Re: UUID v7

On 30.01.24 14:35, Andrey M. Borodin wrote:
>> On 30 Jan 2024, at 15:33, Junwang Zhao <zhjwpku@gmail.com> wrote:
>>
>> It's always good to add a newline at the end of a  source file, though
>> this might be nitpicky.
> 
> Thanks, also fixed warning found by CFBot.

I have various comments on this patch:


- doc/src/sgml/func.sgml

The documentation of the new functions should be broken up a bit.
It's all one paragraph now.  At least make it several paragraphs, or
possibly tables or something else.

Avoid listing the functions twice: Once before the description and
then again in the description.  That's just going to get out of date.
The first listing is not necessary, I think.

The return values in the documentation should use the public-facing
type names, like "timestamp with time zone" and "smallint".

The descriptions of the UUID generation functions use handwavy
language in their descriptions, like "It provides much better data
locality" or "unacceptable security or business intelligence
implications", which isn't useful.  Either we cut that all out and
just say, it creates a UUIDv7, done, look elsewhere for more
information, or we provide some more concretely useful details.

We shouldn't label a link as "IETF standard" when it's actually a
draft.


- src/include/catalog/pg_proc.dat

The description of uuidv4 should be "generate UUID version 4", so that
it parallels uuidv7.

The description of uuid_extract_time says 'extract timestamp from UUID
version 7', the implementation is not limited to version 7.

I think uuid_extract_time should be named uuid_extract_timestamp,
because it extracts a timestamp, not a time.

The functions uuid_extract_ver and uuid_extract_var could be named
uuid_extract_version and uuid_extract_variant.  Otherwise, it's hard
to tell them apart, with only one letter different.


- src/test/regress/sql/uuid.sql

Why are the tests using the input format '{...}', which is not the
standard one?


- src/backend/utils/adt/uuid.c

All this new code should have more comments.  There is a lot of bit
twiddling going on, and I suppose one is expected to follow along in
the RFC?  At least each function should have a header comment, so one
doesn't have to check in pg_proc.dat what it's supposed to do.

I'm suspicious that these functions all appear to return null for
erroneous input, rather than raising errors.  I think at least some
explanation for this should be recorded somewhere.

I think the behavior of uuid_extract_var(iant) is wrong.  The code
takes just two bits to return, but the draft document is quite clear
that the variant is 4 bits (see Table 1).

The uuidv7 function could really use a header comment that explains
the choices that were made.  The RFC draft provides various options
that implementations could use; we should describe which ones we
chose.

I would have expected that, since gettimeofday() provides microsecond
precision, we'd put the extra precision into "rand_a" as per Section 6.2 
method 3.

You use some kind of counter, but could you explain which method that
counter implements?

I don't see any acknowledgment of issues relating to concurrency or
restarts.  Like, how do we prevent duplicates being generated by
concurrent sessions or between restarts?  Maybe the counter or random
stuff does that, but it's not explained.