MySQL常见优化方案汇总

CREATE TABLE `student` (
  `id` int(11) NOT NULL AUTO_INCREMENT COMMENT '主键',
  `name` varchar(50) DEFAULT NULL COMMENT '姓名',
  `age` int(11) DEFAULT NULL COMMENT '年龄',
  `phone` varchar(12) DEFAULT NULL,
  `create_time` datetime DEFAULT NULL COMMENT '创建时间',
  PRIMARY KEY (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

alter table studen add index name_age_phone(name, age, phone);

insert into student(name,age,phone,create_time) values('赛文',1000,'15717177664',now());
insert into student(name,age,phone,create_time) values('雷欧',1200,'15733337664',now());
insert into student(name,age,phone,create_time) values('泰罗',800,'15714447664',now());

#未覆盖索引
EXPLAIN SELECT * FROM student WHERE NAME = '泰罗' and age =1000 and phone='15717177664';
#覆盖了索引
EXPLAIN SELECT name,age,phone FROM student WHERE NAME = '泰罗' and age =1000 and phone='15717177664';
#包含了索引
EXPLAIN SELECT name FROM student WHERE NAME = '泰罗' and age =1000 and phone='15717177664';
#加上主键也还是覆盖索引
EXPLAIN SELECT id, name,age,phone FROM student WHERE NAME = '泰罗' and age =1000 and phone='15717177664';

EXPLAIN SELECT * FROM student WHERE NAME = '赛文';
EXPLAIN SELECT * FROM student WHERE NAME = '雷欧' AND age = 1200;
EXPLAIN SELECT * FROM student WHERE NAME = '泰罗' AND age = 800 AND phone = '15714447664';

EXPLAIN SELECT * FROM student WHERE NAME = '泰罗' AND age = 1000 AND phone = '15717177664';

EXPLAIN SELECT * FROM student WHERE NAME = '泰罗' AND age > 800 AND phone = '15717177664';

EXPLAIN SELECT * FROM student WHERE NAME = '泰罗';
EXPLAIN SELECT * FROM student WHERE left(NAME,1) = '泰罗';

#有使用到索引
EXPLAIN SELECT * FROM student WHERE NAME = '泰罗';
#不等于查询，未使用到索引
EXPLAIN SELECT * FROM student WHERE NAME != '泰罗';
EXPLAIN SELECT * FROM student WHERE NAME <> '泰罗';
 
#如果定要需要使用不等于,请用覆盖索引
EXPLAIN SELECT name,age,phone FROM student WHERE NAME != '泰罗';
EXPLAIN SELECT name,age,phone FROM student WHERE NAME <> '泰罗';

EXPLAIN select * from student where name is null;
EXPLAIN select * from student 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
#索引有效
EXPLAIN select * from student where name ='泰罗';
#索引失效
EXPLAIN select * from student where name like '%泰罗%';
#索引失效
EXPLAIN select * from student where name like '%泰罗';
#索引有效
EXPLAIN select * from student where name like '泰罗%';
 
解决方式：覆盖索引
EXPLAIN select name,age,phone from student where name like '%泰罗%';

未使用索引
EXPLAIN select * from student where name='泰罗' or name = '雷欧';
 
使用索引
EXPLAIN
select * from student where name='泰罗'
UNION
select * from student where name = '雷欧';
 
解决方式：覆盖索引
EXPLAIN select name,age from student where name='泰罗' or name = '雷欧';