mysql联合索引的选择性解析
发布时间：2022-01-17 10:39:46 所属栏目：MySql教程 来源：互联网
导读：本篇内容介绍了mysql联合索引的选择性解析的有关知识，在实际案例的操作过程中，不少人都会遇到这样的困境，接下来就让小编带领大家学习一下如何处理这些情况吧！希望大家仔细阅读，能够学有所成！ 通过下面的实验来探讨下联合索引的使用选择性： 版本：perc
|
本篇内容介绍了“mysql联合索引的选择性解析”的有关知识，在实际案例的操作过程中，不少人都会遇到这样的困境，接下来就让小编带领大家学习一下如何处理这些情况吧！希望大家仔细阅读，能够学有所成！ 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 “mysql联合索引的选择性解析”的内容就介绍到这里了，感谢大家的阅读。 （编辑：晋中站长网） 【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! |
