Thank all very much for great suggestions.
I created function
CREATE OR REPLACE FUNCTION PUBLIC.GOMONTH(DATE, INTEGER, OUT DATE) IMMUTABLE
AS
$_$
SELECT ($1 + ($2 * '1 MONTH'::INTERVAL))::DATE;
$_$ LANGUAGE SQL;
I got errors:
function gomonth(date, numeric) does not exist
and
function gomonth(date, bigint ) does not exist
How to fix those errors ?
Andrus.