[sql]Joins

I am facing a dilemma at my work, I am using postgres first time. For
some reason, my co-workers think that in Postgres joins i.e.
simple joins of two or three tables are so bad that their cost is
exponential. They believe that postgres simply takes Cartesian product
of joining table in order to obtain desired tuples i.e. no optimization
is done.
I refused to believe so. I need your help to convince
them that it is okay to join tables --two or three tables :)-- so we can

eliminate redundancies from the database. I also want to know how
postgres optimizes a join query. Thank you very much your help.
Regards
Najm