我如何在 PostgreSQL 中的数组上按元素应用聚合函数，例如一个组的加权数组总和？

Question

我有一个 table 如下（参见 db<>fiddle）：

grp	n	vals
0	2	{1,2,3,4}
1	5	{3,2,1,2}
1	3	{0,5,4,3}

对于每个组（由 grp 定义），我想执行一些涉及组的标量 n 和数组 vals 的算术运算。我对一种加权求和感兴趣，这样每一行的 vals 乘以它的 n，得到的数组在每个组内按元素求和，每组输出一个数组：

grp	result
0	{2,4,6,8}
1	{15,25,17,19}

这是我试过的方法。这失败并出现错误 (aggregate function calls cannot contain set-returning function calls):

SELECT
    grp,
    ARRAY(SELECT SUM(n * UNNEST(vals)))
FROM
    tbl
GROUP BY
    grp

该错误包含一个提示，但我无法理解它对我的用例的意义。

以下将所需数组汇总为标量：

SELECT
    grp,
    SUM(n * vals[i])
FROM
    tbl,
    generate_series(1, 4) i
GROUP BY
    grp

只有这种作品：

SELECT
    grp,
    SUM(n * vals[1]),
    SUM(n * vals[2]),
    SUM(n * vals[3]),
    SUM(n * vals[4])
FROM
    tbl
GROUP BY
    grp

但它不会产生数组，它涉及分别写出数组的每个元素。在我的例子中，数组比四个元素长得多，所以这太尴尬了。

Answer 1

WITH flattened AS (
    SELECT grp, position, SUM(val * n) AS s
    FROM tbl, unnest(vals) WITH ORDINALITY AS f(val, position)
    GROUP BY grp, position
    ORDER BY grp, position
)
SELECT grp, array_agg(s ORDER BY position)
FROM flattened
GROUP BY grp
;

+---+-------------------------------------------------------------------------------------+
|grp|array_agg                                                                            |
+---+-------------------------------------------------------------------------------------+
|0  |{2.00000000000000000,4.00000000000000000,6.00000000000000000,8.00000000000000000}    |
|1  |{15.00000000000000000,25.00000000000000000,17.00000000000000000,19.00000000000000000}|
+---+-------------------------------------------------------------------------------------+

解释：

您可以使用UNNEST ... WITH ORDINALITY来跟踪每个值的位置：

SELECT grp, position, val, n
FROM tbl, unnest(vals) WITH ORDINALITY AS f(val, position);

+---+--------+---+-+
|grp|position|val|n|
+---+--------+---+-+
|0  |1       |1  |2|
|0  |2       |2  |2|
|0  |3       |3  |2|
|0  |4       |4  |2|
|1  |1       |3  |5|
|1  |2       |2  |5|
|1  |3       |1  |5|
|1  |4       |2  |5|
|1  |1       |0  |3|
|1  |2       |5  |3|
|1  |3       |4  |3|
|1  |4       |3  |3|
+---+--------+---+-+

然后GROUP BY原组和各位置：

SELECT grp, position, SUM(val * n) AS s
FROM tbl, unnest(vals) WITH ORDINALITY AS f(val, position)
GROUP BY grp, position
ORDER BY grp, position;

+---+--------+--+
|grp|position|s |
+---+--------+--+
|0  |1       |2 |
|0  |2       |4 |
|0  |3       |6 |
|0  |4       |8 |
|1  |1       |15|
|1  |2       |25|
|1  |3       |17|
|1  |4       |19|
+---+--------+--+

那么你只需要答案中的ARRAY_AGG。

Answer 2

我会为此编写函数，否则 SQL 会变得非常混乱。

一个函数将所有元素与给定值相乘：

create function array_mul(p_input real[], p_mul int)
  returns real[]
as
$$
  select array(select i * p_mul
               from unnest(p_input) with ordinality as t(i,idx)
               order by idx);
$$
language sql
immutable;

还有一个函数用作汇总具有相同索引的元素的聚合：

create or replace function array_add(p_one real[], p_two real[])
  returns real[]
as
$$
declare
  l_idx int;
  l_result real[];
begin
  if p_one is null or p_two is null then
    return coalesce(p_one, p_two);
  end if;
  
  for l_idx in 1..greatest(cardinality(p_one), cardinality(p_two)) loop
    l_result[l_idx] := coalesce(p_one[l_idx],0) + coalesce(p_two[l_idx], 0);
  end loop;
  
  return l_result;  
end;  
$$
language plpgsql
immutable;

可用于定义自定义聚合：

create aggregate array_element_sum(real[]) (
  sfunc = array_add,
  stype = real[],
  initcond = '{}'
);

然后您的查询就这么简单：

select grp, array_element_sum(array_mul(vals, n))
from tbl
group by grp;

Online example

我如何在 PostgreSQL 中的数组上按元素应用聚合函数，例如一个组的加权数组总和？

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