This is a group_concat(int) replacement for PostgreSQL.

group_concat.sql

CREATE OR REPLACE FUNCTION group_concat_iterate(_state INTEGER[], _value INTEGER)
  RETURNS INTEGER[] AS
$BODY$
  SELECT
    CASE
      WHEN $1 IS NULL THEN ARRAY[$2]
      ELSE $1 || $2
  END
$BODY$
  LANGUAGE SQL VOLATILE;

CREATE OR REPLACE FUNCTION group_concat_finish(_state INTEGER[])
  RETURNS text AS
$BODY$
    SELECT array_to_string($1, ',')
$BODY$
  LANGUAGE SQL VOLATILE;

CREATE AGGREGATE group_concat(int) (SFUNC = group_concat_iterate, STYPE = INTEGER[], FINALFUNC = group_concat_finish);

Adapted from Explain Extended. Here we want to concatenate integers.

Here is a better solution (Not mine!)
https://stackoverflow.com/a/47638417/243233

CREATE OR REPLACE FUNCTION _group_concat_finalize(anyarray)
RETURNS text AS $$
SELECT array_to_string($1,',')
$$ IMMUTABLE LANGUAGE SQL;

CREATE AGGREGATE group_concat(anyelement) (
SFUNC=array_append,
STYPE=anyarray,
FFUNC=_group_concat_finalize,
INITCOND='{}'
);

SELECT group_concat(x) FROM foo;