Re: UUID v7

On Thu, Nov 7, 2024 at 12:34 AM Andrey M. Borodin <x4mmm@yandex-team.ru> wrote:
>
>
>
> > On 7 Nov 2024, at 12:42, Masahiko Sawada <sawada.mshk@gmail.com> wrote:
> >
> > On Wed, Nov 6, 2024 at 10:14 AM Andrey M. Borodin <x4mmm@yandex-team.ru> wrote:
> >>
> >>
> >>
> >>> On 5 Nov 2024, at 23:56, Andrey M. Borodin <x4mmm@yandex-team.ru> wrote:
> >>>
> >>> <v30-0001-Implement-UUID-v7.patch>
> >>
> >> Some more thoughts on this patch version:
> >>
> >> 0. Comment mentioning nanoseconds, while we do not need to carry anything
> >> /* Convert TimestampTz back and carry nanoseconds. */
> >>
> >> 1. There's unnecessary &3 in
> >> uuid->data[7] = uuid->data[7] | ((uuid->data[8] >> 6) & 3);
> >>
> >> 2. Currently we store 0..999 microseconds in 10 bits, so values 1000..1023 are unused. We could use them for
overflow.That would slightly increase non-overflowing capacity when generating more than million UUIDs per second on
onebackend. However, given current performance of our CSPRNG I do not think this feature worth code complexity. 
> >>
> >
> > While using only 10 bits microseconds makes the implementation simple,
> > I'm not sure if 10 bits is enough to generate UUIDs at microsecond
> > granularity without losing monotonicity. Since 10-bit microseconds are
> > used as is in rand_a space, 1000 UUIDs can be generated per
> > millisecond without losing monotonicity.
>
> We won’t loose monotonicity on one backend. We will just accumulate time shift.
> See
> +       us = tv.tv_sec * SECS_PER_DAY * USECS_PER_SEC + tv.tv_usec;
> +       if (previous_us >= us)
> +               us = previous_us + 1;

IIUC the microsecond part is working also as a counter in a sense. It
seems fine to me but I'm slightly concerned that there is no guidance
of such implementation in RFC 9562.

> BTW if we just continue to use nanoseconds patch, zero bits will act exactly as counters.

Yes, but we will lose some randomness on macOS as the nanosecond part
is 0 in most cases.

Regards,

--
Masahiko Sawada
Amazon Web Services: https://aws.amazon.com