Mysql中复合索引使用法则有哪些
发布时间：2021-12-27 12:25:37 所属栏目：MySql教程 来源：互联网
导读：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
|
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 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 | +----+-------------+-------+------+---------------+-------------+---------+-------------------------+------+--------------------------+ 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） 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 | +----+-------------+-------+------+---------------+-------------+---------+-------------+------+------------------------------------+ 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。 （编辑：威海站长网） 【声明】本站内容均来自网络，其相关言论仅代表作者个人观点，不代表本站立场。若无意侵犯到您的权利，请及时与联系站长删除相关内容! |
