Re: What is a typical precision of gettimeofday()?

Peter Eisentraut <peter@eisentraut.org>

From: Peter Eisentraut <peter@eisentraut.org>

To: Aleksander Alekseev <aleksander@timescale.com>, pgsql-hackers <pgsql-hackers@postgresql.org>

Cc: "Andrey M. Borodin" <x4mmm@yandex-team.ru>

Date: 2024-03-20T06:35:22Z

Lists: pgsql-hackers

Commits

Same data as JSON: GET /api/v1/messages/:b64id/commits the thread's linked commits as JSON, with link sources. API reference →

Force LC_NUMERIC to C while running TAP tests.
- f25792c541e5 19 (unreleased) landed
Minor tweaks for pg_test_timing.
- 9dcc7641444f 19 (unreleased) landed
Change pg_test_timing to measure in nanoseconds not microseconds.
- 0b096e379e6f 19 (unreleased) landed

On 19.03.24 10:38, Aleksander Alekseev wrote:
> Considering the number of environments PostgreSQL can run in (OS +
> hardware + virtualization technologies) and the fact that
> hardware/software changes I doubt that it's realistic to expect any
> particular guarantees from gettimeofday() in the general case.

If we want to be robust without any guarantees from gettimeofday(), then 
arguably gettimeofday() is not the right underlying function to use for 
UUIDv7.  I'm not arguing that, I think we can assume some reasonable 
baseline for what gettimeofday() produces.  But it would be good to get 
some information about what that might be.

Btw., here is util-linux saying

     /* Assume that the gettimeofday() has microsecond granularity */

https://github.com/util-linux/util-linux/blob/master/libuuid/src/gen_uuid.c#L232