MySQL关于exists的一个bug

时间：2021-05-01 09:32:23|栏目：Mysql|点击：次

今天碰到一个关于exists很奇怪的问题

第一个语句如下：

SELECT
count(1)
FROM
APPLY t
WHERE
EXISTS (
SELECT
r.APPLY_ID
FROM
RECORD r
WHERE
t.APPLY_ID = r.APPLY_ID
);

产生的结果是：89584

第二个语句如下：

SELECT
count(1)
FROM
APPLY t
WHERE
EXISTS (
SELECT
max(r.FINISH_TIME)
FROM
RECORD r
WHERE
t.APPLY_ID = r.APPLY_ID
);

产生的结果是：432382

确实相当奇怪，对于exist子句来说，其判断的是子查询的值是否存在，也就是说，列名，和对列名求最大值没什么区别啊。

包括MySQL官方文档中也提到

Traditionally, an EXISTS subquery starts with SELECT *, but it could begin with SELECT 5 or SELECT column1 or anything at all. MySQL ignores the SELECT list in such a subquery, so it makes no difference.

大意就是MySQL会自动忽略到SELECT的列表。

后来在自己的环境测试了一下，确实是MySQL的一个bug

测试环境：MySQL 5.6.31，5.7.14

mysql> create table t3(id int,t datetime);
Query OK, 0 rows affected (0.44 sec)
mysql> insert into t3 values(1,'20160812');
Query OK, 1 row affected (0.16 sec)
mysql> select 1 from dual where exists (select id from t3 where id=2);
Empty set (0.15 sec)
mysql> select 1 from dual where exists (select max(id) from t3 where id=2);
+---+
| 1 |
+---+
| 1 |

很明显，id等于2的列不存在，但是第二条语句还是当做TRUE来处理了。

也确认了下两条语句的执行计划和改写后的SQL

第一个语句

mysql> EXPLAIN EXTENDED select 1 from dual where exists (select id from t3 where id=2);
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+------------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+------------------+
| 1 | PRIMARY | NULL | NULL | NULL | NULL | NULL | NULL | NULL | NULL | NULL | Impossible WHERE |
| 2 | SUBQUERY | t3 | NULL | ALL | NULL | NULL | NULL | NULL | 1 | 100.00 | Using where |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+------------------+
2 rows in set, 2 warnings (0.00 sec)
mysql> show warnings;
+---------+------+-------------------------------------------------------------------+
| Level | Code | Message |
+---------+------+-------------------------------------------------------------------+
| Warning | 1681 | 'EXTENDED' is deprecated and will be removed in a future release. |
| Note | 1003 | /* select#1 */ select 1 AS `1` from DUAL where 0 |
+---------+------+-------------------------------------------------------------------+

第二个语句

mysql> EXPLAIN EXTENDED select 1 from dual where exists (select max(id) from t3 where id=2);
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+----------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+----------------+
| 1 | PRIMARY | NULL | NULL | NULL | NULL | NULL | NULL | NULL | NULL | NULL | No tables used |
| 2 | SUBQUERY | t3 | NULL | ALL | NULL | NULL | NULL | NULL | 1 | 100.00 | Using where |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+----------------+
2 rows in set, 2 warnings (0.00 sec)
mysql> show warnings;
+---------+------+-------------------------------------------------------------------+
| Level | Code | Message |
+---------+------+-------------------------------------------------------------------+
| Warning | 1681 | 'EXTENDED' is deprecated and will be removed in a future release. |
| Note | 1003 | /* select#1 */ select 1 AS `1` from DUAL where 1 |
+---------+------+-------------------------------------------------------------------+
2 rows in set (0.00 sec)

执行计划及改写后的SQL确实有所不同，看来，确实是MySQL的一个bug了。

于是，给官方提了个bug

http://bugs.mysql.com/bug.php?id=82562

总结

建议写exists语句时，子查询中直接用*，而不用对列进行任何函数操作，避免碰到官方bug，

事实上，对于abs，floor函数又没问题

mysql> select 1 from dual where exists (select abs(id) from t3 where id=2);
Empty set (0.07 sec)
mysql> select 1 from dual where exists (select floor(id) from t3 where id=2);
Empty set (0.00 sec)

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