MySQL和MariaDB查询执行计划的区别答案

【问题标题】：Difference between MySQL and MariaDB Query execution planMySQL和MariaDB查询执行计划的区别
【发布时间】：2014-06-08 06:14:13
【问题描述】：

MySQL 和 MariaDB 查询执行计划有区别吗？

如果是，哪个更好？

CREATE TABLE `Table1` (
  `ID` int(11) NOT NULL,
  KEY `ID` (`ID`)
);

CREATE TABLE `Table2` (
  `ID` int(11) NOT NULL,
  KEY `ID` (`ID`)
);

CREATE TABLE `Table3` (
  `ID` int(11) NOT NULL,
  PRIMARY KEY (`ID`)
);

在 Maria DB 中，

MariaDB [truepay_psr]> explain select T1.ID FROM Table1 T1 LEFT JOIN (SELECT T1.ID FROM Table3 T1 LEFT JOIN Table2 T2 ON T1.ID = T2.ID WHERE T2.ID IS NULL) T2 ON T1.ID=T2.ID WHERE T2.ID IS NULL;
+------+-------------+-------+--------+---------------+---------+---------+-------------------+------+--------------------------+
| id   | select_type | table | type   | possible_keys | key     | key_len | ref               | rows | Extra                    |
+------+-------------+-------+--------+---------------+---------+---------+-------------------+------+--------------------------+
|    1 | SIMPLE      | T1    | index  | NULL          | ID      | 4       | NULL              |    1 | Using index              |
|    1 | SIMPLE      | T1    | eq_ref | PRIMARY       | PRIMARY | 4       | truepay_psr.T1.ID |    1 | Using where; Using index |
|    1 | SIMPLE      | T2    | ref    | ID            | ID      | 4       | truepay_psr.T1.ID |    1 | Using where; Using index |
+------+-------------+-------+--------+---------------+---------+---------+-------------------+------+--------------------------+
3 rows in set (0.01 sec)

在 MySQL 中，

mysql> explain select T1.ID FROM Table1 T1 LEFT JOIN (SELECT T1.ID FROM Table3 T1 LEFT JOIN Table2 T2 ON T1.ID = T2.ID WHERE T2.ID IS NULL) T2 ON T1.ID=T2.ID WHERE T2.ID IS NULL;
+----+-------------+------------+--------+---------------+---------+---------+------------+------+--------------------------------------+
| id | select_type | table      | type   | possible_keys | key     | key_len | ref        | rows | Extra                                |
+----+-------------+------------+--------+---------------+---------+---------+------------+------+--------------------------------------+
|  1 | PRIMARY     | <derived2> | system | NULL          | NULL    | NULL    | NULL       |    0 | const row not found                  | 
|  1 | PRIMARY     | T1         | index  | NULL          | ID      | 4       | NULL       |    1 | Using index                          | 
|  2 | DERIVED     | T1         | index  | NULL          | PRIMARY | 4       | NULL       |    1 | Using index                          | 
|  2 | DERIVED     | T2         | ref    | ID            | ID      | 4       | test.T1.ID |    1 | Using where; Using index; Not exists | 
+----+-------------+------------+--------+---------------+---------+---------+------------+------+--------------------------------------+
4 rows in set (0.00 sec)

【问题讨论】：

产生正确结果的那个更好。否则，使用这些几乎没有数据的愚蠢的简单表格作为测试方法毫无用处。

标签： mysql mariadb

【解决方案1】：

您看到了“表消除”优化的效果（MySQL 计划有 4 行，而 MariaDB 计划只有 3 行）。 MariaDB 计划应该是更好的计划，因为“要做的工作更少”。两者都应该返回相同的结果。

这里详细解释了该功能：

http://s.petrunia.net/blog/?p=58

这里：

https://mariadb.com/kb/en/mariadb/documentation/optimization-and-tuning/query-optimizations/table-elimination/

如果您希望 MariaDB 使用与 MySQL 相同的计划，您可以通过禁用表消除优化来强制执行此操作：

SET optimizer_switch='table_elimination=off';

【讨论】：