Pairwise array sum aggregate function?

Question

I have a table with arrays as one column, and I want to sum the array elements together:

> create table regres(a int[] not null);
> insert into regres          


        

Answer 1

A general solution in Postgres 9.3+ for any number of arrays with any number of elements.
Individual elements or the the whole array can be NULL, too:

SELECT ARRAY (
   SELECT sum(arr[rn])
   FROM  tbl t
       , generate_subscripts(t.arr, 1) AS rn
   GROUP BY rn
   ORDER BY rn
   );

This makes use of an implicit LATERAL JOIN (Postgres 9.3+).
With your example values:

SELECT ARRAY (
   SELECT sum(arr[rn])
   FROM  (
      VALUES
        ('{1,2,3}'::int[])
       ,('{9,12,13}')
      ) t(arr)
    , generate_subscripts(t.arr, 1) AS rn
   GROUP BY rn
   ORDER BY rn
   );

Non-trivial example:

SELECT ARRAY (
   SELECT sum(arr[rn])
   FROM  (
      VALUES
        ('{1,2,3}'::int[])
       ,('{9,12,13}')
       ,('{1,1,1, 33}')
       ,('{NULL,NULL}')
       ,(NULL)
      ) t(arr)
    , generate_subscripts(t.arr, 1) AS rn
   GROUP BY rn
   ORDER BY rn
   );

Simpler in 9.4+ using WITH ORDINALITY

SELECT ARRAY (
   SELECT sum(elem)
   FROM  tbl t
       , unnest(t.arr) WITH ORDINALITY x(elem, rn)
   GROUP BY rn
   ORDER BY rn
   )

Postgres 9.1

SELECT ARRAY (
   SELECT sum(arr[rn])
   FROM  (
      SELECT arr, generate_subscripts(arr, 1) AS rn
      FROM   tbl t
      ) sub
   GROUP BY rn
   ORDER BY rn
   );

The same works in later versions, but set-returning functions in the SELECT list are not standard SQL and frowned upon by some. So use above alternatives with current Postgres.

SQL Fiddle.

Related answers with more explanation:

• PostgreSQL unnest() with element number
• Is there something like a zip() function in PostgreSQL that combines two arrays?