Oracle SQL Plan operation: WINDOW (NOSORT STOPKEY)

create table tq84_tab (
   num number,
   dat date,
   val varchar2(100)
);

--
-- Without the following index, the plan
-- operation WINDOW SORT PUSHED RANK would be used.
--
create index tab_ix on tq84_tab(num, dat);

explain plan for
select
   dat,
   val
from
   tq84_tab
where
   num = 42
order by
   dat
fetch first 2 row only;

select * from table(dbms_xplan.display(format=>'basic'));
--
-- --------------------------------------------------
-- | Id  | Operation                     | Name     |
-- --------------------------------------------------
-- |   0 | SELECT STATEMENT              |          |
-- |   1 |  VIEW                         |          |
-- |   2 |   WINDOW NOSORT STOPKEY       |          |
-- |   3 |    TABLE ACCESS BY INDEX ROWID| TQ84_TAB |
-- |   4 |     INDEX RANGE SCAN          | TAB_IX   |
-- --------------------------------------------------

drop table tq84_tab;