Question

我有以下数据：

 group_id |         day | amount
----------+-------------+-------
        1 | 15 Nov 2015 |   5.0
        1 | 15 Nov 2015 |   6.0
        1 | 14 Nov 2015 |   3.0
        2 | 17 Nov 2015 |   5.0
        2 | 15 Nov 2015 |   5.0

我想为每个（group_id，day）选择前十个金额。我尝试写过：

Postgres 9.4

select max(x.group_id), max(x.day), max(x.amounts) 
from (select group_id, day, array_agg(amount) over w as amounts,
             row_number() over w as r
      from my_table window w as (partition group_id, day 
                                 order by amount desc)) as x
where x.r<=10 group by x.group_id,x.day

我还想到我可以写一个更简单的查询：

select a.day, a.group_id, array(select amount 
                                from my_table 
                                where day=a.day and group_id=a.group_id
                                order by amount desc limit 10)
from my_table as a group by a.day, a.group_id

这正是我想要的。这引出了我的问题：假设我可以调整第一个例子来获得我想要的，哪个查询会更快？子查询是否比分区慢？

Answer 1

您可能应该使用分析功能。

不知道你为什么在子查询之外也有MAX，MIN。你的查询似乎不是等价物。

您按组排名前十的请求应为：

WITH ranked as (
      SELECT group_id, 
             day,
             row_number() OVER 
                   (partition by group_id, day ORDER BY ammount DESC) rn
      FROM my_table 
) 
SELECT group_id, 
       day,
       array_agg(amount) over (partition by group_id, day ORDER BY rn)
FROM ranked
WHERE rn <=10

哪个更有效，选择array_agg over partition，还是选择array（子查询）？

1 个答案: