The deadlocking pids will be logged and then if you have/had query logging turned on you can see what the pids were executing prior to that. Tracing back to the

The deadlocking pids will be logged and then if you have/had query logging turned on you can see what the pids were executing prior to that. Tracing back to the application involves using the available logged information: client IP, client name (possibly just the default from the pq driver, so the same everywhere), content of the actual query.