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 程式師世界 >> 數據庫知識 >> MYSQL數據庫 >> MySQL綜合教程 >> MySQL關於exists的一個bug,MySQLexistsbug

MySQL關於exists的一個bug,MySQLexistsbug

編輯:MySQL綜合教程

MySQL關於exists的一個bug,MySQLexistsbug


今天碰到一個關於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
);

產生的結果是: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
);

產生的結果是:432382

確實相當奇怪,對於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麼區別啊。

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。

後來在自己的環境測試了一下,確實是MySQL的一個bug

測試環境: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等於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行計劃和改寫後的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) 

執行計劃及改寫後的SQL確實有所不同,看來,確實是MySQL的一個bug了。

於是,給官方提了個bug

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

總結

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操作,避免碰到官方bug,

事實上,對於abs,floor函數又沒問題

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關於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幫客之家網站的支持!

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