Re: deadlock detected errors

Clarence Gardner <clarence(at)silcom(dot)com> writes:
> The difference I'm asking about is that my artificial ones always involve
> waiting on a transaction, while the ones from the real application always
> involve one transaction wait and one tuple wait.

> The first question is, is there any significance to this difference?

Yup.

> If so, the second question is, what is it? :) In looking at some code in
> heapam.c, I see the following comment that sorta kinda sounds like maybe
> there are three parties involved, with two of them waiting on a locked
> tuple?

Got it in one. The guy waiting on the tuple-specific lock is second in
line to actually mung the tuple. Whoever is first in line behind the
current tenant will be blocked trying to acquire ShareLock on the
current tenant's transaction ID.

What you appear to have is a situation where two transactions are trying
to lock or update the same two rows in different orders. Without a lot
more info about your application logic, I couldn't guess why this seems
to be associated with having more than two transaction interested in the
same tuple.

Note that the guy looking for ShareLock on the tuple is evidently either
doing SELECT FOR SHARE on this tuple, or trying to install a new tuple
referencing this one as a foreign key (which does SELECT FOR SHARE under
the hood). But he's blocked by someone who's done either SELECT FOR
UPDATE or an actual UPDATE on that tuple.

regards, tom lane