mysql5.7 可以打破 索引 最左原则?

根据 MYSQL 官网的文档 https://dev.mysql.com/doc/refman/5.7/en/multiple-column-indexes.html

查询条件要符合最左原则才能使用到索引

但是在实际测试(5.7.20 生效 5.6 不可重现)中 是可以打破最左原则的

CREATE TABLE `user`  (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `name` varchar(25),
  `sex` varchar(25) ,
  `city` varchar(25) ,
  PRIMARY KEY (`id`) USING BTREE,
  INDEX `name`(`name`, `sex`, `city`) USING BTREE
)

mysql> EXPLAIN select * from `user` where sex='';
+----+-------------+-------+------------+-------+---------------+------+---------+------+------+----------+--------------------------+
| id | select_type | table | partitions | type  | possible_keys | key  | key_len | ref  | rows | filtered | Extra                    |
+----+-------------+-------+------------+-------+---------------+------+---------+------+------+----------+--------------------------+
|  1 | SIMPLE      | user  | NULL       | index | NULL          | name | 309     | NULL |    3 |    33.33 | Using where; Using index |
+----+-------------+-------+------------+-------+---------------+------+---------+------+------+----------+--------------------------+
1 row in set (0.02 sec)

这个是不是打破了最左原则??

测试中, 再新增一列 test. 再次执行以上 sql, 则无法使用索引.

猜测是因为当字段都被索引时做出的优化?