Postgres UUIDv7 and per-back end monotonicity

The reason for this is simple: the documentation doesn't promise this property. Moreover, even if it did, the RFC for UUIDv7 doesn't promise this property. If you decide to depend on it, you're setting yourself up for a bad time when PostgreSQL decides to change their implementation strategy, or you move to a different database.

Further, the stated motivations for this, to slightly simplify testing code, are massively under-motivating. Saving a single line of code can hardly be said to be worth it, but even if it were, this is a problem far better solved by simply writing a function that will both generate the objects and sort them.

As a profession, I strongly feel we need to do a better job orienting ourselves to the reality that our code has a tendency to live for a long time, and we need to optimize not for "how quickly can I type it", but "what will this code cost over its lifetime".

The "RFC for UUIDv7", RFC 9562, explicitly mentions monotonicity in §6.2 ("Monotonicity and Counters"):

    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.

In the UUIDv7 definition (§5.7) it explicitly mentions the technique that Postgres employs for rand_a:

    rand_a:
        12 bits of pseudorandom data to provide uniqueness as per
        Section 6.9 and/or optional constructs to guarantee additional 
        monotonicity as per Section 6.2. Occupies bits 52 through 63 
        (octets 6-7).

Note: "optional constructs to guarantee additional monotonicity". Pg makes use of that option.

>optional constructs

So it is explicitly mentioned in the RFC as optional, and Pg doesn't state that they guaranty that option. The point still stands, depending on optional behavior is a recipe for failure when the option is no longer taken.

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. ...