《一起学mysql》4

索引的使用

索引太少返回结果很慢，但是索引太多，又会占用空间。每次插入新记录时，索引都会针对变化重新排序

什么时候使用索引

1.where 从句中用到的字段

select * from tb where f1 = 'xx' ,如果f1 进行了索引，那么这条sql 语句的销毁就会提高

2 max(),min()

select max(id) from tb

对id 索引非常有用，因为索引是按照顺序排序的，所以直接返回最后一个值即可

3.当返回的内容是索引的一部分时

这种情况不必扫描全表，只需查看索引即可

这种情况是不行的：f1 char(20),index f1(10),select f1 from tb ,因为索引只有10个字符，返回的却是20个

字符，必须扫描全表，扫描索引是不行的

4.order by

select * from tb order by f1,若 f1 是被索引的，则按照索引排序的顺序返回即可

5.在连接条件处使用

select t1.c1,t1.c2,t2.c3

from t1 join t2

on t1.c4=t2.c5

where t1.c6=xx;

如果 c4 和 c5 加了索引，也可以被使用到

6.不以% 开头的like 语句vs

select * from tb where name like 'jas%'

name 列加了索引也会被使用到

但是这种情况是不行的：select * from tb where name like '%jas%'

因为索引是按照第一个字符的字母顺序排序的

创建索引的一些建议：

1.where 条件中的字段创建索引

2.被索引的字段内容重复率越低越好，主键是最好的索引，每个id 只返回一条记录，枚举就比较差劲，比方

说性别，查询性别‘男’，可能会返回近一半的记录

3.使用前缀索引

4.不要创建太多的索引，太多的索引会拖慢插入和更新速度

最左原则

这个问题单纯的口头描述很难说清楚，直接看伪代码

create table tb (f1 char(20), f2 char(20), f3 char(20), f4 char(20), index(f1,f2,f3))

对于上面的表我们来进行下面几个sql 查询

1. select * from tb where f1='xx1' and f2 ='xx2' and f3='xx3'

2. select * from tb where f1='xx1' and f2 ='xx2'

3. select * from tb where f1='xx1'

4. select * from tb where f2 ='xx2' and f3='xx3'

5. select * from tb where f1='xx1' and f3='xx3'

上面的sql 中 1，2，3 的索引全部其作用

4中索引不会起作用

5中只有索引f1 起作用

也就是说复合索引使用的时候，必须是从最左侧开始，且是连续的，这样所有的索引才会被使用到

explain

下面来说一个很有用的东西，explain

先来建两张表，存储学生和老师信息，

create table student

(

stu_id int(3) zerofill,

name char(10),

majar char(10),

teacher_id int

);

create table teacher

(

teacher_id int,

name char(10)

);

insert into student values

(1,'丽丽','chemistry',1),

(2,'丽娟','english',2),

(3,'芳丽','儒学',3),

(4,'光头强','佛学',1),

(5,'熊大','math',2),

(6,'熊二','地理',3),

(7,'陶峰','高分子',4),

(8,'波波','机电',1),

(9,'米老鼠','土木',2);

insert into teacher values

(1,'刘老师'),

(2,'王老师'),

(3,'李老师'),

(4,'柴老师');

赶紧来试试explain

MariaDB [jason]> explain select * from student where stu_id=1;

+------+-------------+---------+------+---------------+------+---------+------+------+-------------+

+------+-------------+---------+------+---------------+------+---------+------+------+-------------+

+------+-------------+---------+------+---------------+------+---------+------+------+-------------+

给stu_id 加上索引再来试一下

MariaDB [jason]> alter table student add index(stu_id);

Query OK, 0 rows affected (0.02 sec)

Records: 0 Duplicates: 0 Warnings: 0

MariaDB [jason]> explain select * from student where stu_id=1;

+------+-------------+---------+------+---------------+--------+---------+-------+------+-------+

+------+-------------+---------+------+---------------+--------+---------+-------+------+-------+

| 1 | SIMPLE | student | ref | stu_id | stu_id | 5 | const | 1 | |

+------+-------------+---------+------+---------------+--------+---------+-------+------+-------+

有木有发现不一样？下面我们来解释一下explain 各个字段含义

说实话，我对参考资料中的的解释有些困惑，我想先自己造几个例子看看

MariaDB [jason]> show indexes from student ;

