MySql和subselect，为什么这么慢？

Question

我在这里选择:

select parent_id from sales_flat_order_status_history where status like '%whatever%' group by parent_id having count(parent_id) > 1

这个查询只运行了几秒钟。现在我想在另一个选择中使用它，像这样:

select increment_id from sales_flat_order where entity_id in(
select parent_id from sales_flat_order_status_history where status like '%whatever%' group by parent_id having count(parent_id) > 1)

这会永远运行，所以我尝试一个一个地插入 ID:

select increment_id from sales_flat_order where entity_id in(329,523,756,761,763,984,1126,1400,1472,2593,3175,3594,3937,...)

这运行得很快，区别在哪里？我怎样才能使我的第一种方法运行得更快？

谢谢!

Answer 1

您的查询需要很长时间才能运行，因为它正在执行子查询并查找 sales_flat_order 表的每一行。

加入可能会更快:

select increment_id 
from sales_flat_order
  inner join (select parent_id 
              from sales_flat_order_status_history 
              where status like '%whatever%' 
              group by parent_id having count(parent_id) > 1) Sub 
  on sales_flat_order.entity_id  = Sub.parent_ID

这强制子查询只执行一次