 * This code looks for deadlocks involving the given process.  If any
 * are found, it tries to rearrange lock wait queues to resolve the
 * deadlock.  If resolution is impossible, return DS_HARD_DEADLOCK ---
 * the caller is then expected to abort the given proc's transaction.
…
DeadLockState
DeadLockCheck(PGPROC *proc)
{
…
	/* Search for deadlocks and possible fixes */
	if (DeadLockCheckRecurse(proc))
	{
		/*
		 * Call FindLockCycle one more time, to record the correct
		 * deadlockDetails[] for the basic state with no rearrangements.
		 */
		int			nSoftEdges;

		TRACE_POSTGRESQL_DEADLOCK_FOUND();

		nWaitOrders = 0;
		if (!FindLockCycle(proc, possibleConstraints, &nSoftEdges))
			elog(FATAL, "deadlock seems to have disappeared");

		return DS_HARD_DEADLOCK;	/* cannot find a non-deadlocked state */
	}
…
	/* Return code tells caller if we had to escape a deadlock or not */
	if (nWaitOrders > 0)
		return DS_SOFT_DEADLOCK;
	else if (blocking_autovacuum_proc != NULL)
		return DS_BLOCKED_BY_AUTOVACUUM;
	else
		return DS_NO_DEADLOCK;
}
…
	ereport(ERROR,
			(errcode(ERRCODE_T_R_DEADLOCK_DETECTED),
			 errmsg("deadlock detected"),
			 errdetail_internal("%s", clientbuf.data),
			 errdetail_log("%s", logbuf.data),
			 errhint("See server log for query details.")));

Warm-up