eventid in (2,3,6,7,10,12,13,15,18,20,22,24,26,28,30,32,34,36,38,40,42,44,46,48,50) order by eventid;
+------+-------------+-------------+-------+---------------+---------------+---------+------+------+--------------------------+
| id   | select_type | table       | type  | possible_keys | key           | key_len | ref  | rows | Extra                    |
+------+-------------+-------------+-------+---------------+---------------+---------+------+------+--------------------------+
|    1 | SIMPLE      | problem_tag | index | problem_tag_3 | problem_tag_3 | 1542    | NULL |   19 | Using where; Using index |
+------+-------------+-------------+-------+---------------+---------------+---------+------+------+--------------------------+
1 row in set (0.00 sec)