Question

I’ve a table that looks like:

 id |   t   
----+-------
  1 | {1,2}
  2 | {5,2}
  3 | {6,2}
  4 | {1,7}
  5 | {1,8}
  6 | {1,9}

I’m looking for a SELECT query that will return me array of t like {{1,2}, {5,2}, ... {1,9}}.

Had t not been an array data type, it would have been easy like:

SELECT ARRAY (SELECT t from tbl_foo);

Can same be done if data type is int[]?

Answer 1

I don’t know if there is some simpler way (I hope so), but this works (PostgreSQL doesn’t have array of arrays, so array_agg aproach doesn’t work here):

CREATE OR REPLACE FUNCTION func()
RETURNS int[] AS $$
DECLARE
    arr int[];
    res int[];
    n int;  
BEGIN
    res := '{{0, 0}}';
    FOR arr IN SELECT t FROM tbl_foo
    LOOP
        res := res || arr;
    END LOOP;
    n := array_length(res, 1);
    RETURN res[2:n];
END $$
LANGUAGE 'plpgsql';

Example:

CREATE TABLE tbl_foo (id serial, t int[]);
INSERT INTO tbl_foo (t) VALUES
    ('{1, 2}'),
    ('{5, 2}'),
    ('{6, 2}'),
    ('{1, 7}'),
    ('{1, 8}'),
    ('{1, 9}');

SELECT func();
                 func                  
---------------------------------------
 {{1,2},{5,2},{6,2},{1,7},{1,8},{1,9}}
(1 row)

EDIT:

Second solution is based on new aggregate function, called let’s say array2_agg:

CREATE OR REPLACE FUNCTION array2_agg_cutFirst(res anyarray)
RETURNS anyarray AS $$ 
BEGIN
    RETURN res[2:array_length(res, 1)];
END $$
LANGUAGE 'plpgsql';

CREATE AGGREGATE array2_agg(anyarray)
(
    SFUNC = array_cat,
    STYPE = anyarray,
    FINALFUNC = array2_agg_cutFirst,
    INITCOND = '{{0, 0}}'
);

SELECT array2_agg(t) FROM tbl_foo;
              array2_agg
---------------------------------------
 {{1,2},{5,2},{6,2},{1,7},{1,8},{1,9}}
(1 row)

I need that array2_agg_cutFirst function (simply cutting first '{0, 0}' subarray), because INITCOND = '{{}}' is not allowable.