PL/SQL FETCH NEXT or rownum

Question

Not much to elaborate, what should be used? they looks to perform the same task.

rownum?

select * from IDENTIFIKATOR
  where rownum <= 10;

or fetch next?

select * from IDENTIFIKATOR
  FETCH NEXT 10 ROWS ONLY;

Upon further expection rownum, might on the surface look to be faster

select * from (select * from IDENTIFIKATOR order by IDENTIFIKATORID )
  where rownum <= 10;  

has a cardinality of 10 and a cost of 10 (plain execution time of 0.15-0.2s (asc vs desc))

select * from IDENTIFIKATOR 
order by IDENTIFIKATORID 
FETCH NEXT 10 ROWS ONLY;

has a cardinality of 10 and a cost of 158869 (plain execution time of 1.9s-2s(asc vs desc))

Answer 1

If you are using Oracle 12c+, then I would suggest that you get used to the FETCH clause. It is ANSI-standard syntax and available in other databases. Plus, it is more flexible. For instance, you can do:

select i.*
from IDENTIFIKATOR i
order by col2 desc
fetch first 10 rows only;

And, it supports the OFFSET modifier.

Doing this using rownum requires a subquery.

On the other hand, if your code needs to work with earlier versions of Oracle, then rownum is basically your only choice.