Recovery/Rollback question

Hi,

I am new to the list and want to say "hello" first.
We are migrating to Postgres and therefore my question might be simple for
you.
We run several application databases under one db server. Now we are looking
for a mechanism to rollback unwanted user command without impact for other
databases. Is there a posiibilty to use something like WAL on tablespaces or
databases not only on db servers?
Does Postgres provide other utilities for this? Which strategies to you use?
Scenatio running databases a and b under the server.
It is now 10 o 'clock, user made a mistake at 8 o'clock in database a, so
we need to roll database a back to 8 o'clock, database b keeps current data
status.
How do you solve this under Postgres? Any links to useful docu chapters ?

Regards,
A.Stejskal