create function korektaCenyNalisnika(idnalesnika int) returns void as
$$
declare
float avg;
begin
  create temp table times as select id_bazowego_nalesnika as id_nalesnika,count(*) as sztuk from kompozycje group by id_nalesnika;

  select avg(sztuk) into avg from times;

  update bazowe_nalesniki b
      set koszt = case
              when avg<sztuk  then koszt*1.05
              else koszt *0.95 end

      from times t
      where t.id_nalesnika = b.id_nalesnika and b.id_nalesnika=idnalesnika;

  drop table times;
end;
$$
language plpgsql;