多个单列索引和联合索引的区别
MySql版本为：5.7.14
联合索引测试
创建一张10w数据的测试表：
CREATE TABLE `10w` (
`id` int(11) NOT NULL AUTO_INCREMENT,
`t1` int(11) NOT NULL DEFAULT '0',
`t2` varchar(255) NOT NULL DEFAULT '',
`t3` int(11) NOT NULL DEFAULT '0',
`t4` varchar(255) NOT NULL DEFAULT '',
PRIMARY KEY (`id`),
KEY `index_3` (`t1`,`t2`,`t3`) USING BTREE
) ENGINE=InnoDB DEFAULT CHARSET=utf8;
依据结构，我们为t1,t2,t3三个字段添加了联合索引！
1、查询条件为t1时
mysql> explain select * from 10w where t1=1000;
+----+-------------+-------+------------+------+---------------+---------+---------+-------+------+----------+-------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+------+---------------+---------+---------+-------+------+----------+-------+
| 1 | SIMPLE | 10w | NULL | ref | index_3 | index_3 | 4 | const | 1 | 100 | NULL |
+----+-------------+-------+------------+------+---------------+---------+---------+-------+------+----------+-------+
联合索引有效
2、查询条件为t2时
mysql> explain select * from 10w where t2='a1000';
+----+-------------+-------+------------+------+---------------+------+---------+------+--------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+------+---------------+------+---------+------+--------+----------+-------------+
| 1 | SIMPLE | 10w | NULL | ALL | NULL | NULL | NULL | NULL | 100039 | 10 | Using where |
+----+-------------+-------+------------+------+---------------+------+---------+------+--------+----------+-------------+
联合索引无效
3、查询条件为t3时
mysql> explain select * from 10w where t3=1000;
+----+-------------+-------+------------+------+---------------+------+---------+------+--------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+------+---------------+------+---------+------+--------+----------+-------------+
| 1 | SIMPLE | 10w | NULL | ALL | NULL | NULL | NULL | NULL | 100039 | 10 | Using where |
+----+-------------+-------+------------+------+---------------+------+---------+------+--------+----------+-------------+
联合索引无效
4、查询条件为t1 and t2时
mysql> explain select * from 10w where t1=1000 and t2='a1000';
+----+-------------+-------+------------+------+---------------+---------+---------+-------------+------+----------+-------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+------+---------------+---------+---------+-------------+------+----------+-------+
| 1 | SIMPLE | 10w | NULL | ref | index_3 | index_3 | 771 | const,const | 1 | 100 | NULL |
+----+-------------+-------+------------+------+---------------+---------+---------+-------------+------+----------+-------+
联合索引有效
5、查询条件为t2 and t1时
mysql> explain select * from 10w where t2='a1000' and t1=1000;
+----+-------------+-------+------------+------+---------------+---------+---------+-------------+------+----------+-------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+------+---------------+---------+---------+-------------+------+----------+-------+
| 1 | SIMPLE | 10w | NULL | ref | index_3 | index_3 | 771 | const,const | 1 | 100 | NULL |
+----+-------------+-------+------------+------+---------------+---------+---------+-------------+------+----------+-------+
在4的基础上调换了查询条件的顺序，发现联合索引依旧有效
6、查询条件为t1 or t2时
mysql> explain select * from 10w where t1=1000 or t2='a1000';
+----+-------------+-------+------------+------+---------------+------+---------+------+--------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+------+---------------+------+---------+------+--------+----------+-------------+
| 1 | SIMPLE | 10w | NULL | ALL | index_3 | NULL | NULL | NULL | 100039 | 10 | Using where |
+----+-------------+-------+------------+------+---------------+------+---------+------+--------+----------+-------------+
把 and 换成 or，发现联合所索引无效
7、查询条件为t1 and t3时
mysql> explain select * from 10w where t1=1000 and t3=1000;
+----+-------------+-------+------------+------+---------------+---------+---------+-------+------+----------+-----------------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+------+---------------+---------+---------+-------+------+----------+-----------------------+
| 1 | SIMPLE | 10w | NULL | ref | index_3 | index_3 | 4 | const | 1 | 10 | Using index condition |
