> explain select * from activity where parent_id in (1,2,3);
+----+-------------+----------+-------+---------------+--------+---------+------+------+-----------------------+
| id | select_type | table    | type  | possible_keys | key    | key_len | ref  | rows | Extra                 |
+----+-------------+----------+-------+---------------+--------+---------+------+------+-----------------------+
|  1 | SIMPLE      | activity | range | parent        | parent | 9       | NULL |    3 | Using index condition |
+----+-------------+----------+-------+---------------+--------+---------+------+------+-----------------------+
1 row in set (0.07 sec)

> set eq_range_index_dive_limit=2;
Query OK, 0 rows affected (0.07 sec)

> explain select * from activity where parent_id in (1,2,3);
+----+-------------+----------+-------+---------------+--------+---------+------+--------+-----------------------+
| id | select_type | table    | type  | possible_keys | key    | key_len | ref  | rows   | Extra                 |
+----+-------------+----------+-------+---------------+--------+---------+------+--------+-----------------------+
|  1 | SIMPLE      | activity | range | parent        | parent | 9       | NULL | 269697 | Using index condition |
+----+-------------+----------+-------+---------------+--------+---------+------+--------+-----------------------+
1 row in set (0.07 sec)