MySQL的EXPLAIN會修改數據測試

在博客“Explain命令可能會修改MySQL數據”瞭解到MySQL中EXPLAIN可能會修改數據,這個現象確實挺讓人意外和震驚的，像SQL Server或Oracle數據庫，查看執行計劃是不會真的執行的SQL語句的，但是MySQL確實有點“古怪”。

 

下面，我們簡單準備一下測試環境數據。

 

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 ;

 

接下來，我們測試驗證一下 

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)

 

 

隨後翻看官方文檔，發現官方文檔其實也有簡單介紹，只不過一句話帶過，不細心的話，還真給忽略了，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”.

 

所以這麼說它還不算是一個Bug，也就是說MySQL中使用EXPLAIN查看執行計劃時，對應的子查詢中調用函數是會執行的。例如，官方文檔中還有這麼一段描述

 

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())));

 

估計這事只能那些高手深入代碼研究才能搞清楚這種機制，暫且記錄一下這個現象。

 

參考資料：

 

https://www.cnblogs.com/abclife/p/14101191.html

https://www.docs4dev.com/docs/zh/mysql/5.7/reference/derived-tables.html

https://www.docs4dev.com/docs/zh/mysql/5.7/reference/explain-output.html

https://dev.mysql.com/doc/refman/8.0/en/explain-output.html

https://dev.mysql.com/doc/refman/8.0/en/derived-tables.html