MySQL關(guān)于exists的一個bug

更新時間：2016年08月12日 16:12:38 作者：iVictor

今天小編給大家分享一個mysql關(guān)于exists的一個bug問題，非常不錯，感興趣的朋友一起學習下

今天碰到一個關(guān)于exists很奇怪的問題

第一個語句如下：

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

產(chǎn)生的結(jié)果是：89584

第二個語句如下：

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

產(chǎn)生的結(jié)果是：432382

確實相當奇怪，對于exist子句來說，其判斷的是子查詢的值是否存在，也就是說，列名，和對列名求最大值沒什么區(qū)別啊。

包括MySQL官方文檔中也提到

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會自動忽略到SELECT的列表。

后來在自己的環(huán)境測試了一下，確實是MySQL的一個bug

測試環(huán)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 |

很明顯，id等于2的列不存在，但是第二條語句還是當做TRUE來處理了。

也確認了下兩條語句的執(zhí)行計劃和改寫后的SQL

第一個語句

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 |
+---------+------+-------------------------------------------------------------------+

第二個語句

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)

執(zhí)行計劃及改寫后的SQL確實有所不同，看來，確實是MySQL的一個bug了。

于是，給官方提了個bug

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

總結(jié)

建議寫exists語句時，子查詢中直接用*，而不用對列進行任何函數(shù)操作，避免碰到官方bug，

事實上，對于abs，floor函數(shù)又沒問題

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)

以上所述是小編給大家介紹的MySQL關(guān)于exists的一個bug ，希望對大家有所幫助，如果大家有任何疑問請給我留言，小編會及時回復大家的。在此也非常感謝大家對腳本之家網(wǎng)站的支持！

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