Re: UUID v7

Andrey M. Borodin wrote:

> I'm sending amendments addressing your review as a separate step in patch
> set. Step 1 of this patch set is identical to v39.

Some comments about the implementation of monotonicity:

+/*
+ * Get the current timestamp with nanosecond precision for UUID generation.
+ * The returned timestamp is ensured to be at least SUBMS_MINIMAL_STEP
greater
+ * than the previous returned timestamp (on this backend).
+ */
+static inline int64
+get_real_time_ns_ascending()
+{
+ static int64 previous_ns = 0;

[...]

+ /* Guarantee the minimal step advancement of the timestamp */
+ if (previous_ns + SUBMS_MINIMAL_STEP_NS >= ns)
+ ns = previous_ns + SUBMS_MINIMAL_STEP_NS;
+ previous_ns = ns;

In the case of parallel execution (uuidv7() being parallel-safe), if
there have been previous calls to uuidv7() in that backend,
previous_ns will be set in the backend process,
but zero in a newly spawned worker process.
If (previous_ns + SUBMS_MINIMAL_STEP_NS >= ns) ever happens
to be true in the main process, it will start at false in the workers,
leading to non-monotonic results within the same query.

Also in the case of a backward clock change, we can end up with some
backends sticking to the "old time" plus increment per invocation
until they die, while some other backends spawned after the clock
change are on the "new time". These backends may produce series of
UUIDv7 that would be completely out of sync with each others.
A backward clock change is an abnormality, but if it occurs, what's
the best choice? Take the bullet and switch to the new time , or
stick to a time that is permanently decorrelated from the OS
clock? I would think that the latter is worse.

Best regards,
--
Daniel Vérité
https://postgresql.verite.pro/
Twitter: @DanielVerite