mysql索引优化
索引优化
# 创建表 和数据
create table IF NOT EXISTS staffs(
id int primary key auto_increment,
`NAME` varchar(24) not null default '' COMMENT'姓名',
age int not null default 0 COMMENT'年龄',
pos varchar(24) not null default '' COMMENT'职位',
add_time TIMESTAMP not null default CURRENT_TIMESTAMP COMMENT'入职时间'
);
insert into staffs(`NAME`,age,pos,add_time) values('z3',22,'manager',NOW());
insert into staffs(`NAME`,age,pos,add_time) values('judy',22,'dev',NOW());
insert into staffs(`NAME`,age,pos,add_time) values('gc',22,'dev',NOW());
# 添加索引
alter table staffs add index idx(name , age,pos);
分析该数据列：
explain select * from staffs where name = ‘z3’ and age = ‘z3’ and pos = ‘123’
explain select * from staffs where name = ‘z3’
explain select * from staffs where name = ‘z3’ and pos = ‘123’
explain select * from staffs where age = ‘z3’ and pos = ‘123’
1，复合索引，需要按照建索引的顺序列查询，如果中间去掉了一列后，则后面的索引会失效，如果第一列索引都不匹配，则索引全失效
explain select * from staffs where name = ‘z3’
explain select * from staffs where left(name,4) = ‘z3’
2，在索引列上做任何操作（计算，函数、（自动或手动）的类型转换），会导致索引失效而转向全表的扫描
explain select * from staffs where name = ‘z3’ and age > ‘3’ and pos = ‘123’
3，存储引擎不能使用索引中范围条件右边的列（也就是范围条件后面的索引会失效）
explain select name,age,pos from staffs where name = ‘z3’ and age = ‘3’ and pos = ‘123’
4，尽量使用覆盖索引（只访问索引列上存在的字段），减少select * 的查询（同时如果此时以3的示例，效率也会更高）
explain select * from staffs where name <> ‘z3’
5，mysql在使用不等号（！= 或 <>） 的时候，会导致无法使用索引会导致全表扫描（但是 select 索引列 from … 不会导致索引失效，因为用到覆盖索引了）
explain select * from staffs where name is null
explain select * from staffs where name is not null
6,is null 或 is not null 也无法使用索引
explain select * from staffs where name like ‘judy%’
explain select * from staffs where name like ‘%judy%’
explain select * from staffs where name like ‘%judy’
like 百分号最好放右边，才会避免索引失效，且后面的索引也不会失效
那么如果愣头轻，两遍都要百分号怎么办
explain select name from staffs where name like ‘%judy%’
7，两遍百分号，查询字段如果是覆盖索引，那么索引页不会失效了
explain select * from staffs where name = ‘33’
explain select * from staffs where name = 33
8，字符串查询不加单引号索引会失效（同时也就是2演示所说，（自动或手动）的类型转换）
explain select * from staffs where name = ‘33’ or name = ‘123’
9，少用or,用它连接的时候索引会失效
