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MySQL5.7对in用法有什么黑科技

构建测试环境

Part1:创建测试数据库

[root@HE1 ~]# mysql -uroot -p

Enter password: 

Welcome to the MySQL monitor.  Commands end with ; or \g.

Your MySQL connection id is 5

Server version: 5.7.15-log MySQL Community Server (GPL)

Copyright (c) 2000, 2016, Oracle and/or its affiliates. All rights reserved.

Oracle is a registered trademark of Oracle Corporation and/or its

affiliates. Other names may be trademarks of their respective

owners.

Type 'help;' or '\h' for help. Type '\c' to clear the current input statement.

mysql> select version();

+------------+

| version()  |

+------------+

| 5.7.15-log |

+------------+

1 row in set (0.00 sec)

[root@HE3 ~]# /usr/local/mariadb/bin/mysql -uroot -S /tmp/mariadb.sock 

Welcome to the MariaDB monitor.  Commands end with ; or \g.

Your MariaDB connection id is 3

Server version: 10.1.16-MariaDB MariaDB Server

Copyright (c) 2000, 2016, Oracle, MariaDB Corporation Ab and others.

Type 'help;' or '\h' for help. Type '\c' to clear the current input statement.

MariaDB [helei]> select version();

+-----------------+

| version()       |

+-----------------+

| 10.1.16-MariaDB |

+-----------------+

1 row in set (0.00 sec)

如何构建MySQL5.7测试环境和MariaDB10.1测试环境本文不做赘述，如有需要可移步：

一分钟完成MySQL5.7安装

http://suifu.blog.51cto.com/9167728/1855415

MariaDB10.1自动化部署

http://suifu.blog.51cto.com/9167728/1830575

Part2:构建测试表

CREATE TABLE helei (

id INT (10) UNSIGNED NOT NULL AUTO_INCREMENT,

c1 INT (10) NOT NULL DEFAULT '0',

c2 INT (10) UNSIGNED DEFAULT NULL,

c5 INT (10) UNSIGNED NOT NULL DEFAULT '0',

c3 TIMESTAMP NOT NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP,

c4 VARCHAR (200) NOT NULL DEFAULT '',

PRIMARY KEY (id),

KEY idx_c1 (c1),

KEY idx_c2 (c2)

) ENGINE = INNODB;

对比测试结果

Part1:MyriaDB10.1.16

MariaDB [helei]> explain select * from helei where (id,c1) in ((100,2684),(101,3566));

+------+-------------+-------+------+---------------+------+---------+------+------+-------------+

| id   | select_type | table | type | possible_keys | key  | key_len | ref  | rows | Extra       |

+------+-------------+-------+------+---------------+------+---------+------+------+-------------+

|    1 | SIMPLE      | helei | ALL  | NULL          | NULL | NULL    | NULL | 5198 | Using where |

+------+-------------+-------+------+---------------+------+---------+------+------+-------------+

1 row in set (0.00 sec)

可以看到，MariaDB对这条SQL总共扫描了5198行，且没有用到任何的索引。

Part2:MySQL5.7.15

mysql> explain select * from helei where (id,c1) in ((100,2684),(101,3566));

+----+-------------+-------+------------+-------+----------------+---------+---------+------+------+----------+-------------+

| id | select_type | table | partitions | type  | possible_keys  | key     | key_len | ref  | rows | filtered | Extra       |

+----+-------------+-------+------------+-------+----------------+---------+---------+------+------+----------+-------------+

|  1 | SIMPLE      | helei | NULL       | range | PRIMARY,idx_c1 | PRIMARY | 4       | NULL |    2 |    20.00 | Using where |

+----+-------------+-------+------------+-------+----------------+---------+---------+------+------+----------+-------------+

1 row in set, 1 warning (0.29 sec)

可以看到，MySQL5.7版本已经用到了索引，一共扫描条数为2行。这是因为该语句已经被MySQL5.7的内部优化器改写为如下图所示：

wKiom1f7NeGxpFDTAAFAu7CKX0c704.jpg

——总结——

可以看出，MariaDB版本内部优化器无法改写，因此对于这类查询只能全表扫描。经测试，MySQL5.6和MariaDB10.0/10.1都无法改写。由于笔者的水平有限，编写时间也很仓促，文中难免会出现一些错误或者不准确的地方，不妥之处恳请读者批评指正。

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