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It's mentioned in the RFC as being explicitly monotonic based the time-based design.

Implementations that need monotonicity beyond the resolution of a timestamp-- like when you allocate 30 UUIDs at one instant in a batch-- can optionally use those additional bits for that purpose.

> Implementations SHOULD employ the following methods for single-node UUID implementations that require batch UUID creation or are otherwise concerned about monotonicity with high-frequency UUID generation.

(And it goes on to recommend the obvious things you'd do: use a counter in those bits when assigning a batch; use more bits of time precision; etc.)

The comment in PostgreSQL before the implementation makes it clear that they chose the third option for this in the RFC:

     * variant bits. To ensure monotonicity in scenarios of high-
     * frequency UUID generation, we employ the method "Replace
     * LeftmostRandom Bits with Increased Clock Precision (Method 3)",
     * described in the RFC. ...

> It's mentioned in the RFC as being explicitly monotonic based the time-based design.

It's explicitly partially monotonic.

Or as other people would call it, "not monotonic".

People are talking past each other based on their use of the word "monotonic".

It's explicitly monotonic, except for apps that have a very fast ID rate, in which case there are recommended approaches (the word "SHOULD" is used in an RFC) to make it work. And PostgreSQL used one of these recommended approaches and documented it.

> It's explicitly monotonic, except for apps that have a very fast ID rate

"might generate two IDs in the same millisecond" is not a very exotic occurrence. It makes a big difference whether the rest is guaranteed or not.

> And PostgreSQL used one of these recommended approaches and documented it.

Well that's the center of the issue, right? OP's interpretation was that PostgreSQL did not document such, and so it shouldn't be relied upon. If it is a documented promise, then things are fine.

But is it actually in the documentation? A source code comment saying it uses a certain method isn't a promise that it will always use that method.

> Well that’s the center of the issue, right? OP’s interpretation was that PostgreSQL did not document such, and so it shouldn’t be relied upon.

And the correct answer is…we don’t know. We have a commit that landed and the explanation of the commit; we don’t have the documentation for the corresponding release of Postgres, because…it doesn’t exist yet. Because monoticity is an important feature for UUIDv7, it would be very odd if Postgres used an implementation that took the extra effort to use a nanosecond-level time value as the high-order portion of the variant part of the UUID instead of the minimum millisecond-level time, but not document that, but any assumption about what will be the documented, reliable-going-forward advertised feature is speculative until the documentation exists and is finalized.

OTOH, its perfectly fine to talk about what the implementation allows now, because that kind of thing is important to the decision about what should be documented and committed to going forward.

The point of the extra bits is to allow the application developer to keep monotonicity in the "not very exotic occurrence" scenario. The purpose is to be monotonic. I feel like you are missing the core concept.

I'm not missing anything, the problem is a lot of people using sloppy wording or mixing up the two modes.

This comment thread is about the guaranteed level of monotonicity. Yes, those bits exist. But you can't depend on them from something that only promises "UUIDv7". You need an additional promise that it's configured that way and actually using those bits to maintain monotonicity.

> It's explicitly partially monotonic.

From the RFC:

    Monotonicity (each subsequent value being greater than the last) is 
    the backbone of time-based sortable UUIDs. Normally, time-based UUIDs 
    from this document will be monotonic due to an embedded timestamp

"time-based UUIDs from this document will be monotonic". "will be monotonic".

I'm not sure how much more explicit this can be made.

The intent of UUIDv7 is monotonicity. If an implementation is not monotonic then it's a bug in the implementation.

> So it is explicitly mentioned in the RFC as optional […]

The use of rand_a for extra monotonicity is optional. The monotonicity itself is not optional.

§5.7 states:

    Alternatively, implementations MAY fill the 74 bits, 
    jointly, with a combination of the following subfields, 
    in this order from the most significant bits to the least, 
    to guarantee additional monotonicity within a millisecond:

Guaranteeing additional monotonicity means that there is already a 'base' level of monotonicity, and there are provisions for even more ("additional") levels of it. This 'base level' is why §6.2 states:

    Monotonicity (each subsequent value being greater than the last) is 
    the backbone of time-based sortable UUIDs. Normally, time-based UUIDs 
    from this document will be monotonic due to an embedded timestamp; 
    however, implementations can guarantee additional monotonicity via 
    the concepts covered in this section.

"Backbone of time-based sortable UUIDs"; "additional monotonicity". Additional: adding to what's already there.

* https://datatracker.ietf.org/doc/html/rfc9562

"this monotonicity" that OP suggests people not use is specifically the additional monotonicity.

Or to put it another way: OP is suggesting you don't depend on it being properly monotonic, because the default is that it is only partially monotonic.

> Normally, time-based UUIDs from this document will be monotonic due to an embedded timestamp; however, implementations can guarantee additional monotonicity via the concepts covered in this section.

“Normally, I am at home because I do not have a reason to go out; however, sometimes I am at home because I am sleeping.”

Notice how this statement does not actually mean that I am always at home.

I was recently bit doing a Postgres upgrade by the Postgres team considering statements like `select 1 group by true` fine to silently break in Postgres 15. See https://postgrespro.com/list/thread-id/2661353 - and this behavior remains undocumented in https://www.postgresql.org/docs/release/ . It's an absolutely incredible project, and I don't disagree with the decision to classify it as wontfix - but it's an anecdote to not rely on undefined behavior!

The "optional" portion is this part of the spec, not the time part.

> implementations can guarantee additional monotonicity via the concepts covered in this section

The “time part” is actually two different parts: the required millisecond-level ordering and the optional use of part of rand_a (which postgres does) to provide higher-resolution (nanosecond, in the postgres case) time ordering when combined with the required portion.

So, no, the “time part” of the postgres implementation is, in part, one of the options discussed in the spec, not merely the “time part” required in the spec.

Relying on an explicitly documented implementation behavior that the specification explicitly describes as an option is not an issue. Especially if the behavior is only relied on in a test, where the worst outcome is a failed testcase that is easily fixed.

Even if the behavior went away, UUIDs unlike serials can always be safely generated directly by the application just as well as they can be generated by the database.

Going straight for that would arguably be the "better" path, and allows mocking PRNG to get sequential IDs.

Software is arbitrary. Any so-called "guarantee" is only as good as the developers and organizations maintaining a piece of software want to make it regardless of prior statements. At some point, the practical likelihood of a documented, but not guaranteed, process being violated vs. the willful abandonment of a guarantee start to look very similar.... at which point nothing saves you.

Sometimes the best you can do is recognize who you're working with today, know how they work, and be prepared for those people to be different in the future (or of a different mind) and for things to change regardless to expressed guarantees.

....unless we're talking about the laws of physics... ...that's different...