I am going to use them as primary key of the table, so I'll surely need them unique :)
Eduoardo, I REALLY suggest you don't use them at all. You should make a primary key like this:
CREATE TABLE blah ( id SERIAL PRIMARY KEY, ... );
Also note that by default, OIDs are NOT dumped by pg_dump. You will need to add extra switches to your pg_dump backup to ensure that they are.
Chris
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