SQL> create table t as 
  2  select object_id, object_name from dba_objects where object_id is not null;

Table created.

SQL> alter table t add constraint t_pk primary key (object_id);

Table altered.

SQL> set autotrace traceonly explain
SQL> select * 
  2  from (select object_id from t order by object_id desc) 
  3  where rownum<=100
  4  /

Execution Plan
----------------------------------------------------------
Plan hash value: 330171726

-------------------------------------------------------------------------------------
| Id  | Operation                    | Name | Rows  | Bytes | Cost (%CPU)| Time     |
-------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT             |      |   100 |  1300 |     2   (0)| 00:00:01 |
|*  1 |  COUNT STOPKEY               |      |       |       |            |          |
|   2 |   VIEW                       |      | 36149 |   458K|     2   (0)| 00:00:01 |
|   3 |    INDEX FULL SCAN DESCENDING| T_PK | 36149 |   458K|     2   (0)| 00:00:01 |
-------------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

   1 - filter(ROWNUM<=100)