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?