MySQL 更新不支持索引条件下推？

Question

CREATE TABLE `t2` (
  `a` int(11) DEFAULT NULL,
  `b` int(11) DEFAULT NULL,
  `c` char(10) DEFAULT NULL,
  `d` varchar(256) DEFAULT NULL,
  `e` text,
  KEY `ii1` (`a`,`b`)
) ENGINE=InnoDB DEFAULT CHARSET=latin1;

sql1: update t2 set b = 3 where a = 1;
sql2: select b from t2 where a = 1;

sql2会使用索引条件下推，而sql1不会，但是为什么，引擎和服务器之间转换的数据格式是这样的？

mysql> explain select b from t2 where a = 1;
+----+-------------+-------+------------+------+---------------+------+---------+-------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key  | key_len | ref   | rows | filtered | Extra       |
+----+-------------+-------+------------+------+---------------+------+---------+-------+------+----------+-------------+
|  1 | SIMPLE      | t2    | NULL       | ref  | ii1           | ii1  | 5       | const |    1 |   100.00 | Using index |
+----+-------------+-------+------------+------+---------------+------+---------+-------+------+----------+-------------+
1 row in set, 1 warning (0.00 sec)

mysql> explain update t2 set b = 3 where a = 1;
+----+-------------+-------+------------+-------+---------------+------+---------+-------+------+----------+------------------------------+
| id | select_type | table | partitions | type  | possible_keys | key  | key_len | ref   | rows | filtered | Extra                        |
+----+-------------+-------+------------+-------+---------------+------+---------+-------+------+----------+------------------------------+
|  1 | UPDATE      | t2    | NULL       | range | ii1           | ii1  | 5       | const |    1 |   100.00 | Using where; Using temporary |
+----+-------------+-------+------------+-------+---------------+------+---------+-------+------+----------+------------------------------+
1 row in set (0.00 sec)

Answer 1

注释Using index 表示查询正在使用覆盖索引。这意味着 SELECT 查询可以通过仅读取索引来返回结果。这对于 INSERT/UPDATE/DELETE 是不可能的，因为索引和索引所属的表必须同时更新。

注释Using index condition 用于索引条件下推。这是将一些预过滤委托给存储引擎的功能。

EXPLAIN 输出中的单词相似，但这两个注释指的是完全不同的特征。