+---------+------------+------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+

+---------+------------+------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+

| student | 1 | stu_id | 1 | stu_id | A | 9 | NULL | NULL | YES | BTREE | | |

| student | 1 | teacher_id | 1 | teacher_id | A | 9 | NULL | NULL | YES | BTREE | | |

+---------+------------+------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+

MariaDB [jason]> drop index stu_id on student;

MariaDB [jason]> alter table student add primary key(stu_id);

MariaDB [jason]> show indexes from teacher;

+---------+------------+------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+

+---------+------------+------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+

| teacher | 1 | teacher_id | 1 | teacher_id | A | 1 | NULL | NULL | YES | BTREE | | |

+---------+------------+------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+

1 row in set (0.00 sec)

MariaDB [jason]> drop index teacher_id on teacher;

Query OK, 0 rows affected (0.00 sec)

Records: 0 Duplicates: 0 Warnings: 0

MariaDB [jason]> show indexes from teacher;

+---------+------------+------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+

+---------+------------+------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+

| teacher | 1 | teacher_id | 1 | teacher_id | A | 1 | NULL | NULL | YES | BTREE | | |

+---------+------------+------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+

1 row in set (0.00 sec)

MariaDB [jason]> drop index teacher_id on teacher;

Query OK, 0 rows affected (0.00 sec)

Records: 0 Duplicates: 0 Warnings: 0

MariaDB [jason]> alter talbe teacher add primary key(teacher_id);

ERROR 1064 (42000): You have an error in your SQL syntax; check the manual that corresponds to your MariaDB server version for the right syntax to use near 'talbe teacher add primary key(teacher_id)' at line 1

MariaDB [jason]> alter table teacher add primary key(teacher_id);

我们先把student表中的stu_id 和tercher表中的teacher_id 变成了主键

example1：

MariaDB [jason]> explain select * from student where stu_id = 1;

+------+-------------+---------+-------+---------------+---------+---------+-------+------+-------+

+------+-------------+---------+-------+---------------+---------+---------+-------+------+-------+

+------+-------------+---------+-------+---------------+---------+---------+-------+------+-------+

example2：

MariaDB [jason]> explain select * from student where stu_id > 2;

+------+-------------+---------+-------+---------------+---------+---------+------+------+-------------+

+------+-------------+---------+-------+---------------+---------+---------+------+------+-------------+

+------+-------------+---------+-------+---------------+---------+---------+------+------+-------------+

example3：

MariaDB [jason]> explain select * from student where teacher_id = 1;

+------+-------------+---------+------+---------------+------------+---------+-------+------+-------+

+------+-------------+---------+------+---------------+------------+---------+-------+------+-------+

| 1 | SIMPLE | student | ref | teacher_id | teacher_id | 5 | const | 3 | |

+------+-------------+---------+------+---------------+------------+---------+-------+------+-------+

example4：

MariaDB [jason]> explain select * from student where teacher_id>2;

+------+-------------+---------+-------+---------------+------------+---------+------+------+-----------------------+

+------+-------------+---------+-------+---------------+------------+---------+------+------+-----------------------+

+------+-------------+---------+-------+---------------+------------+---------+------+------+-----------------------+

example5：

ariaDB [jason]> explain select s.stu_id,s.name,s.majar,s.teacher_id,t.name

-> from student s join teacher t

-> on s.teacher_id = t.teacher_id;

+------+-------------+-------+------+---------------+------+---------+------+------+-------------------------------------------------+

+------+-------------+-------+------+---------------+------+---------+------+------+-------------------------------------------------+

| 1 | SIMPLE | t | ALL | PRIMARY | NULL | NULL | NULL | 4 | |

+------+-------------+-------+------+---------------+------+---------+------+------+-------------------------------------------------+

example6：

MariaDB [jason]> explain select s.stu_id,s.name,s.majar,s.teacher_id,t.name

-> from student s left join teacher t

-> on s.teacher_id = t.teacher_id;

+------+-------------+-------+--------+---------------+---------+---------+--------------------+------+-------+

