Question

I want to retrieve the last 20 rows that have been updated from a table in an Oracle database without using order by
then rownum function like

select * 
  from EMP 
 order by last_Updated 
 where rownum < 20

This query takes lot of time.

Answer 1

The query you posted doesn’t do what you think it does. That query will fetch 20 arbitrary rows and then order by the last_updated column. If you want the 20 most recent rows, you’d need to ORDER BY last_updated in a nested query and then apply the rownum predicate in an outer query.

SELECT *
  FROM (SELECT *
          FROM emp
         ORDER BY last_updated desc)
 WHERE rownum <= 20

Assuming that last_updated is indexed, that should be a very efficient query. You should be able to do a min/max scan on the last_updated index and then do a count stopkey to fetch only the first 20 rows. If you are having problems with the performance of this query, what sort of query plan are you getting?