How to identify the source of a deadlock?

There is an application A doing some things in a database. In the middle
of the program, application B is called which does some other things.
Now for some reason application B hangs for certain inputs and I have to
find out the reason for this. The sequence is:

| A: BEGIN
| A: [does some things]
| A: [calls B]
|
| B: BEGIN
| B: [does some things]
| B: update bmeproduct set manufacturerpid='40913', leadtime='1' where idproduct=9681
| B: [waits forever]

When I look at pg_stat_activity, I can see the update statement with
"waiting == 't'" for process B, which would not change for at least
several hours. And I can see "<IDLE> in transaction" for process A
which, of course, I know by the nature of the program.

However, to eliminate the problem I would need the actual source of the
lock. Program A does not ever touch the table "bmeproduct" (it does only
things in a completely different part of the database - or at least: it
_should_ do), so the lock must be introduces via some foreign key. But:
is there any chance to find out WHICH table (or even record) is the
cause of the trouble? This would be so helpful...

Stefan

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