Mysql中复合索引使用规则有哪些

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联合索引验证:从左向右发挥作用
索引：(c1,c2,c3,c4):找到c1的基础上，可以找到c2，找到c3的基础上，可以找到c4
a:select * from t where c1=x and c2=x and c3=x and c4=x;
b:select * from t where c1=x and c2=x and c4>x and c3=x; 用到了c1+c2+c3+c4
c:select * from t where c1=x and c2=x and c4=x order by c3; C1+C2用到了索引查找，C3只发挥了排序的作用，C3不用（order by c3:发挥作用了，排序不用作了），C4的索引就不用，4块木板，中间断了，后面也就用不上了
d:select * from t where c1=x and c4=x group by c3,c2;
e:select * from t where c1=x and c5=x order by c2,c3;
f:select * from t where c1=x and c2=x and c5=? order by c2,c3;


create table t (c1 char(10),c2 char(10),c3 char(10),C4 char(10),c5 char(10));
insert into t values('a1','a2','a3','a4','a5'),('b1','b2','b3','b4','b5');
insert into t values('a1','a2','a3','a4','a5'),('b1','b2','b3','b4','b5');
insert into t values('a1','a2','a3','a4','a5'),('b1','b2','b3','b4','b5');
insert into t values('a1','a2','a3','a4','a5'),('b1','b2','b3','b4','b5');
insert into t values('a1','a2','a3','a4','a5'),('b1','b2','b3','b4','b5');
insert into t values('a1','a2','a3','a4','a5'),('b1','b2','b3','b4','b5');
insert into t values('a1','a2','a3','a4','a5'),('b1','b2','b3','b4','b5');


create index idx_t_c1234 on t(c1,c2,c3,c4);
create index idx_t_c1 on t(c1);
create index idx_t_c2 on t(c2);
create index idx_t_c3 on t(c3);
create index idx_t_c4 on t(c4);

alter table t drop index idx_t_c1234;

a:
explain select * from t where c1='a1' and c2='b2' and c3='a3' and c4='a';
+----+-------------+-------+------+---------------+-------------+---------+-------------------------+------+--------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+------+---------------+-------------+---------+-------------------------+------+--------------------------+
| 1 | SIMPLE | t | ref | idx_t_c1234 | idx_t_c1234 | 44 | const,const,const,const | 1 | Using where; Using index |
+----+-------------+-------+------+---------------+-------------+---------+-------------------------+------+--------------------------+


key_len: 44 // CHAR(10)*4 + 4 * NULL:说明全用到了3个索引，且都是等值查询的索引：c1,c2,c3,c4


删除了复合索引后：发现只用到c1索引，c2,c3,c4索引全没用上
explain select * from t where c1='a1' and c2='b2' and c3='a3' and c4='a';
+----+-------------+-------+------+-------------------------------------+----------+---------+-------+------+------------------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+------+-------------------------------------+----------+---------+-------+------+------------------------------------+
| 1 | SIMPLE | t | ref | idx_t_c1,idx_t_c2,idx_t_c3,idx_t_c4 | idx_t_c1 | 11 | const | 1 | Using index condition; Using where |
+----+-------------+-------+------+-------------------------------------+----------+---------+-------+------+------------------------------------+


删除了复合索引后：发现只使用了一个索引c4，没有用c1索引，这是因为优化器发现c4='a1'一条也没找到，用这个索引查询是最快的
explain select * from t where c1='a1' and c2='b2' and c3='a3' and c4='a1';
+----+-------------+-------+------+-------------------------------------+----------+---------+-------+------+------------------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+------+-------------------------------------+----------+---------+-------+------+------------------------------------+
| 1 | SIMPLE | t | ref | idx_t_c1,idx_t_c2,idx_t_c3,idx_t_c4 | idx_t_c4 | 11 | const | 1 | Using index condition; Using where |
+----+-------------+-------+------+-------------------------------------+----------+---------+-------+------+------------------------------------+


