Oracle TOP N ordered rows

Question

I would like to get the top N rows from an Oracle table sorted by date.

The common way to do this, and this solution returns for every question I could find on SO/go

Answer 1

Question is, is there a way to limit the number of ORDERED rows returned in the subquery ?

The following is what I typically use for top-n type queries (pagination query in this case):

select * from (
  select a.*, rownum r
  from (
    select *
    from your_table
    where ...
    order by ...
  ) a
  where rownum <= :upperBound
)
where r >= :lowerBound;

I usually use an indexed column to sort in inner query, and the use of rownum means Oracle can use the count(stopkey) optimization. So, not necessarily going to do full table scan:

create table t3 as select * from all_objects;
alter table t3 add constraint t_pk primary key(object_id);
analyze table t3 compute statistics;

delete from plan_table;
commit;
explain plan for
select * from (
  select a.*, rownum r
  from (
    select object_id, object_name
    from t3
    order by object_id
  ) a
  where rownum <= 2000
)
where r >= 1;

select operation, options, object_name, id, parent_id, position, cost, cardinality, other_tag, optimizer
from plan_table
order by id;

You'll find Oracle does a full index scan using t_pk. Also note the use of stopkey option.

Hope that explains my answer ;)