mysql - 为什么组合计数查询执行时间太长？

Question

服务器不会对此查询进行字节处理，执行时间太长:

select prodavac.id, count(artikl.id) as brojartikala, count(poruceno.id) as brojporudzbina from prod_prodavac prodavac 
inner join prod_artikl artikl 
    on prodavac.id=artikl.prodavacid 
inner join prod_poruceno poruceno 
    on prodavac.id=poruceno.prodavacid 
group by prodavac.id 

另一方面，两个半查询都运行得非常快:

select prodavac.id, count(artikl.id) as brojartikala from prod_prodavac prodavac 
inner join prod_artikl artikl 
    on prodavac.id=artikl.prodavacid 
group by prodavac.id 

还有另一个:

select prodavac.id, count(poruceno.id) as brojporudzbina from prod_prodavac prodavac 
inner join prod_poruceno poruceno 
    on prodavac.id=poruceno.prodavacid 
group by prodavac.id 
order by prodavac.id asc 

我真的很想在一个查询中完成它，那么如何正确地合并它们呢？所有 ID 都是索引整数。

解释选择显示:

Answer 1

根据表和数据之间的关系，您的组合查询甚至可能不会返回所需的结果。对于简单的关系计数，您可以在 SELECT 子句中使用相关子查询:

select prodavac.id, (
    select count(*)
    from prod_artikl artikl 
    where artikl.prodavacid = prodavac.id
  ) as brojartikala, (
    select count(*)
    from prod_poruceno poruceno 
    where poruceno.prodavacid = prodavac.id
  ) as brojporudzbina
from prod_prodavac prodavac
order by prodavac.id asc