删除了复合索引后：发现只使用了一个索引c1，没有用其它索引，这是因为优化器没有发现哪个条件取值记录最少（c2,c3,c4='等值连接也是匹配多条）就选第最左列索引
explain select * from t where c1='a1' and c2='b2' and c3='a3' and c4='a4';
+----+-------------+-------+------+-------------------------------------+----------+---------+-------+------+------------------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+------+-------------------------------------+----------+---------+-------+------+------------------------------------+
| 1 | SIMPLE | t | ref | idx_t_c1,idx_t_c2,idx_t_c3,idx_t_c4 | idx_t_c1 | 11 | const | 18 | Using index condition; Using where |
+----+-------------+-------+------+-------------------------------------+----------+---------+-------+------+------------------------------------+

a:
explain select * from t where c4='a1' and c2='b2' and c3='a3' and c1='a1';
+----+-------------+-------+------+---------------+-------------+---------+-------------------------+------+--------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+------+---------------+-------------+---------+-------------------------+------+--------------------------+
| 1 | SIMPLE | t | ref | idx_t_c1234 | idx_t_c1234 | 44 | const,const,const,const | 1 | Using where; Using index |
+----+-------------+-------+------+---------------+-------------+---------+-------------------------+------+--------------------------+
where条件后面的顺序无关

b:
explain select * from t where c1='a1' and c2='b2' and c4>'a' and c3='a3';
+----+-------------+-------+-------+---------------+-------------+---------+------+------+-----------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+-------+---------------+-------------+---------+------+------+-----------------------+
| 1 | SIMPLE | t | range | idx_t_c1234 | idx_t_c1234 | 44 | NULL | 1 | Using index condition |
+----+-------------+-------+-------+---------------+-------------+---------+------+------+-----------------------+


key_len: 44 // CHAR(10)*4 + 4 * NULL:说明全用到了4个索引，且都是等值查询的索引：c1,c2,c3,c4,全通过
Using index condition：5.6新特性，Where条件过滤是在innodb引擎层就可做掉了，这样innodb发送给server层的会少很多，如果不启用该功能，则数据通过索引访问后，数据要发送到server层进行where过滤


b:
explain select * from t where c1='a1' and c2='b2' and c3='a3' and c4>'a';
+----+-------------+-------+-------+---------------+-------------+---------+------+------+-----------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+-------+---------------+-------------+---------+------+------+-----------------------+
| 1 | SIMPLE | t | range | idx_t_c1234 | idx_t_c1234 | 44 | NULL | 1 | Using index condition |
+----+-------------+-------+-------+---------------+-------------+---------+------+------+-----------------------+


range：代表c4采用索引了，且使用到范围查找


c:
explain select * from t where c1='a1' and c2='b2' and c4='b4' order by c3;
+----+-------------+-------+------+---------------+-------------+---------+-------------+------+------------------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+------+---------------+-------------+---------+-------------+------+------------------------------------+
| 1 | SIMPLE | t | ref | idx_t_c1234 | idx_t_c1234 | 22 | const,const | 1 | Using index condition; Using where |
+----+-------------+-------+------+---------------+-------------+---------+-------------+------+------------------------------------+


key_len: 22 // CHAR(10)*2 + 2 * NULL:说明全用到了c1,c2索引,且都是等值查询的索引：c1,c2
Using where:说明c4在server层进行where过滤操作
c3：用到了索引排序

ref 需要与索引比较的列 列名或者const（常数，where id = 1的时候就是const了）



删除了复合索引后：只用到了c1索引，也就是只用一个索引，其它索引也没用上，排序也没用上
explain select * from t where c1='a1' and c2='b2' and c4='b4' order by c3;
+----+-------------+-------+------+----------------------------+----------+---------+-------+------+----------------------------------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+------+----------------------------+----------+---------+-------+------+----------------------------------------------------+
| 1 | SIMPLE | t | ref | idx_t_c1,idx_t_c2,idx_t_c4 | idx_t_c1 | 11 | const | 2 | Using index condition; Using where; Using filesort |
+----+-------------+-------+------+----------------------------+----------+---------+-------+------+----------------------------------------------------+

d:
explain select * from t where c1='a1' and c4='c4' group by c3,c2;
+----+-------------+-------+------+---------------+-------------+---------+-------+------+---------------------------------------------------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+------+---------------+-------------+---------+-------+------+---------------------------------------------------------------------+
| 1 | SIMPLE | t | ref | idx_t_c1234 | idx_t_c1234 | 11 | const | 1 | Using index condition; Using where; Using temporary; Using filesort |


key_len: 11 // CHAR(10)*1 + 1 * NULL:说明全用到了c1索引,且都是等值查询的索引：c1
Using temporary：DISTINCT，或者使用了不同的ORDER BY 和GROUP BY 列,且没用到索引，才会用临时表来排序，该临时表是内存临时表，还不是最糟糕的，最怕的是Using disk temporary
Using filesort：当我们试图对一个没有索引的字段进行排序时，就是filesoft
c3,c2由于与(c1,c2,c3,c4)索引不连续，无法用到索引排序

