Database Features Questions,

I've never used PostgreSQL before, and I have some questions I couldn't
locate the answers to in any of the postgresql manuals I've found.
Forgive me if they've been asked a dozen times before.

Can I create nested transactions?
EG: Can I create transaction1
do some work
create transaction2
do some work
commit/or/rollback transaction2
do some work
commit/or/rollback transaction1

If not, how can I simulate this?

Can I create transactions which other connections can access to?
Server 1 starts Transaction named "WebRequest-1234"
Server 1 does work in transaction "WebRequest-1234"
Server 2 does work in transaction "WebRequest-1234"
Server 2 signals completion to Server 1
Server 1 commits/or/rollsback transaction "WebRequest-1234"

Another example of this would be a multithreaded database application
trying to achieve parallelism in reporting inside the context of the
uncommitted changes in a transaction:

Thread 1 starts Transaction "Transaction-1234"
Thread 1 does some work
Thread 1 dispatches Threads 2,3,4
Thread 2 does some heavy reporting inside context of
Transaction-1234
Thread 3 does some heavy reporting inside context of
Transaction-1234
Thread 4 does some heavy reporting inside context of
Transaction-1234
Thread 1 waits for threads to complete
Thread 1 commits/rollsback Transaction-1234

In this case they could should share a single DBConnection, but that
would not allow any parallelism in the DB.

If this is not possible, how I can simulate this?

I see that to create unique identifiers for a column, I can use the OID
value or a sequence. If I use a sequence, is there any way of having
the sequence value returned after an insert, instead of the OID? Or for
every insert I do, do I need to do two requests -- One to insert, and
one to do a lookup by OID to fetch the sequence number of the row I just
inserted?

Semirelated question, what is the maximum bound of the OID value? If I
fetch this value using JDBC, what type does it come back as? And what
happens when that overflows?

Another question is, if postgre does not support these features, does
Interbase, Oracle or another DB support these types of features?

Thanks,
-Joe