How to troubleshoot a halted postgres 8.3 ?

Hi,

I have a postgres 8.3 db in which I execute a transaction with a lot of alter tabel etc, update etc statements in a single transaction.
I also execute several select statements in parallel.
This puts my database in a state which I would describe as halted, i.e. the sql interface does not respond any more, pg_admin works and shows me several transactions running.

How do I proceed to analyze what's happened? Of course, my first thought is some kind of transaction deadlock, but how do I check for this ?

In the status section of pg_admin, I can see a total of 6 transactions running, 3 in idle mode, the other 3 doing some kind of select. The lock section has several hundred lines listed, but they don't tell me what exactly is locked.

So, please give me some general idea on how to proceed with such a situation.

Thanks,

Stefan