A question: why include a timestamp? Why not just 80 or 128 bits of pure randomness---if somebody needed a timestamp they could always just append it themselves. And, it seems to me, there would be many instances where you didn't need or want the timestamp.
It seem that, they want it to be sortable, like the title says. Due to that, it is helpful to include a timestamp. (If you do want a random identifier instead, then you can use that instead, I suppose.)
Yeah, but timestamp is only one of the keys you might like to sort on. To sort on anything else you'd have to augment the UUID anyways....why is a timestamp so important that it is put in every UUID, whether the user needs it or not?
Clustering. Without which indexes would be less efficient.
Thx. That's a good answer.
Unless you have a single monotonic clock, these will be partially ordered. It's risky to design around sorting that won't always work.