+------+-------------+-------+--------+---------------+---------+---------+--------------------+------+-------+

| 1 | SIMPLE | s | ALL | NULL | NULL | NULL | NULL | 9 | |

| 1 | SIMPLE | t | eq_ref | PRIMARY | PRIMARY | 4 | jason.s.teacher_id | 1 | |

+------+-------------+-------+--------+---------------+---------+---------+--------------------+------+-------+

example7：

MariaDB [jason]> explain select s.stu_id,s.name,s.majar,s.teacher_id,t.name

-> from student s left join teacher t

-> on s.teacher_id = t.teacher_id

-> where t.name is not null;

+------+-------------+-------+------+---------------+------+---------+------+------+-------------------------------------------------+

+------+-------------+-------+------+---------------+------+---------+------+------+-------------------------------------------------+

+------+-------------+-------+------+---------------+------+---------+------+------+-------------------------------------------------+

example8：

MariaDB [jason]> explain select s.stu_id,s.name,s.majar,s.teacher_id,t.name

-> from student s right join teacher t

-> on s.teacher_id = t.teacher_id;

+------+-------------+-------+------+---------------+------------+---------+--------------------+------+-------+

+------+-------------+-------+------+---------------+------------+---------+--------------------+------+-------+

| 1 | SIMPLE | t | ALL | NULL | NULL | NULL | NULL | 4 | |

| 1 | SIMPLE | s | ref | teacher_id | teacher_id | 4 | jason.t.teacher_id | 1 | |

+------+-------------+-------+------+---------------+------------+---------+--------------------+------+-------+

我尝试过使用full outer join ，但是mysql 不支持外连接

example9：

MariaDB [jason]> explain select stu_id,teacher_id from student;

+------+-------------+---------+-------+---------------+------------+---------+------+------+-------------+

+------+-------------+---------+-------+---------------+------------+---------+------+------+-------------+

+------+-------------+---------+-------+---------------+------------+---------+------+------+-------------+

example10：

MariaDB [jason]> explain select max(teacher_id) from student;

+------+-------------+-------+------+---------------+------+---------+------+------+------------------------------+

+------+-------------+-------+------+---------------+------+---------+------+------+------------------------------+

+------+-------------+-------+------+---------------+------+---------+------+------+------------------------------+

example11：

MariaDB [jason]> explain select * from student order by teacher_id desc;

+------+-------------+---------+------+---------------+------+---------+------+------+----------------+

+------+-------------+---------+------+---------------+------+---------+------+------+----------------+

+------+-------------+---------+------+---------------+------+---------+------+------+----------------+

example12：

MariaDB [jason]> explain select * from student force index(teacher_id) order by teacher_id desc;

+------+-------------+---------+-------+---------------+------------+---------+------+------+-------+

+------+-------------+---------+-------+---------------+------------+---------+------+------+-------+

+------+-------------+---------+-------+---------------+------------+---------+------+------+-------+

example13：

MariaDB [jason]> alter table student add index(name);

MariaDB [jason]> explain select * from student where name like '丽%';

+------+-------------+---------+-------+---------------+------+---------+------+------+-----------------------+

+------+-------------+---------+-------+---------------+------+---------+------+------+-----------------------+

+------+-------------+---------+-------+---------------+------+---------+------+------+-----------------------+

example14：

MariaDB [jason]> explain select * from student where teacher_id +5 < 7;

+------+-------------+---------+------+---------------+------+---------+------+------+-------------+

+------+-------------+---------+------+---------------+------+---------+------+------+-------------+

+------+-------------+---------+------+---------------+------+---------+------+------+-------------+

1 row in set (0.01 sec)

MariaDB [jason]> explain select * from student where teacher_id < 2;

+------+-------------+---------+-------+---------------+------------+---------+------+------+-----------------------+

+------+-------------+---------+-------+---------------+------------+---------+------+------+-----------------------+

+------+-------------+---------+-------+---------------+------------+---------+------+------+-----------------------+

1 row in set (0.00 sec)

example 15:

MariaDB [jason]> explain select s.stu_id,s.name,s.majar,s.teacher_id,t.name

-> from student s left join teacher t

-> on s.teacher_id = t.teacher_id

-> where t.teacher_id is null;

+------+-------------+-------+--------+---------------+---------+---------+--------------------+------+-------------------------+

+------+-------------+-------+--------+---------------+---------+---------+--------------------+------+-------------------------+

| 1 | SIMPLE | s | ALL | NULL | NULL | NULL | NULL | 9 | |

+------+-------------+-------+--------+---------------+---------+---------+--------------------+------+-------------------------+

