| From: | "Andrey M(dot) Borodin" <x4mmm(at)yandex-team(dot)ru> |
|---|---|
| To: | Aleksander Alekseev <aleksander(at)timescale(dot)com> |
| Cc: | pgsql-hackers mailing list <pgsql-hackers(at)postgresql(dot)org>, Nikolay Samokhvalov <nik(at)postgres(dot)ai>, "David G(dot) Johnston" <david(dot)g(dot)johnston(at)gmail(dot)com>, Przemysław Sztoch <przemyslaw(at)sztoch(dot)pl>, Sergey Prokhorenko <sergeyprokhorenko(at)yahoo(dot)com(dot)au>, Jelte Fennema-Nio <postgres(at)jeltef(dot)nl>, Nick Babadzhanian <pgnickb(at)gmail(dot)com>, Mat Arye <mat(at)timescaledb(dot)com>, Peter Eisentraut <peter(dot)eisentraut(at)enterprisedb(dot)com>, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, Daniel Gustafsson <daniel(at)yesql(dot)se>, Matthias van de Meent <boekewurm+postgres(at)gmail(dot)com>, Nikolay Samokhvalov <samokhvalov(at)gmail(dot)com>, "Kyzer Davis (kydavis)" <kydavis(at)cisco(dot)com>, Andres Freund <andres(at)anarazel(dot)de>, "brad(at)peabody(dot)io" <brad(at)peabody(dot)io>, Kirk Wolak <wolakk(at)gmail(dot)com> |
| Subject: | Re: UUID v7 |
| Date: | 2024-01-24 15:29:36 |
| Message-ID: | 54A6B150-7B2E-433C-8369-F1AEEF7D6F66@yandex-team.ru |
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| Lists: | pgsql-hackers |
> On 24 Jan 2024, at 18:29, Aleksander Alekseev <aleksander(at)timescale(dot)com> wrote:
>
> Hi,
>
>> Function to extract timestamp does not provide any guarantees at all. Standard states this, see Kyzer answers upthread.
>> Moreover, standard urges against relying on that if uuidX was generated before uuidY, then uuidX<uuid. The standard is doing a lot to make this happen, but does not guaranty that.
>> All what is guaranteed is the uniqueness at certain conditions.
>>
>>> Otherwise you can calculate crc64(X) or sha256(X)
>>> internally in order to generate an unique ID and claim that it's fine.
>>>
>>> Values that violate named invariants should be rejected with an error.
>>
>> Think about the value that you pass to uuid generation function as an entropy. It’s there to ensure uniqueness and promote ordering (but not guarantee).
>
> If the standard doesn't guarantee something it doesn't mean it forbids
> us to give stronger guarantees.
No, the standard makes these guarantees impossible.
If we insist that uuid_extract_time(uuidv7(time))==time, we won't be able to generate uuidv7 most of the time. uuidv7(now()) will always ERROR-out.
Standard implies more coarse-grained timestamp that we have.
Also, please not that uuidv7(time+1us) and uuidv7(time) will have the same internal timestamp, so despite time+1us > time, still second uuid will be greater.
Both invariants you proposed cannot be reasonably guaranteed. Upholding any of them greatly reduces usability of UUID v7.
Best regards, Andrey Borodin.
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