mysql联合索引的选择性解析
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通过下面的实验来探讨下联合索引的使用选择性：
版本：percona 5.6.27
create table test(
a int,
b int,
c int,
name varchar(32),
PRIMARY key(a),
key index_a_b_c(a,b,c)) ENGINE=INNODB
insert into test values(1,1,1,3,'leo');
insert into test values(2,1,2,1,'mike' );
insert into test values(3,1,3,1,'exo' );
insert into test values(4,1,2,3,'jhon' );
insert into test values(5,1,1,3,'lucy' );
insert into test values(6,2,2,3,'leo' );
insert into test values(7,3,1,2,'dv' );
insert into test values(8,2,1,3,'men' );
一：where条件对联合索引的选择性
mysql> explain select * from test where a=2;
+----+-------------+-------+------+---------------+-------------+---------+-------+------+-------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+------+---------------+-------------+---------+-------+------+-------+
| 1 | SIMPLE | test | ref | index_a_b_c | index_a_b_c | 5 | const | 2 | NULL |
+----+-------------+-------+------+---------------+-------------+---------+-------+------+-------+
mysql> explain select * from test where a=2 and b=1;
+----+-------------+-------+------+---------------+-------------+---------+-------------+------+-------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+------+---------------+-------------+---------+-------------+------+-------+
| 1 | SIMPLE | test | ref | index_a_b_c | index_a_b_c | 10 | const,const | 1 | NULL |
+----+-------------+-------+------+---------------+-------------+---------+-------------+------+-------+
mysql> explain select * from test where a=2 and b=2 and c=3;
+----+-------------+-------+------+---------------+-------------+---------+-------------------+------+-------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+------+---------------+-------------+---------+-------------------+------+-------+
| 1 | SIMPLE | test | ref | index_a_b_c | index_a_b_c | 15 | const,const,const | 1 | NULL |
+----+-------------+-------+------+---------------+-------------+---------+-------------------+------+-------+
这三个是正常的使用方法,都走了索引
mysql> explain select * from test where a=2 and c=3;
+----+-------------+-------+------+---------------+-------------+---------+-------+------+-----------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+------+---------------+-------------+---------+-------+------+-----------------------+
| 1 | SIMPLE | test | ref | index_a_b_c | index_a_b_c | 5 | const | 2 | Using index condition |
+----+-------------+-------+------+---------------+-------------+---------+-------+------+-----------------------+
1 row in set (0.00 sec)
如果把b漏掉，同样走了索引
mysql> explain select * from test where b=2 and c=3;
+----+-------------+-------+------+---------------+------+---------+------+------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+------+---------------+------+---------+------+------+-------------+
| 1 | SIMPLE | test | ALL | NULL | NULL | NULL | NULL | 8 | Using where |
+----+-------------+-------+------+---------------+------+---------+------+------+-------------+
1 row in set (0.00 sec)
mysql> explain select * from test where c=3;
+----+-------------+-------+------+---------------+------+---------+------+------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+------+---------------+------+---------+------+------+-------------+
| 1 | SIMPLE | test | ALL | NULL | NULL | NULL | NULL | 8 | Using where |
+----+-------------+-------+------+---------------+------+---------+------+------+-------------+
如果把a漏掉，则不会走索引
结论：必须有联合索引的第一个字段作为wehre条件
二：联合索引排序选择性
联合索引的排序会按照（a,b,c）的顺序进行排序
测试数据在联合索引的排序会是(1,1,3), (1,2,1), (1,2,3), (1,3,1), (2,1,3), (2,2,3), (3,1,2)顺序存储
mysql> explain select * from test where a=2 order by b;
+----+-------------+-------+------+---------------+-------------+---------+-------+------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+------+---------------+-------------+---------+-------+------+-------------+
| 1 | SIMPLE | test | ref | index_a_b_c | index_a_b_c | 5 | const | 2 | Using where |
+----+-------------+-------+------+---------------+-------------+---------+-------+------+-------------+
mysql> explain select * from test where a=2 order by c;
+----+-------------+-------+------+---------------+-------------+---------+-------+------+-----------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+------+---------------+-------------+---------+-------+------+-----------------------------+
| 1 | SIMPLE | test | ref | index_a_b_c | index_a_b_c | 5 | const | 2 | Using where; Using filesort |
+----+-------------+-------+------+---------------+-------------+---------+-------+------+-----------------------------+
可以看出第二个Using filesort使用了临时表排序了，效率低。从联合索引的排序就可以知道当指定a的值的时候，这些值会按b的值排序，不是按c的值排序，故order by b不用再filesort排序，反之order by b需要重新排序。
所以select * from test where a=2 and b=2 order by c;不会 filesort排序
mysql> explain select * from test where a=2 and b=2 order by c;
+----+-------------+-------+------+---------------+-------------+---------+-------------+------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+------+---------------+-------------+---------+-------------+------+-------------+
| 1 | SIMPLE | test | ref | index_a_b_c | index_a_b_c | 10 | const,const | 1 | Using where |
+----+-------------+-------+------+---------------+-------------+---------+-------------+------+-------------+
结论：当针对联合索引中的某个字段进行排序的时候，最优的方法是有联合索引排序字段之前的字段过滤条件
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