Does using "fetch first N rows only" make /*+ FIRST_ROWS([N]) */ hint redundant?

Question

I know the FIRST_ROWS hint tells the optimizer to minimize time to first row.  I know the new 12c functionality for "FETCH [FIRST|NEXT] [N] ROWS [ONLY|WITH TIES]"fetch first/next N rows only/with ties" will implement the query using ROW_NUMBER().  Should I leave hint in case it improves performance, or does the FETCH FIRST clause make this hint redundant?

Answer 1

maybe the answer to this question is more complicated then I assumed at first: as Randolf Geist wrote in his presentation Everything You Wanted To Know About FIRST_ROWS_n But Were Afraid To Ask a rownum predicate implicitly defines a first rows n optimization. So I guessed that the fetch first clause would trigger a similar optimization. But a rownum predicate and a fetch first clause are not transformed into the same operation:
create table t
as
select rownum id
  from dual
connect by level <= 1000000;
select id
  from t
where rownum <= 5;
   ID
    1
    2
    3
    4
    5
| Id  | Operation          | Name | Rows  | Bytes | Cost (%CPU)| Time     |
|   0 | SELECT STATEMENT   |      |     5 |    25 |     2   (0)| 00:00:01 |
|*  1 |  COUNT STOPKEY     |      |       |       |            |          |
|   2 |   TABLE ACCESS FULL| T    |     5 |    25 |     2   (0)| 00:00:01 |
Predicate Information (identified by operation id):
   1 - filter(ROWNUM<=5)
select id
  from t
fetch first 5 rows only;
   ID
    1
    2
    3
    4
    5
| Id  | Operation              | Name | Rows  | Bytes | Cost (%CPU)| Time     |
|   0 | SELECT STATEMENT       |      |  1000K|    24M|   427   (1)| 00:00:01 |
|*  1 |  VIEW                  |      |  1000K|    24M|   427   (1)| 00:00:01 |
|*  2 |   WINDOW NOSORT STOPKEY|      |  1000K|  4882K|   427   (1)| 00:00:01 |
|   3 |    TABLE ACCESS FULL   | T    |  1000K|  4882K|   427   (1)| 00:00:01 |
Predicate Information (identified by operation id):
   1 - filter("from$_subquery$_002"."rowlimit_$$_rownumber"<=5)
   2 - filter(ROW_NUMBER() OVER ( ORDER BY  NULL )<=5)
Looking at a CBO trace for the two operations I see signs of a first rows optimization only for the query with the rownum predicate:
Final query after transformations:******* UNPARSED QUERY IS *******
SELECT "T"."ID" "ID" FROM "C##TEST"."T" "T" WHERE ROWNUM<=5
First K Rows: Setup begin
And so I think there is no first rows optimization for the fetch first clause. Of course the analytics would make an optimization for fast response almost impossible if there is an order in the query since in this case the analytics have to read the complete set before the filtering takes place.
Regards
Martin Preiss