MySQL join 连表查询索引问题

首先先创建两个临时表，并加一条基础数据进去

create table user
(
  id              int auto_increment comment \'自增主键\' primary key,
  name            varchar(30) null comment \'用户名称\',
  create_time     datetime    not null  comment \'注册时间\',
  last_login_time datetime    null  comment \'最后登录时间\'
)  comment \'测试表\';
create table article
(
  id          int auto_increment  comment \'自增主键\'    primary key,
  user_id     int          not null  comment \'用户id\',
  name        varchar(30)  null  comment \'帖子名称\',
  topic       varchar(30)  null  comment \'帖子关键词\',
  content     varchar(500) null  comment \'帖子内容\',
  create_time datetime     not null  comment \'创建时间\'
)  comment \'测试表2\';*/insert into article values(1,1, \'name_1\', \'topic_1\', \'content_1\', \'2019-01-01 00:00:00\');
insert into user values(1,\'user_1\', \'2019-01-01 00:00:00\', \'2019-03-01 12:00:00\');

 

为了能模拟大查询的情况，给每个表插入一些数据，user要有万级数据量，article要有百万级数据量，下面的sql每执行一次，数据量翻倍，谨慎执行！

set @i=1;
set @time=1;
insert into user(name, create_time, last_login_time)
select
       concat(\'user_\',@i:=@i+1),
       date_add(create_time,interval +@time*cast(rand()*100 as signed) SECOND),
       null
from user;
select count(1) from user;

set @i=1;
set @time=1;
insert into article(user_id, name, topic, content, create_time)
select
       round(rand()*(select max(id) from user)),
       concat(\'name_\',@i:=@i+1),
       concat(\'topic_\',@i:=@i+1),
       concat(\'content_\',@i:=@i+1),
       date_add(create_time,interval +@time*cast(rand()*100 as signed) SECOND)
from article;
select count(1) from article;

 

看下查询的SQL语句

select sql_no_cache  * from user left join article on(user.id = article.user_id)
where  user.name like \'user_4%\';

没有使用缓存，user表的id是主键，article表除主键外没有任何索引，这种情况下，百万级数据查询情况如下

sql> select sql_no_cache  * from user left join article on(user.id = article.user_id)
     where  user.name like \'user_4%\'
[2020-05-17 13:24:45] 500 rows retrieved starting from 1 in 4 s 681 ms (execution: 1 s 312 ms, fetching: 3 s 369 ms)

 

给article表加个索引

CREATE INDEX user_id ON article (user_id);

 

再执行一下看下效果

sql> select sql_no_cache  * from user left join article on(user.id = article.user_id)
     where  user.name like \'user_4%\'
[2020-05-17 13:27:22] 500 rows retrieved starting from 1 in 142 ms (execution: 112 ms, fetching: 30 ms)

 

可以看出，加了索引后，时间缩短了97%

结论：

A表与B表使用Join联合查询的时候，针对on里面的字段加与不加索引的效率（假设on的条件是A.id=B.aid）：

1、两个字段都不加索引，效率极低

2、A表的字段加了索引，B表的字段没有加索引，效果同上

3、A表B表的字段都加了索引，效果很明显

4、A表不加索引，B表加了索引，效果同上

 

参考：

资料一

资料二