MySQL 复合索引测试

对MySQL复合索引结合具体示例，各条件下索引使用情况的运行结果及分析。 

目录

复合索引示例

创建表

新增数据

查询数据

选项A

SQL查询

explain分析

选项B

SQL查询

explain分析

选项C

SQL查询

explain分析

选项D

SQL查询

explain分析

选项E

SQL查询

explain分析

总结

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复合索引示例

假设某个表有一个联合索引（c1,c2,c3,c4）。

A where c1 = ? and c2 = ? and c4 > ? and c3 = ?

B where c1 = ? and c2 = ? and c4 = ? order by c3

C where c1 = ? and c4 = ? group by c3, c2

D where c1 = ? and c5 = ? order by c2, c3

E where c1 = ? and c2 = ? and c5=? order by c2, c3

有谁知道下面A-E能否可以使用索引！！为什么？

创建表

创建一个表，表引擎为MYISAM，并设置包含四个列的复合索引。

SQL语句如下：

CREATE TABLE `fuhe` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `c1` char(1) DEFAULT '',
  `c2` char(1) DEFAULT '',
  `c3` char(1) DEFAULT '',
  `c4` char(1) DEFAULT '',
  `c5` char(1) DEFAULT '',
  PRIMARY KEY (`id`),
  KEY `c1` (`c1`,`c2`,`c3`,`c4`) USING BTREE
) ENGINE=MyISAM DEFAULT CHARSET=utf8;

新增数据

在上述表中新增部分数据，语句如下：

INSERT INTO `test`.`fuhe` (`id`, `c1`, `c2`, `c3`, `c4`, `c5`) VALUES ('1', 'a', 'b', 'c', 'd', 'e');
INSERT INTO `test`.`fuhe` (`id`, `c1`, `c2`, `c3`, `c4`, `c5`) VALUES ('2', 'A', 'b', 'c', 'd', 'e');
INSERT INTO `test`.`fuhe` (`id`, `c1`, `c2`, `c3`, `c4`, `c5`) VALUES ('3', 'a', 'B', 'c', 'd', 'e');

查询数据

选项A

SQL查询

where c1 = ? and c2 = ? and c4 > ? and c3 = ?

# where c1 = ? and c2 = ? and c4 > ? and c3 = ?
SELECT * FROM `fuhe` where c1 = 'a' and c2 = 'b' and c4 > 'a' and c3 = 'c';

运行结果：

explain分析

explain SELECT * FROM `fuhe` where c1 = 'a' and c2 = 'b' and c4 > 'a' and c3 = 'c';

运行结果：

说明：

通过key_len属性12可知，utf8每个索引长度为3，使用了4列的索引；故复合索引的c1 c2 c3 c4列索引都使用上了。因为c4是范围查找，所以type类型为range。

选项B

SQL查询

where c1 = ? and c2 = ? and c4 = ? order by c3

# where c1 = ? and c2 = ? and c4 = ? order by c3
SELECT * FROM `fuhe` where c1 = 'a' and c2 = 'b' and c4 = 'd' order by c3;

 运行结果：

explain分析

explain SELECT * FROM `fuhe` where c1 = 'a' and c2 = 'b' and c4 = 'd' order by c3;

运行结果：

说明：

使用了复合索引的c1 c2列，c3列只是参与了排序。如果c3列没有索引就会进行文件排序。

选项C

SQL查询

where c1 = ? and c4 = ? group by c3, c2

# where c1 = ? and c4 = ? group by c3, c2
SELECT * FROM `fuhe` where c1 = 'a' and c4 = 'd' group by c3, c2;

运行结果：

explain分析

explain SELECT * FROM `fuhe` where c1 = 'a' and c4 = 'd' group by c3, c2;

运行结果：

说明：

只使用了复合索引的c1列，而且由于group by并不是按照索引顺序进行分组的，导致使用了临时表和文件排序。

选项D

SQL查询

where c1 = ? and c5 = ? order by c2, c3

# where c1 = ? and c5 = ? order by c2, c3
SELECT * FROM `fuhe` where c1 = 'a' and c5 = 'e' order by c2, c3;

运行结果：

explain分析

explain SELECT * FROM `fuhe` where c1 = 'a' and c5 = 'e' order by c2, c3;

运行结果：

说明：

通过索引长度判断，只使用到了复合索引的第一列c1，c2 c3列参与了排序，因为c5列未创建索引故using where。

选项E

SQL查询

 where c1 = ? and c2 = ? and c5=? order by c2, c3

# where c1 = ? and c2 = ? and c5=? order by c2, c3
SELECT * FROM `fuhe` where c1 = 'a' and c2 = 'b' and c5 = 'e' order by c2, c3;

运行结果：

explain分析

explain SELECT * FROM `fuhe` where c1 = 'a' and c2 = 'b' and c5 = 'e' order by c2, c3;

运行结果：

说明：

使用了复合索引的前两列，因为是按照索引顺序进行排序的，c2 c3列参与了排序，最后的c5没有创建索引，故使用了where条件，其他都是在索引树上扫描的。

总结

对MySQL复合索引结合具体示例，各条件下索引使用情况的运行结果及分析。