Christophe Pettus <x...@thebuild.com> writes: >> On Apr 9, 2018, at 07:33, Thomas Poty <thomas.p...@gmail.com> wrote: >> ok, and long answer ? is it random?
> It's not literally random, but from the application point of view, it's not > predictable. For example, it's not always the one that opened first, or any > other consistent measure. It's whichever one runs the deadlock detector first after the circular wait becomes established. For instance: * Process A takes lock L1 * Process B takes lock L2 * Process A tries to take lock L2, blocks * Process B tries to take lock L1, blocks (now a deadlock exists) Process A will run the deadlock detector one deadlock_timeout after blocking. If that happens before B has blocked, then A will see no deadlock and will go back to waiting. In that case, when B's own deadlock_timeout expires and it runs the deadlock detector, it will see the deadlock and fix it by canceling its own wait. On the other hand, if B started to wait less than one deadlock_timeout after A did, then A will be first to observe the deadlock and it will cancel itself, not B. So you can't predict it unless you have a lot of knowledge about the timing of events. You could probably make it more predictable by making deadlock_timeout either very short or very long, but neither of those are desirable things to do. regards, tom lane