5分钟了解MySQL5.7对in用法有什么黑科技

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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：

　　

　　

　　

——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。