in型子查询陷阱

Posted 2020-10-07 Microtiger

in 型子查询引出的陷阱 select goods_id from goods where cat_id in (1,2,3) 直接用id,不包含子查询,不会中陷阱

题: 在ecshop商城表中,查询6号栏目的商品, (注,6号是一个大栏目)

最直观的: mysql> select goods_id,cat_id,goods_name from  goods where cat_id in (select

cat_id from ecs_category where parent_id=6);

误区: 给我们的感觉是, 先查到内层的6号栏目的子栏目,如7,8,9,11

然后外层, cat_id in (7,8,9,11)

 

事实: 如下图, goods表全扫描, 并逐行与category表对照,看parent_id=6是否成立

原因: mysql的查询优化器,针对In型做优化,被改成了exists的执行效果.

当goods表越大时, 查询速度越慢.

 

select goods_id,cat_id,goods_name from  goods where cat_id in (select cat_id from ecs_category where parent_id=6)

改进: 用连接查询来代替子查询

 explain select goods_id,g.cat_id,g.goods_name from  goods as g

 inner join (select cat_id from ecs_category where parent_id=6) as t

 using(cat_id) \\G

 

exists子查询

题: 查询有商品的栏目.

按上面的理解,我们用join来操作,如下:

mysql> select c.cat_id,cat_name from ecs_category as c inner join  goods as g

 on c.cat_id=g.cat_id group by cat_name; (见36)

 

优化1:  在group时, 用带有索引的列来group, 速度会稍快一些,另外,

用int型 比 char型 分组,也要快一些.(见37)

 

 

优化2: 在group时, 我们假设只取了A表的内容,group by 的列,尽量用A表的列,

会比B表的列要快.(见38)

 

优化3: 从语义上去优化

select cat_id,cat_name from ecs_category where exists(select *from  goods where  goods.cat_id=ecs_category.cat_id) (见40)

 

|       36 | 0.00039075 | select c.cat_id,cat_name from ecs_category as c inner

join  goods as g on c.cat_id=g.cat_id group by cat_name

              |

|       37 | 0.00038675 | select c.cat_id,cat_name from ecs_category as c inner

join  goods as g on c.cat_id=g.cat_id group by cat_id

              |

|       38 | 0.00035650 | select c.cat_id,cat_name from ecs_category as c inner

join  goods as g on c.cat_id=g.cat_id group by c.cat_id

              |

|       40 | 0.00033500 | select cat_id,cat_name from ecs_category where exists

(select * from  goods where  goods.cat_id=ecs_category.cat_id)

              |

 

from 型子查询:

注意::内层from语句查到的临时表, 是没有索引的. select id from (select id from t1 where cat_id=100) where id>100  其中id>100是没有索引的,from的是一个临时表

所以: from的返回内容要尽量少.