2 rows in set (0.00 sec)

example 16:

MariaDB [jason]> explain select *

-> from t2 left join t1 on t2.f2=t1.f1

-> left join t3 on t3.f3=t2.f2

-> left join t4 on t4.f4=t3.f3

-> where t1.f1=1;

+------+-------------+-------+-------+---------------+---------+---------+-------+------+-------------+

+------+-------------+-------+-------+---------------+---------+---------+-------+------+-------------+

+------+-------------+-------+-------+---------------+---------+---------+-------+------+-------------+

这里没有列出最左原则的例子，大家可以自己个儿试一下，

关于expalin 各列的解释可以看这里

analyze table

mysql 在执行时select 或join 时会使用索引的分布信息来决定表的关联顺序、或使用那个索引，

analyze table tablename 就可以分析并存储索引的分布信息，

MariaDB [jason]> analyze table student;

+---------------+---------+----------+----------+

+---------------+---------+----------+----------+

+---------------+---------+----------+----------+

可以通过 show index from tb_name 来查看索引的情况

MariaDB [jason]> show index from student;

+---------+------------+------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+

+---------+------------+------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+

| student | 1 | name | 1 | name | A | 10 | NULL | NULL | YES | BTREE | | |

+---------+------------+------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+

各字段的解释看这里

重点解释一下Cardinality

1. 列值代表的是此列中存储的唯一值的个数（如果此列为primary key 则值为记录的行数）

2. 列值只是个估计值，并不准确。

3. 列值不会自动更新，需要通过Analyze table来更新一张表或者mysqlcheck -Aa来进行更新整个数据库。

4. 列值的大小影响Join时是否选用这个Index的判断。

5. 创建Index时，MyISAM的表Cardinality的值为null，InnoDB的表Cardinality的值大概为行数。

6. MyISAM与InnoDB对于Cardinality的计算方式不同。

optimize

如果对表进行了大量的删除和更新操作，那么表中就会留下缝隙，optimize table tb_name 可以将缝隙删除

将分割的记录连接在一起

MariaDB [jason]> optimize table student;

+---------------+----------+----------+-------------------------------------------------------------------+

+---------------+----------+----------+-------------------------------------------------------------------+

+---------------+----------+----------+-------------------------------------------------------------------+

什么鬼？原来是innodb 不支持optimize, 百度找到了方法

ALTER TABLE table.name ENGINE='InnoDB';

This will create a copy of the original table, and drop the original table, and replace to the original place.

Although this is safe, but I suggest you do backup and test first before doing this.

