Comment by toraway

You could do that, but now you're like 90% of the way to maintaining a monotonically increasing number you that could just use as a unique ID instead without any randomness required (and without the additional 128 bits for collision protection via the appended UUID).

So your ID would take like 64 bits for the time unique to the nanosecond plus 128 bits for the UUIDv4 = 192 bits which is a pretty beefy sized ID.

(I know you said just append a second count but you will want a predictable/fixed size for your data structure in pretty much any use case so need to decide the upper bound and precision ahead of time)

Especially when the alternative is a 128 bit UUIDv4 that's guaranteed unique with proper usage of high quality RNG or a 128 bit UUIDv7 if you have a clock (that's needed for your method anyway) that will be much more forgiving of a flaky source of randomness and more sortable than your monotonic-ish ID for 1/3 fewer bits.

Basically, stapling anything onto a UUID is a waste of space if you don't trust it, so might as well drop it completely and use a significantly smaller source of randomness at that point.