CREATE TABLE 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(20) not null default "" comment'职位',
add_time timestamp not null default current_timestamp comment '入职时间'
)CHARACTER SET utf8 comment '员工记录表';
插入几条数据,如下:
insert into staffs(name,age,pos,add_time) values('z3',22,'manage',now());
insert into staffs(name,age,pos,add_time) values('july',23,'dev',now());
insert into staffs(name,age,pos,add_time) values('2000',23,'dev',now());
添加复合索引,包括三列:
alter table staffs add index idx_staffs_nameAgePos(name,age,pos);
EXPLAIN SELECT * FROM staffs WHERE NAME = 'July';
EXPLAIN SELECT * FROM staffs WHERE NAME = 'July' AND age = 25;
EXPLAIN SELECT * FROM staffs WHERE NAME = 'July' AND age = 25 AND pos = 'dev'
上面使用执行计划来看索引是否生效,如图:
Key_len表示索引中使用的字节数,从上面三个数能够看出,每一条语句使用的索引的个数是不相同的,另外从ref中也能够看出。
当建立了索引列后,能在where条件中使用索引的尽量所用。
如果索引了多列,要遵守最左前缀法则。指的是查询从索引的最左前列开始并且不跳过索引中的列。
EXPLAIN SELECT * FROM staffs WHERE age = 25 AND pos = 'dev'(不会使用索引)
EXPLAIN SELECT * FROM staffs WHERE pos = 'dev'(不会使用索引)
EXPLAIN SELECT * FROM staffs WHERE NAME = 'July' (会使用索引)
不在索引列上做任何操作(计算、函数、(自动or手动)类型转换),会导致索引失效而转向全表扫描
EXPLAIN SELECT * FROM staffs WHERE NAME = 'July';
EXPLAIN SELECT * FROM staffs WHERE left(NAME,4) = 'July';
EXPLAIN SELECT * FROM staffs WHERE NAME = 'July' ;
EXPLAIN SELECT * FROM staffs WHERE NAME = 'July' and age >22;
EXPLAIN SELECT * FROM staffs WHERE NAME = 'July' and age >22 and pos='manager';
中间有范围查询会导致后面的索引列全部失效,age本身索引并不会失效。
尽量使用覆盖索引(只访问索引的查询(索引列和查询列一致)),减少select *
EXPLAIN SELECT * FROM staffs WHERE NAME = 'July' and age =22 and pos='manager'
EXPLAIN SELECT name,age,pos FROM staffs WHERE NAME = 'July' and age =22 and pos='manager'
EXPLAIN SELECT * FROM staffs WHERE NAME = 'July' and age >22 and pos='manager'
EXPLAIN SELECT name,age,pos FROM staffs WHERE NAME = 'July' and age >22 and pos='manager'
mysql 在使用不等于(!= 或者<>)的时候无法使用索引会导致全表扫描
EXPLAIN SELECT * FROM staffs WHERE NAME = 'July';(使用了索引)
EXPLAIN SELECT * FROM staffs WHERE NAME != 'July';(索引失效)
EXPLAIN SELECT * FROM staffs WHERE NAME <> 'July';(索引失效)
如果定要需要使用不等于,请用覆盖索引
EXPLAIN SELECT name,age,pos FROM staffs WHERE NAME != 'July';
EXPLAIN SELECT name,age,pos FROM staffs WHERE NAME <> 'July';
如果创建表的时候name字段自定定义为NOT NULL
EXPLAIN select * from staffs where name is null(索引失效,根本不会扫描)
EXPLAIN select * from staffs where name is not null(索引失效)
在字段为not null的情况下,使用is null 或 is not null 会导致索引失效
解决方式:覆盖索引
EXPLAIN select name,age,pos from staffs where name is not null
如果创建表的时候name字段自定义为NULL或者不定义
EXPLAIN select * from staffs2 where name is null(索引有效)
EXPLAIN select * from staffs2 where name is not null (索引失效)
Is not null 的情况会导致索引失效
解决方式:覆盖索引
EXPLAIN select name,age,pos from staffs where name is not null
like以通配符开头(’%abc…’)mysql索引失效会变成全表扫描的操作
EXPLAIN select * from staffs where name ='july'(索引有效)
EXPLAIN select * from staffs where name like '%july%'(索引失效)
EXPLAIN select * from staffs where name like '%july'(索引失效)
EXPLAIN select * from staffs where name like 'july%'(索引有效)
解决方式:覆盖索引
EXPLAIN select name,age,pos from staffs where name like ‘%july%’
字符串不加单引号索引失效
EXPLAIN select * from staffs where name = 917 (索引失效)
解决方式:请加引号
EXPLAIN
select * from staffs where name='July' or name = 'z3' (索引失效)
EXPLAIN
select * from staffs where name='July'
UNION
select * from staffs where name = 'z3' (索引有效)
解决方式:覆盖索引
EXPLAIN select name,age from staffs where name='July' or name = 'z3'
记忆总结:
全值匹配我最爱,最左前缀要遵守;
带头大哥不能死,中间兄弟不能断;
索引列上少计算,范围之后全失效;
LIKE百分写最右,覆盖索引不写*;
不等空值还有OR,索引影响要注意;
varchar引号不可丢, SQL优化有诀窍。