《一起学mysql》4的更多相关文章

《从零开始做一个MEAN全栈项目》（1）
欢迎关注本人的微信公众号"前端小填填",专注前端技术的基础和项目开发的学习. 在本系列的开篇,我打算讲一下全栈项目开发的优势,以及MEAN项目各个模块的概览. 为什么选择全栈开发? ...
《从零开始做一个MEAN全栈项目》（2）
欢迎关注本人的微信公众号"前端小填填",专注前端技术的基础和项目开发的学习. 上一节简单介绍了什么是MEAN全栈项目,这一节将简要介绍三个内容:(1)一个通用的MEAN项目的技 ...
《从零开始做一个MEAN全栈项目》（3）
欢迎关注本人的微信公众号"前端小填填",专注前端技术的基础和项目开发的学习. 上一篇文章给大家讲了一下本项目的开发计划,这一章将会开始着手搭建一个MEAN项目.千里之行,始于足下, ...
《从零开始做一个MEAN全栈项目》（4）
欢迎关注本人的微信公众号"前端小填填",专注前端技术的基础和项目开发的学习. 在上一篇中,我们讲了如何去构建第一个Express项目,总结起来就是使用两个核心工具,express和 ...
一个关于vue+mysql+express的全栈项目（一）
最近学了mysql数据库,寻思着能不能构思一个小的全栈项目,思来想去,于是就有了下面的项目: 先上几张效果图吧目前暂时前端只有这几个页面,后端开发方面,有登录,注册,完善用户信息,获取用 ...
Vue、Nuxt服务端渲染，NodeJS全栈项目，面试小白的博客系统~~
Holle,大家好,我是李白!! 一时兴起的开源项目,到这儿就告一段落了. 这是一个入门全栈之路的小项目,从设计.前端.后端.服务端,一路狂飙的学习,发量正在欣喜若狂~~ 接触过WordPress,H ...
Vue、Node全栈项目~面向小白的博客系统~
个人博客系统前言 ❝ 代码质量问题轻点喷(去年才学的前端),有啥建议欢迎联系我,联系方式见最下方,感谢! 页面有啥bug也可以反馈给我,感谢! 这是一套包含前后端代码的个人博客系统,欢迎各位提出建议 ...
SpringBoot 整合 Elastic Stack 最新版本(7.14.1)分布式日志解决方案，开源微服务全栈项目【有来商城】的日志落地实践
一. 前言日志对于一个程序的重要程度不用过多的言语修饰,本篇将以实战的方式讲述开源微服务全栈项目有来商城是如何整合当下主流日志解决方案 ELK +Filebeat . 话不多说,先看实现的效果图 ...
全栈项目|小书架|服务器端-NodeJS+Koa2实现首页图书列表接口
通过上篇文章全栈项目|小书架|微信小程序-首页水平轮播实现我们实现了前端(小程序)效果图的展示,这篇文章来介绍服务器端的实现. 首页书籍信息先来回顾一下首页书籍都有哪些信息: 从下面的图片可以看 ...
全栈项目|小书架|服务器开发-NodeJS 使用 JWT 实现登录认证
通过这篇全栈项目|小书架|服务器开发-JWT 详解文章我们对JWT有了深入的了解,那么接下来介绍JWT如何在项目中使用. 安装 $ npm install jsonwebtoken 生成 Toke ...

随机推荐

IT兄弟连 HTML5教程 HTML5的基本语法了解Web
HTML也是计算机编程语言,但由于功能简单易用,不涉及业务逻辑,算是编程语言中最简单的了.其实学习HTML这门语言,就是在学习一个个HTML标记的使用,标记的名称和使用不是自定义的,它的功能及用法是已 ...
【Linux】Linux 性能瓶颈阈值分析
Linux系统资源包括:CPU.IO(磁盘和网络).内存等利用率达到三个阶段时: 1)50% 引起注意 2)70% 密切关注 3)90% 严重情况 vmstat.sar.iostat.mpstat. ...
黄聪：table自适应宽度和高度
自适应宽度: td { width: 1px; white-space: nowrap; /* 自适应宽度*/ word-break: keep-all; /* 避免长单词截断,保持全部 */ } 自 ...
Django日志记录详细的报错信息
当服务器500错误的时候,普通日志只会记录一行500的request信息,并不会记录详细的报错定位 [ERROR] 2019-06-12 15:07:03,597 "GET /api/v1/ ...
一则SQL优化案例
原始sql: select CASE ) counts ,) else deadline end as deadline from t_product_credit) c group by sort ...
[转]C++字符串操作函数_tcschr、_tcsrchr
原文出处:https://blog.csdn.net/eickandy/article/details/50083169 C++标准库函数提供了字符和字符串的操作函数,并提供了其UNICODE版本,本 ...
Saltstack_使用指南10_配置管理-状态模块
1. 主机规划 salt 版本 [root@salt100 ~]# salt --version salt (Oxygen) [root@salt100 ~]# salt-minion --versi ...
PHP代码篇（三）--常用方法
模块下不间断更新,PHP常用方法,欢迎留言! 一.递归删除指定目录下所有文件及文件夹 /** * 递归删除指定目录下所有文件及文件夹 * @param unknown $path,删除路径 */ fu ...
python 中文分词库 jieba库
jieba库概述: jieba是优秀的中文分词第三方库中文文本需要通过分词获得单个的词语 jieba是优秀的中文分词第三方库,需要额外安装 jieba库分为精确模式.全模式.搜索引擎模式原理 1. ...
HTML&CSS基础-子元素和后代元素选择器
HTML&CSS基础-子元素和后代元素选择器作者:尹正杰版权声明:原创作品,谢绝转载!否则将追究法律责任. 一.html源代码 <!DOCTYPE html> <html ...

《一起学mysql》4

《一起学mysql》4的更多相关文章

随机推荐

热门专题