-- concat function (plpgsql) postgresql 8.0
  CREATE FUNCTION concat (text, text) RETURNS text AS $$
    DECLARE
      t text;
    BEGIN
      IF character_length($1) > 0 THEN
        t = $1 ||', '|| $2;
      ELSE
        t = $2;
      END IF;
      RETURN t;
    END;
  $$ LANGUAGE plpgsql;

-- aggregate function pegar
  CREATE AGGREGATE pegar (
    sfunc = concat,
    basetype = text,
    stype = text,
    initcond = ''
  );

-- usage example
  SELECT pais.pais, pegar(ciudad) FROM ciudad JOIN pais ON ciudad.idpais=pais.idpais GROUP BY pais.pais;