Examples of the use of MySQL indexes

Preface

These are the notes I took while listening to the teacher's lecture. This is a video address
Author: Chenyun Zhi
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1. Slow query log

//查看是否开启慢查询日志
mysql> show variables like '%slow%';
//临时开启慢查询日志
mysql> set global slow_query_log=ON;
//查看是否开启慢查询日志
mysql> show variables like '%slow%';

//查询超过多少时间就可以记录，上面是如果超过10秒就要记录
mysql> show variables like '%long%';
//改成一秒，如果超过一秒就写到慢日志里面去(一般一秒是最好的)
mysql> set long_query_time=1;
//查看日记存储方式，默认FILE
mysql> show variables like '%log_output%';
// 慢查询日志文件所在位置
mysql> show variables like '%datadir%';

//响应时间是3秒，超过了原先设定的一秒
mysql> select sleep(3);

When we went to check in the folder, we found that it has been stored in the slow query diary

This part explains how to find the slower SQL through the slow log . The latter part will talk about why it is slow and how it can be faster.

2. Query Analyzer-explain

Role : Through this we can know where to view SQL is slow, and which aspects need to be optimized

Column: we create an employee data table

create table employee(
	id int not null auto_increment primary key,
	name varchar(30) comment '姓名',
	sex varchar(1) comment '性别',
	salary int comment '薪资（元）',
	dept varchar(30) comment '部门'
);

insert into employee(name, sex, salary, dept) values('张三', '男', 5500, '部门A');
insert into employee(name, sex, salary, dept) values('李洁', '女', 4500, '部门C');
insert into employee(name, sex, salary, dept) values('李小梅', '女', 4200, '部门A');
insert into employee(name, sex, salary, dept) values('欧阳辉', '男', 7500, '部门C');
insert into employee(name, sex, salary, dept) values('李芳', '女', 8500, '部门A');
insert into employee(name, sex, salary, dept) values('张江', '男', 6800, '部门A');
insert into employee(name, sex, salary, dept) values('李四', '男', 12000, '部门B');
insert into employee(name, sex, salary, dept) values('王五', '男', 3500, '部门B');
insert into employee(name, sex, salary, dept) values('马小龙', '男', 6000, '部门A');
insert into employee(name, sex, salary, dept) values('龙五', '男', 8000, '部门B');
insert into employee(name, sex, salary, dept) values('冯小芳', '女', 10000, '部门C');
insert into employee(name, sex, salary, dept) values('马小花', '女', 4000, '部门B');
insert into employee(name, sex, salary, dept) values('柳峰', '男', 8800, '部门A');

//通过explain解读他，后面加一个\G便于阅读
mysql> explain select * from employee where name='柳峰'\G;
//扫描快捷
mysql> explain select * from employee where id=13\G;

Effect: As shown in the figure below, you can see why it was so slow before and it took four seconds to respond

3. Basic use of index

mysql> show index from employee\G;
//主键会默认建一个id索引

Improved index creation efficiency

//查询分析
mysql> explain select * from employee where name='柳峰';
//创建普通索引
mysql> create index idx_name on employee(name);

//删除
mysql> drop index idx_name on employee;

Teacher's list:

If you have used like to search, the efficiency remains the same, so it depends on how you use it

4. Composite Index

//查的时候可以看到一个主键索引
mysql> show index from employee\G;

Currently all global scan

select * from employee where name ='柳峰';
//查询分析
explain select * from employee where name ='柳峰'\G;

Create index

//创建索引
create index idx_name_salary_dept on employee(name,salary,dept);
//查询分析
explain select * from employee where name ='柳峰'\G;

Verify that there is a name to index

// name和salary
mysql> explain select * from employee where name ='柳峰' and salary=8800\G;
//name和dept
mysql> explain select * from employee where name ='柳峰' and dept='部门A'\G;

Cannot use index without name

mysql> explain select * from employee where  salary=8800;

mysql> explain select * from employee where  dept='部门A';

V. Covering Index

According to the above steps, we can see four indexes, the first is the primary key index , followed by the composite indexname_salary_dept

mysql> show index from employee;

How to trigger

We use id as query data

mysql> select * from employee;

mysql> select * from employee where id =11;

Only check id

mysql> explain select id from employee  employee where id=11\G;

mysql> explain select id from employee\G;

//查name,salary
mysql> explain select name,salary from employee;

//查name,salary,dept
mysql> explain select name,salary,dept from employee;

//因为没有sxe条件，所以只能做全部扫描type为null
mysql> explain select name,sex,salary,dept from employee;

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