Question

我有4张桌子：域：：有很多=＆gt;渠道：has_many =＆gt;销售：has_many =＆gt;访问

我正在尝试获取此查询中每个域的所有域名，渠道数，销售次数和访问次数：

SELECT
    kd.id, kd.domain,
    COUNT(distinct kc.id) AS channels_count,
    COUNT(distinct kv.id) AS visits_count,
    COUNT(distinct kv.ip_address) AS visitors_count,
    COUNT(distinct ks.id) AS sales_count
FROM
    domains AS kd
LEFT JOIN
    channels AS kc
    ON(kc.domain_id=kd.id)
LEFT JOIN
    sales AS ks
    ON(ks.channel_id=kc.id)
LEFT JOIN
    visits AS kv
    ON(kv.channel_id=kc.id)
GROUP BY kd.id

我在销售和访问表中有几千条记录，没有什么大的，但我的查询需要永远。它永远不会回来。知道为什么吗？

db：postgresql

Answer 1

COUNT（DISTINCT）可能是资源匮乏。此外，您的表可能在“kd.id”列上有许多重复项，导致生成数以万计的行。

更有效的方法是首先进行聚合，然后将结果连接在一起。我认为以下查询可以执行您想要的操作（我假设每个表中的id列都是唯一的）：

SELECT kd.id, kd.domain, channels_count, visits_count,
       visitors_count, sales_count
FROM domains kd LEFT JOIN
     (select kc.domain_id, count(*) as channels_count
      from channels kc
      group by kc.domain_id
     ) kc
     ON(kc.domain_id=kd.id) LEFT JOIN
     (select ks.channel_id, count(*) as sales_count
      from sales ks
      group by ks.channel_id
     ) ks
     ON(ks.channel_id=kc.id) LEFT JOIN
     (select kv.channel_id, count(*) as visits_count,
             count(distinct ipaddress) as visitors_count
      from visits kv
      on kv.channel_id
     )
     ON(kv.channel_id=kc.id)

GROUP BY kd.id

使用嵌套连接进行查询需要很长时间

1 个答案: