How to identify the transaction of a lock?

Hello.  I am having a problem with a locked table that causes postmaster
to hang on inserts.  I went to pg_locks and got:

transaction   6536720
pid 15992
mode ExclusiveLock
granted t

How do I go from this transaction identifier to finding out which
transaction caused it to lock, so I can resolve it in my code?

Thanks!

Si Chen