CREATE OR REPLACE function test_func(id int)
 RETURNS TABLE(results_q int4,flag character varying)
 LANGUAGE plpgsql
AS $function$
declare
    act_res int:= null;

begin
 with tab as (
     select 4/id  as res 
     where id = 2
    )

    select res into act_res from tab;

    if (act_res is not null)
    then
    return query 
    select res,
    case when res = 2 then 'Y'::varchar  end as flag
    from tab
    ;
    else 
  return query  with tab2 as (
     select 4+id  as res 
     )  
     select res,
     'N'::varchar as flag
    from tab2;

end if;
end;
$function$
;
select * from test_func(2)
;
;