Aggregate functions over arrays

Question

I have a table like this:

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

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,...}

Answer 1

Try something like this:

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

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).

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

Next - just aggregate the avg_val into array:

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

Details: unnest , ntile , array_agg , OVER (PARTITION BY )

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;

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

SELECT id, array300_to_100(array300)
FROM table1;

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

Answer 2

Putting the pieces of Igor into another form:

 select id, array300, (
    select array_agg(z) from
    (
        select avg(x) from 
        (
            select x, ntile(array_length(array300,1)/3) over() from unnest(array300) x
        ) y 
        group by ntile
    ) z
) array100
from your_table

For a small example table like this

 id |       array300        
----+-----------------------
  1 | {110,25,53,110,25,53}
  2 | {56,75,59,110,25,53}
  3 | {65,93,82,110,25,53}
  4 | {75,70,80,110,25,53}

the result is:

 id |       array300        |                   array100                    
----+-----------------------+-----------------------------------------------
  1 | {110,25,53,110,25,53} | {(62.6666666666666667),(62.6666666666666667)}
  2 | {56,75,59,110,25,53}  | {(63.3333333333333333),(62.6666666666666667)}
  3 | {65,93,82,110,25,53}  | {(80.0000000000000000),(62.6666666666666667)}
  4 | {75,70,80,110,25,53}  | {(75.0000000000000000),(62.6666666666666667)}
(4 rows)

Edit My first version used a fixes ntile(2). This only worked for source arrays of size 6. I've fixed that by using array_length(array300,1)/3 instead.