CREATE TABLE Sales(
    DATE_            date,
    ProdID_         integer,
    OrderID_            integer,
    ManagerID_      integer,
    Sum_            integer
);
DROP FUNCTION testfunc(integer, date);
insert into Sales VALUES (make_date(2019, 2, 4), 1, 1, 1, 63000);

CREATE OR REPLACE FUNCTION testfunc(ID_ integer, select_month_ date) RETURNS TABLE (
month_ date,
ManagerID_ integer,
OrderID_ integer,
Sum_ integer
) AS $$

begin
    RETURN QUERY select ManagerID_ from Sales where orderID = 1;
end;
$$ LANGUAGE plpgsql;


DO $$
declare
    d date:= make_date(2020, 1, 1);
BEGIN
WHILE d < make_date(2020, 1, 1)
    LOOP
        PERFORM "testfunc"(3, d);
        d:= d + interval '1 month';
    END LOOP;
END $$;


Drop Table Sales;