Not related to the main question, but that query could use a little adjustment. Something like:
WITH x AS (
select kelt from javaink_forgalma
where en_kaptam is true and az_aru_neve = 'nyugdíjam'
order by kelt desc limit 2
)
,y AS (select min(kelt) from x)
,z AS (select max(kelt) from x)
INSERT INTO public.havonkenti_megtakaritasaink (ezen_idokozben, megtakaritva_rsd, kelt)
VALUES (
(select tsrange(min,max,'[)') FROM y,z)
, (select sum(egysegar * ennyi_egyseg) from javaink_forgalma, y, z
where a_penzem = 'RSD' and kelt >= min and kelt < max)
, DEFAULT
);
Also, it's best to use "timestamp with time zone" NOT "timestamp without time zone" whenever possible (i.e. timestamptz not timestamp)
Cheers,
Greg