+----+-------------+-------+------------+------+---------------+---------+---------+-------+------+----------+-----------------------+
这两个条件分别位于联合索引位置的第一和第三，测试联合索引依旧有效
8、查询条件为t2 and t3时
mysql> explain select * from 10w where t2='a1000' and t3=1000;
+----+-------------+-------+------------+------+---------------+------+---------+------+--------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+------+---------------+------+---------+------+--------+----------+-------------+
| 1 | SIMPLE | 10w | NULL | ALL | NULL | NULL | NULL | NULL | 100039 | 1 | Using where |
+----+-------------+-------+------------+------+---------------+------+---------+------+--------+----------+-------------+
这两个条件分别位于联合索引位置的第二和第三，发现联合索引无效
9、查询条件为t1 and t2 and t3时
mysql> explain select * from 10w where t1=1000 and t2='a1000' and t3=1000;
+----+-------------+-------+------------+------+---------------+---------+---------+-------------------+------+----------+-------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+------+---------------+---------+---------+-------------------+------+----------+-------+
| 1 | SIMPLE | 10w | NULL | ref | index_3 | index_3 | 775 | const,const,const | 1 | 100 | NULL |
+----+-------------+-------+------------+------+---------------+---------+---------+-------------------+------+----------+-------+
所有条件一起查询，联合索引有效
单列索引测试
将上面三个联合索引字段改成单个单列索引
1、查询条件为t1 and t2 and t3时
mysql> explain select * from 10w where t1=1000 and t2='a1000' and t3=1000;
+----+-------------+-------+------------+------+---------------+-----+---------+-------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+------+---------------+-----+---------+-------+------+----------+-------------+
| 1 | SIMPLE | 10w | NULL | ref | t1,t2,t3 | t1 | 4 | const | 1 | 5 | Using where |
+----+-------------+-------+------------+------+---------------+-----+---------+-------+------+----------+-------------+
我们发现三个单列索引只有 t1 有效（位置为查询条件第一个），其他两个都没有用上
2、查询条件为t2 and t3时
mysql> explain select * from 10w where t2='a1000' and t3=1000;
+----+-------------+-------+------------+------+---------------+-----+---------+-------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+------+---------------+-----+---------+-------+------+----------+-------------+
| 1 | SIMPLE | 10w | NULL | ref | t2,t3 | t2 | 767 | const | 1 | 5 | Using where |
+----+-------------+-------+------------+------+---------------+-----+---------+-------+------+----------+-------------+
我们发现此处两个查询条件只有 t2 有效（位置也为查询条件第一个），后面的无效
3、查询条件为t1 or t2时
mysql> explain select * from 10w where t1=1000 or t2='a1000';
+----+-------------+-------+------------+-------------+---------------+-------+---------+------+------+----------+---------------------------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+-------------+---------------+-------+---------+------+------+----------+---------------------------------+
| 1 | SIMPLE | 10w | NULL | index_merge | t1,t2 | t1,t2 | 4,767 | NULL | 2 | 100 | Using union(t1,t2); Using where |
+----+-------------+-------+------------+-------------+---------------+-------+---------+------+------+----------+---------------------------------+
这次把 and 换成 or，发现两个查询条件都用上索引了
结论
通俗理解：
利用索引中的附加列，您可以缩小搜索的范围，但使用一个具有两列的索引 不同于使用两个单独的索引。
复合索引的结构与电话簿类似，人名由姓和名构成，电话簿首先按姓氏对进行排序，然后按名字对有相同姓氏的人进行排序。如果您知道姓，电话簿将非常有用；如果您知道姓和名，电话簿则更为有用，但如果您只知道名不姓，电话簿将没有用处。
所以说创建复合索引时，应该仔细考虑列的顺序。对索引中的所有列执行搜索或仅对前几列执行搜索时，复合索引非常有用；仅对后面的任意列执行搜索时，复合索引则没有用处。
重点：
多个单列索引在多条件查询时只会生效第一个索引！所以多条件联合查询时最好建联合索引！
最左前缀原则：
顾名思义是最左优先，以最左边的为起点任何连续的索引都能匹配上， 注：如果第一个字段是范围查询需要单独建一个索引 注：在创建联合索引时，要根据业务需求，where子句中使用最频繁的一列放在最左边。这样的话扩展性较好，比如 userid 经常需要作为查询条件，而 mobile 不常常用，则需要把 userid 放在联合索引的第一位置，即最左边
思考
同时存在联合索引和单列索引（字段有重复的），这个时候查询mysql会怎么用索引呢？
这个涉及到mysql本身的查询优化器策略了，当一个表有多条索引可走时, Mysql 根据查询语句的成本来选择走哪条索引；mysql执行优化器会对其进行优化，当不考虑索引时，where条件顺序对效率没有影响，真正有影响的是是否用到了索引！
联合索引本质：
当创建(a,b,c)联合索引时，相当于创建了(a)单列索引，(a,b)联合索引以及(a,b,c)联合索引 想要索引生效的话,只能使用 a和a,b和a,b,c三种组合；当然，我们上面测试过，a,c组合也可以，但实际上只用到了a的索引，c并没有用到！ 注：这个可以结合上边的 通俗理解 来思考！
其他知识点：
1、需要加索引的字段，要在where条件中 2、数据量少的字段不需要加索引；因为建索引有一定开销，如果数据量小则没必要建索引（速度反而慢） 3、如果where条件中是OR关系，加索引不起作用 4、联合索引比对每个列分别建索引更有优势，因为索引建立得越多就越占磁盘空间，在更新数据的时候速度会更慢。另外建立多列索引时，顺序也是需要注意的，应该将严格的索引放在前面，这样筛选的力度会更大，效率更高。