Mysql中复合索引使用规则有哪些

技术Mysql中复合索引使用规则有哪些这篇文章主要介绍了Mysql中复合索引使用规则有哪些,具有一定借鉴价值,感兴趣的朋友可以参考下,希望大家阅读完这篇文章之后大有收获,下面让小编带着大家一起了解一下。联合索引验证:从左

这篇文章主要介绍了关系型数据库中复合索引使用规则有哪些,具有一定借鉴价值,感兴趣的朋友可以参考下,希望大家阅读完这篇文章之后大有收获,下面让小编带着大家一起了解一下。

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,可以找到补体第四成份缺乏

a:select *来自t,其中c1=x,c2=x,c3=x,C4=x;

b:select *来自t,其中c1=x,c2=x,c4x,C3=x;用到了c1 c2 c3 c4

c:select * from t其中c1=x和c2=x和c4=x按c3排序;C1C2用到了索引查找,C3只发挥了排序的作用,C3不用(c3:订购发挥作用了,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其中c1=x和c4=x被c3、c2分组;

e:select * from t其中c1=x和c5=x按c2、c3排序;

来自t的f:select *其中c1=x和c2=x和c5=?按c2、c3排序;

创建表t (c1炭(10)、c2炭(10)、c3炭(10)、C4炭(10)、C5炭(10));

插入t值(' a1 ',' a2 ',' a3 ',' a4 ',' a5 ',(' b1 ',' b2 ',' b3 ',' b4 ',' b5 ');

插入t值(' a1 ',' a2 ',' a3 ',' a4 ',' a5 ',(' b1 ',' b2 ',' b3 ',' b4 ',' b5 ');

插入t值(' a1 ',' a2 ',' a3 ',' a4 ',' a5 ',(' b1 ',' b2 ',' b3 ',' b4 ',' b5 ');

插入t值(' a1 ',' a2 ',' a3 ',' a4 ',' a5 ',(' b1 ',' b2 ',' b3 ',' b4 ',' b5 ');

插入t值(' a1 ',' a2 ',' a3 ',' a4 ',' a5 ',(' b1 ',' b2 ',' b3 ',' b4 ',' b5 ');

插入t值(' a1 ',' a2 ',' a3 ',' a4 ',' a5 ',(' b1 ',' b2 ',' b3 ',' b4 ',' b5 ');

插入t值(' a1 ',' a2 ',' a3 ',' a4 ',' a5 ',(' b1 ',' b2 ',' b3 ',' b4 ',' b5 ');

在t(c1,c2,c3,c4)上创建索引idx _ t _ c1234

在t(c1)上创建索引idx _ t _ c1

在t(c2)上创建索引idx _ t _ c2

在t(c3)上创建索引idx _ t _ c3

在t(c4)上创建索引idx _ t _ c4

更改表t删除索引idx _ t _ c1234

a:

解释从t中选择*其中c1='a1 '和c2='b2 '和c3='a3 '和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

感谢你能够认真阅读完这篇文章,希望小编分享的“Mysql中复合索引使用规则有哪些”这篇文章对大家有帮助,同时也希望大家多多支持,关注行业资讯频道,更多相关知识等着你来学习!

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