在数组上聚合函数 [英] Aggregate functions over arrays

问题描述

我有一个这样的表：

+-----+----------------+
| ID  |  array300      |
+-----+----------------+
| 100 | {110,25,53,..} |
| 101 | {56,75,59,...} |
| 102 | {65,93,82,...} |
| 103 | {75,70,80,...} |
+-----+----------------+

array300 列是300个元素的数组。我需要有100个元素的数组，每个元素代表 array300 的3个元素的平均值。对于此示例，答案将类似于：

array100

{62.66，...}

{63.33，...}

{80，...}

{78.33，...}

array300 column is an array of 300 elements. I need to have arrays of 100 elements with every element representing the average of 3 elements of array300. For this example the answer will be like:
array100
{62.66,...}
{63.33,...}
{80,...}
{78.33,...}

推荐答案

尝试如下操作：

SELECT id, unnest(array300) as val, ntile(100) OVER (PARTITION BY id) as bucket_num
FROM your_table

此选择将为每个 array300 提供300条具有相同 id 的记录，并将它们分配给 bucket_num （第1个元素3个元素，第2个元素3个，依此类推）。

This SELECT will give you 300 records per array300 with same id and assing them the bucket_num (1 for firs 3 elements, 2 for next 3, and so on).

然后使用此选择获取存储桶中元素的 avg ：

Then use this select to get the avg of elements in the bucket:

SELECT id, avg(val) as avg_val
FROM (...previous select here...)
GROUP BY id, bucket_num

下一步-只需将 avg_val 汇总到数组中即可：

Next - just aggregate the avg_val into array:

SELECT id, array_agg(avg_val) as array100
FROM (...previous select here...)
GROUP BY id

详细信息：不必要， ntile ， array_agg ， OVER（按 PARTITION BY）

UPD：尝试此函数：

UPD: Try this function:

CREATE OR REPLACE FUNCTION public.array300_to_100 (
  p_array300 numeric []
)
RETURNS numeric [] AS
$body$
DECLARE
  dim_start int = array_length(p_array300, 1); --size of input array
  dim_end int = 100; -- size of output array
  dim_step int = dim_start / dim_end; --avg batch size
  tmp_sum NUMERIC; --sum of the batch
  result_array NUMERIC[100]; -- resulting array
BEGIN

  FOR i IN 1..dim_end LOOP --from 1 to 100.
    tmp_sum = 0;

    FOR j IN (1+(i-1)*dim_step)..i*dim_step LOOP --from 1 to 3, 4 to 6, ...
      tmp_sum = tmp_sum + p_array300[j];  
    END LOOP; 

    result_array[i] = tmp_sum / dim_step;
  END LOOP; 

  RETURN result_array;
END;
$body$
LANGUAGE 'plpgsql'
IMMUTABLE
RETURNS NULL ON NULL INPUT;

需要一个 array300 并输出一个 array100 。要使用它：

It takes one array300 and outputs one array100. To use it:

SELECT id, array300_to_100(array300)
FROM table1;

如果您在理解它时遇到任何问题，请问我。

If you have any problems understanding it - just ask me.

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