MySQL的EXPLAIN会修改数据测试

文章转载自：https://www.cnblogs.com/kerrycode/p/14138626.html

在博客“Explain命令可能会修改MySQL数据”了解到MySQL中EXPLAIN可能会修改数据,这个现象确实挺让人意外和震惊的，像SQL Server或Oracle数据库，查看执行计划是不会真的执行的SQL语句的，但是MySQL确实有点“古怪”。

下面，我们简单准备一下测试环境数据。

mysql> create table test(id int, name varchar(12));
Query OK, 0 rows affected (0.33 sec)
 
mysql> insert into test
    -> select 1, 'kerry' from dual union all
    -> select 2, 'ken'   from dual union all
    -> select 3, 'jerry' from dual;

Query OK, 3 rows affected (0.08 sec)

Records: 3  Duplicates: 0  Warnings: 0

mysql> DELIMITER &&
mysql> create function cleanup() returns char(50) charset utf8mb4
    -> DETERMINISTIC
    -> begin
    -> delete from test;
    -> return 'OK';
    -> end &&
Query OK, 0 rows affected (0.07 sec)

mysql> DELIMITER ;

接下来，我们测试验证一下

mysql> select version();
+-----------+
| version() |
+-----------+
| 8.0.18    |
+-----------+
1 row in set (0.00 sec)
 
mysql> select * from test;
+------+-------+
| id   | name  |
+------+-------+
|    1 | kerry |
|    2 | ken   |
|    3 | jerry |
+------+-------+
3 rows in set (0.00 sec)
 
mysql> explain select * from (select cleanup()) as t;
+----+-------------+------------+------------+--------+---------------+------+---------+------+------+----------+----------------+
| id | select_type | table      | partitions | type   | possible_keys | key  | key_len | ref  | rows | filtered | Extra          |
+----+-------------+------------+------------+--------+---------------+------+---------+------+------+----------+----------------+
|  1 | PRIMARY     | <derived2> | NULL       | system | NULL          | NULL | NULL    | NULL |    1 |   100.00 | NULL           |
|  2 | DERIVED     | NULL       | NULL       | NULL   | NULL          | NULL | NULL    | NULL | NULL |     NULL | No tables used |
+----+-------------+------------+------------+--------+---------------+------+---------+------+------+----------+----------------+
2 rows in set, 1 warning (0.07 sec)
 
mysql> select * from test;
Empty set (0.00 sec)

随后翻看官方文档，发现官方文档其实也有简单介绍，只不过一句话带过，不细心的话，还真给忽略了，原文如下：

It is possible in some cases to execute statements that modify data when EXPLAIN SELECT is used with a subquery; for more information, see Section 13.2.11.8, “Derived Tables”.

所以这么说它还不算是一个Bug，也就是说MySQL中使用EXPLAIN查看执行计划时，对应的子查询中调用函数是会执行的。例如，官方文档中还有这么一段描述

This also means that an EXPLAIN SELECT statement such as the one shown here may take a long time to execute because the BENCHMARK() function is executed once for each row in t1:

EXPLAIN SELECT * FROM t1 AS a1, (SELECT BENCHMARK(1000000, MD5(NOW())));

估计这事只能那些高手深入代码研究才能搞清楚这种机制，暂且记录一下这个现象。