Postgresql and Top N Analysis?

Hello all,

Can anyone give me an hit how to do Top 'N' Analysis in postgresql (if it is 
possible)?
I see that postgresql does not have an pseudocolumn rownum, therefore I 
can't use query like this:

SELECT   *
FROM     (SELECT DISTINCT session_type_id FROM user_sessions ORDER BY 
session_type_id) as dummy
GROUP BY dummy;

Thank you for your time.

Daniel

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