I have a table with this structure:

    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    |                |
    +----------+----------------------+------+-----+---------+----------------+

primary key of this table is : PRIMARY(user_id, group_id)
I have simple query:

    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 |
    +----+-------------+-----------+-------+---------------+---------+---------+------+------+--------------------------+


it has `using where;` , according it has additional disk seek,I know `using where;` concept, and Also I know it is not terrible , but conceptually I want to know why this query has `using where;`, although the condition is on primary key directly and it doesn't have order or anything that causes `using where;`.
How can I fix this `using where;`?