General key issues when comparing performance between PostgreSQL and oracle

Hi all (Hopefully this is the correct mailing list for this).

I'm working on performance tuning a host of queries on PostgreSQL 9.2
from an application, each query having its own issues and fixes, however
from what I understand this application runs the exact same queries on
the exact same data in half the time on oracle and SQL server.

Are there any known differences between the database systems in terms of
query planners or general operations (sorting, unions) that are notable
different between the systems that would make postgres slow down when
executing the exact same queries?

It's worth noting that the queries are not that good, they have issues
with bad sub-selects, Cartesian products, and what looks like bad query
design in general, so the blame isn't completely with the database being
slow, but I wonder what makes oracle preform better when given
not-so-great queries?

I know this is rather general and high level, but any tips or experience
anyone has would be appreciated.

Thanks,
- Brian F