How to delete duplicate rows?

This one must be obvious for most here.

I have a 170 million rows table from which I want to eliminate
duplicate "would be" keys and leave only uniques.

I found a query in http://www.jlcomp.demon.co.uk/faq/duplicates.html
for the oracle database but can't figure out how to refer to the row
id in postgresql:

delete from test where rowid not in
(select min(rowid) from test group by a,b);

How to refer to the row id? Any better way to do it?

Regards, Clodoaldo Pinto