EXPLAIN results with PRIMARY KEY (`id`, `fenwick`)

+------+-------------+---------+-------+---------------+---------+---------+------+------+--------------------------+
| id   | select_type | TABLE   | TYPE  | possible_keys | KEY     | key_len | REF  | ROWS | Extra                    |
+------+-------------+---------+-------+---------------+---------+---------+------+------+--------------------------+
|    1 | SIMPLE      | entries | INDEX | NULL          | PRIMARY | 9       | NULL | 2000 | USING WHERE; USING INDEX |
+------+-------------+---------+-------+---------------+---------+---------+------+------+--------------------------+
1 ROW IN SET (0.00 sec)