Depending on your configuration and needs, using a method to generate primary keys in a way unique to each system might still yield better performance. For example, in MySQL you can use auto_increment_increment and auto_increment_offset to force one server use insert using even integers and one odd, or any combination you need. It does eat bits from the key size though. Changing it later can be a pain, but it possible if you think it through. I don't doubt something like this is possible in Postgres as well.