[SQL] Issues with lag command

I use the LAG function and the below code



CREATE OR REPLACE FUNCTION public.update_test_dates()
  RETURNS SETOF test AS
$BODY$
DECLARE

    r test%rowtype;
BEGIN
    FOR r IN SELECT * FROM test  order by id
  
   
    LOOP
        -- can do some processing here
        if r.time_id is  null
        then
        update test set time_id= (select lag(time_id) OVER (ORDER BY id) from test where id=r.id) where id=r.id;
        end if;
       
        RETURN NEXT r; -- return current row of SELECT
    END LOOP;
    RETURN;
END
$BODY$
  LANGUAGE plpgsql;

  select * from update_test_dates();