Question

下面的查询将first的结果分组为4个等间隔的日期分箱，并汇总每个分箱中the_value的平均值。

WITH first as(
SELECT
    extract(EPOCH FROM foo.t_date) as the_date,
    foo_val as the_value
FROM bar
INNER JOIN foo
ON
    foo.user_id = bar.x_id
    and
    foo.user_name = 'xxxx'
)
SELECT bin, round(sum(bin_sum) OVER w /sum(bin_ct) OVER w, 2) AS running_avg
FROM  (
   SELECT width_bucket(first.the_date
                     , x.min_epoch, x.max_epoch, x.bins) AS bin
        , sum(first.the_value) AS bin_sum
        , count(*)   AS bin_ct
   FROM   first
       , (SELECT MIN(first.the_date) AS min_epoch
               , MAX(first.the_date) AS max_epoch
               , 4 AS bins
          FROM  first
         ) x
   GROUP  BY 1
   ) sub
WINDOW w AS (ORDER BY bin)
ORDER  BY 1;

我希望能够只计算每个箱子中最低20 the_value的平均值。从Stackoverflow上的其他帖子我已经看到这是可能的，也许ORDER BY the_value和rank()是最好的方法。但我的困难在于我不确定应该修改当前查询的位置以实现此目的。

任何见解都将受到赞赏。

Postgres版本9.3

Answer 1

在每个垃圾箱上使用row_number() 首先计算行号rn，然后在下一步中应用WHERE rn < 21：

WITH first AS (
   SELECT extract(EPOCH FROM foo.t_date) AS the_date
        , foo_val AS the_value
   FROM bar
   JOIN foo ON foo.user_id = bar.x_id
           AND foo.user_name = 'xxxx'
   )
, x AS (
   SELECT MIN(the_date) AS min_epoch
        , MAX(the_date) AS max_epoch
   FROM  first
   )
, y AS (
   SELECT width_bucket(f.the_date, x.min_epoch, x.max_epoch, 4) AS bin, *
   FROM   first f, x
   )
, z AS (
   SELECT row_number() OVER (PARTITION BY bin ORDER BY the_value) AS rn, *
   FROM   y
   )
SELECT bin, round(sum(bin_sum) OVER w / sum(bin_ct) OVER w, 2) AS running_avg
FROM  (
   SELECT bin
        , sum(the_value) AS bin_sum
        , count(*)       AS bin_ct
   FROM   z
   WHERE  rn < 21   -- max 20 lowest values
   GROUP  BY 1
   ) sub
WINDOW w AS (ORDER BY bin)
ORDER  BY 1;

CTE y和z可能会混淆。同样，first和x也可以混为一谈但这样更清楚。

未经测试，因为我们没有测试数据。

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