删除了复合索引后：只用到了c1索引，也就是只用一个索引，其它索引也没用上，group by 也没用上
explain select * from t where c1='a1' and c4='c4' group by c3,c2;
+----+-------------+-------+------+-------------------+----------+---------+-------+------+---------------------------------------------------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+------+-------------------+----------+---------+-------+------+---------------------------------------------------------------------+
| 1 | SIMPLE | t | ref | idx_t_c1,idx_t_c4 | idx_t_c4 | 11 | const | 1 | Using index condition; Using where; Using temporary; Using filesort |
+----+-------------+-------+------+-------------------+----------+---------+-------+------+---------------------------------------------------------------------+


d:
explain select * from t where c1='a1' and c4='c4' group by c2,c3;
+----+-------------+-------+------+---------------+-------------+---------+-------+------+------------------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+------+---------------+-------------+---------+-------+------+------------------------------------+
| 1 | SIMPLE | t | ref | idx_t_c1234 | idx_t_c1234 | 11 | const | 1 | Using index condition; Using where |
+----+-------------+-------+------+---------------+-------------+---------+-------+------+------------------------------------+


c2,c3用到了(c1,c2,c3,c4)索引排序，与c1相连

e:
explain select * from t where c1='a3' and c5='a5' order by c2,c3;
+----+-------------+-------+------+---------------+-------------+---------+-------+------+------------------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+------+---------------+-------------+---------+-------+------+------------------------------------+
| 1 | SIMPLE | t | ref | idx_t_c1234 | idx_t_c1234 | 11 | const | 1 | Using index condition; Using where |
+----+-------------+-------+------+---------------+-------------+---------+-------+------+------------------------------------+


key_len: 11 // CHAR(10)*1 + 1 * NULL:说明全用到了c1索引,且都是等值查询的索引：c1

f:
explain select * from t where c1='a1' and c2='a2' and c5='a5' order by c2,c3;
+----+-------------+-------+------+---------------+-------------+---------+-------------+------+------------------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+------+---------------+-------------+---------+-------------+------+------------------------------------+
| 1 | SIMPLE | t | ref | idx_t_c1234 | idx_t_c1234 | 22 | const,const | 1 | Using index condition; Using where |
+----+-------------+-------+------+---------------+-------------+---------+-------------+------+------------------------------------+


key_len: 11 // CHAR(10)*2 + 2 * NULL:说明全用到了c1索引,且都是等值查询的索引：c1,c2

group by 中能通过索引避免排序的原理：
explain select * from t where c1='a1' and c4='c4' group by c3,c2;
explain select * from t where c1='a1' and c2='b2' and c4='b4' order by c3;
where条件只是过虑数据，在过滤的过程中，如果c3,c2有索引，就可直接使用
在查找的过程中，己可得到c3在一起的数据，此时可以sum,avg等，不用排序了

删除了复合索引后：只用到了c1索引，也就是只用一个索引，其它索引也没用上， order by 也没用上
explain select * from t where c1='a1' and c2='a2' and c5='a5' order by c2,c3;
+----+-------------+-------+------+-------------------+----------+---------+-------+------+----------------------------------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+------+-------------------+----------+---------+-------+------+----------------------------------------------------+
| 1 | SIMPLE | t | ref | idx_t_c1,idx_t_c2 | idx_t_c1 | 11 | const | 2 | Using index condition; Using where; Using filesort |
+----+-------------+-------+------+-------------------+----------+---------+-------+------+----------------------------------------------------+


g:
explain select * from t where c3='a%';
+----+-------------+-------+------+---------------+------+---------+------+------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+------+---------------+------+---------+------+------+-------------+
| 1 | SIMPLE | t | ALL | NULL | NULL | NULL | NULL | 36 | Using where |
+----+-------------+-------+------+---------------+------+---------+------+------+-------------+
全表扫，没用到了复合索引idx_t_c1234，除非Where条件后面有c1,c2

explain select * from t where c1='a%';
+----+-------------+-------+------+---------------+-------------+---------+-------+------+-----------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+------+---------------+-------------+---------+-------+------+-----------------------+
| 1 | SIMPLE | t | ref | idx_t_c1234 | idx_t_c1234 | 11 | const | 1 | Using index condition |
+----+-------------+-------+------+---------------+-------------+---------+-------+------+-----------------------+

用到了复合索引idx_t_c1234

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