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MySQL关于exists的一个bug

2017年12月12日  | 移动技术网IT编程  | 我要评论

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


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)

以上所述是小编给大家介绍的mysql关于exists的一个bug ，希望对大家有所帮助

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