简单查询中奇怪的“使用哪里”

我有一个具有这种结构的表：

mysql> describe user_groups;

+----------+----------------------+------+-----+---------+----------------+
| Field    | Type                 | Null | Key | Default | Extra          |
+----------+----------------------+------+-----+---------+----------------+
| id       | int(11) unsigned     | NO   | MUL | NULL    | auto_increment |
| user_id  | int(11) unsigned     | NO   | PRI | NULL    |                |
| group_id | smallint(5) unsigned | NO   | PRI | NULL    |                |
+----------+----------------------+------+-----+---------+----------------+

该表的主键是：PRIMARY(user_id, group_id) 我有一个简单的查询：

mysql> EXPLAIN SELECT `UserGroup`.`user_id` , `UserGroup`.`group_id` FROM `user_groups` AS `UserGroup` WHERE `UserGroup`.`user_id` IN ( 1, 2 );
+----+-------------+-----------+-------+---------------+---------+---------+------+------+--------------------------+
| id | select_type | table     | type  | possible_keys | key     | key_len | ref  | rows | Extra                    |
+----+-------------+-----------+-------+---------------+---------+---------+------+------+--------------------------+
|  1 | SIMPLE      | UserGroup | range | PRIMARY       | PRIMARY | 4       | NULL |    4 | Using where; Using index |
+----+-------------+-----------+-------+---------------+---------+---------+------+------+--------------------------+

它有using where;，根据它有额外的磁盘搜索，我知道using where;概念，而且我也知道这并不可怕，但从概念上我想知道为什么这个查询有using where;，虽然条件直接在主键上并且它没有顺序或任何导致using where;. 我该如何解决这个问题using where;？