update...where没有索引的字段为什么会走PRIMARY索引？

Question

name字段没有索引，但是会提示走PRIMARY索引，但是也确实锁表了，有大佬能解释一下MySQL的逻辑吗？

CREATE TABLE `users` (
  `id` int(11) unsigned NOT NULL AUTO_INCREMENT,
  `name` varchar(32) NOT NULL DEFAULT '',
  `mobile` char(11) NOT NULL DEFAULT '',
  `rank` int(10) unsigned NOT NULL DEFAULT '0',
  PRIMARY KEY (`id`),
  UNIQUE KEY `mobile` (`mobile`),
  KEY `rank` (`rank`)
) ENGINE=InnoDB AUTO_INCREMENT=8 DEFAULT CHARSET=utf8mb4;

explain update users set rank=rank-1 where name="zhangsan"

mysql> explain update users set rank=rank-1 where name="zhangsan";
+----+-------------+-------+------------+-------+---------------+---------+---------+------+------+----------+-------------+
| id | select_type | table | partitions | type  | possible_keys | key     | key_len | ref  | rows | filtered | Extra       |
+----+-------------+-------+------------+-------+---------------+---------+---------+------+------+----------+-------------+
|  1 | UPDATE      | users | NULL       | index | NULL          | PRIMARY | 4       | NULL |    7 |   100.00 | Using where |
+----+-------------+-------+------------+-------+---------------+---------+---------+------+------+----------+-------------+
1 row in set (0.03 sec)

mysql> explain select * from users where name="zhangsan";
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key  | key_len | ref  | rows | filtered | Extra       |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+
|  1 | SIMPLE      | users | NULL       | ALL  | NULL          | NULL | NULL    | NULL |    7 |    14.29 | Using where |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+
1 row in set (0.03 sec)

mysql>