[gnaritas]

> most UUIDs are going to be non-matches in their lower bits, assuming a random distribution

But you can't assume a random distribution because anyone using UUID's for database keys isn't using random UUID's, they're using sequential ones.

[jsmthrowaway]

Which would still make the low-order quadword not match.

[gnaritas]

It's a database though, you don't get to control how comparisons are done. Guid's are larger than ints, so using them as keys or joining on them is going to be slightly slower (not that it makes them bad, I'm not against GUID's). Just as using a bigint will be slower than an int.

[jsmthrowaway]

If I were implementing the uuid type in Postgres and did anything other than `if(unlikely(LO(uuid1) == LO(uuid2)) && unlikely(HI(uuid1) == HI(uuid2)))` I'd consider myself doing the wrong thing, personally. Two 64-bit compares, two flag checks, and a branch, which is about as fast as it's going to get in the absence of a 128-bit architecture.

You could actually vectorize the compares in SSE, I believe (but I stopped writing assembly before SSE existed and don't know much about it at a low level). This is why it's so critical to treat a UUID as an integer instead of a string, and I see a lot of code that does the latter.

[gnaritas]

While all of that is true, it's really not relevant to a programmer deciding on which to use as a key; he has no control over any of those things.