CREATE OR REPLACE FUNCTION pension.qwerty(int4)
  RETURNS SETOF record AS
'
declare
  _year alias for $1;
  _result record;
begin
  if _year = 2003 then
    for _result in
      select p_certificate from pension.employee_donations
       where extract(year from date_donation) = 2003
       group by p_certificate
      having count(p_certificate) = 1
   intersect 
      select p_certificate from pension.employee_donations
       where date_donation = \'2003- 08 - 01 \'::date
       group by p_certificate
    loop
      return next _result;
    end loop;
  else
    for _result in
      select p_certificate from pension.pension_staff ps
       where exists(
         select p_certificate from pension.employee_donations
          where extract(year from date_donation) = _year
            and p_certificate = ps.p_certificate)
    loop
      return next _result;
    end loop;
  end if;
  return;
end;
'
  LANGUAGE 'plpgsql' VOLATILE;

select * from pension.qwerty( 2003 ) as (p_certificate varchar)

ERROR:  wrong record type supplied in RETURN NEXT
CONTEXT:  PL/pgSQL function "qwerty" line  16  at return next

RETURN NEXT does not actually return from the function;
it simply saves away the value of the expression (or record or row variable,
as appropriate for the data type being returned).

_result record
и
RETURNS SETOF record

_result pension.employee_donations%ROWTYPE
и
RETURNS SETOF pension.employee_donations

      select p_certificate from pension.pension_staff ps
       where exists(
         select p_certificate from pension.employee_donations
          where extract(year from date_donation) =  2003 
            and p_certificate = ps.p_certificate)
      except -- вычитаем
      select p_certificate
        from pension.employee_donations
       where extract(year from date_donation) =  2003 
       group by p_certificate
      having count(p_certificate) =  1 
   intersect 
      select p_certificate
        from pension.employee_donations
       where date_donation = '2003-08-01'::date
       group by p_certificate

_result pension.pension_staff%ROWTYPE
и
RETURNS SETOF pension.pension_staff

...
  RETURNS SETOF record AS
...
  _result record;
...
      return next _result;