Question

So I noticed that postgres (9.0) doesn’t like it when you dereference a 2d array with only 1 dimension. Here’s a funny example

WITH my_table(arr) AS ( VALUES (ARRAY[[10,11],[20,21]]) )
SELECT arr[2][1] AS good, arr[1] AS bad FROM my_table;

 good | bad
------+-----
   20 |
(1 row)

As you can see this returns null when you don’t specify the second dimension. Ideally I’d like it to return the inner array {10,11}. So to solve this I wrote this function

CREATE OR REPLACE FUNCTION deref_2d(orig_arr numeric[][], inner_arr int)
RETURNS numeric[] AS $$
DECLARE
  index int;
  len int;
  return_arr numeric[];
BEGIN
  len := array_upper(orig_arr,2);
  FOR index IN 1 .. len LOOP
    return_arr[index] := orig_arr[inner_arr][index];
  END LOOP;
  RETURN return_arr;
END
$$ LANGUAGE plpgsql;

And now I can write:

WITH my_table(arr) AS ( VALUES (ARRAY[[10,11],[20,21]]) )
SELECT deref_2d(arr,1) FROM my_table;
 deref_2d
----------
 {10,11}

But multiple things about this make me feel uneasy about my solution. Is there a better way to do this?

Answer 1

Consider the full cast of this Western before you write it off:

WITH tbl(arr) AS (SELECT (ARRAY[[10,11],[20,21]]))
SELECT arr[2][1]    AS the_good
      ,arr[1]       AS the_bad
      ,arr[1:1]     AS the_ugly   -- first slice of 2D-array
      ,arr[1][1:2]  AS the_ugly_twin -- 2D-array with elements 1 & 2 of 1st slice
      ,ARRAY((SELECT unnest(arr[1:1]))) AS the_righteous -- 1D-array of 1st slice
FROM   tbl;

->sqlfiddle with more examples.

Some background in the manual here and here.