我建立一个组合索引，在查询的时候不使用这个这个组合索引的前导列进行查询但还是走索引，请教各位！

-----------------------------------------------------------+
| Table | Create Table                                                                                                                                                                                       |
+-------+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| t     | CREATE TABLE `t` (
  `name` varchar(20) DEFAULT NULL,
  `age` int(29) DEFAULT NULL,
  `local` varchar(30) DEFAULT NULL,
  KEY `xxoo` (`name`,`age`,`local`)
) ENGINE=InnoDB DEFAULT CHARSET=latin1 |

 select count(*) from t;
+----------+
| count(*) |
+----------+
|    10000 |
+----------+
1 row in set (0.02 sec)


 explain select name from t where local='xx33';
+----+-------------+-------+------------+-------+---------------+------+---------+------+-------+----------+--------------------------+
| id | select_type | table | partitions | type  | possible_keys | key  | key_len | ref  | rows  | filtered | Extra                    |
+----+-------------+-------+------------+-------+---------------+------+---------+------+-------+----------+--------------------------+
|  1 | SIMPLE      | t     | NULL       | index | NULL          | xxoo | 61      | NULL | 10161 |    10.00 | Using where; Using index |
+----+-------------+-------+------------+-------+---------------+------+---------+------+-------+----------+--------------------------+
1 row in set, 1 warning (0.00 sec)