If you're generating random UUIDs as the primary key, how do you not run into key collisions? Having to search the entire table before inserting is slow, and catching the error and trying again is also annoying.
And, it’s so random that if you ever do see a collision you should immediately start looking for a compromised system or bug. This is basically how GitHub discovered the OpenSSL bug that had removed too much entropy from the RNG setup.
Read and take notes. This is crazy in untrusted environments.
You do not need to search. I have generated hundreds of millions and have never hit a duplicate. Its technically possible, but its so vanishingly rare it will be something to brag about, not worry about. If you're really worried about it, and building something that cannot tolerate an error, put a try/catch around it and detect a PK failure and try again. But it will be a waste of time.
So if there's a collision you'll get an error. You can write application code to handle the error (by re-generating a new ID and re-trying). Or you can figure it's so unlikely (googling for uuidv4 says "1 in 2.71 x 1018", which is pretty huge; not sure for uuidv7 which will be somewhat more likely) that you aren't going to worry about it, and if an error happens, oh well, errors happen sometimes, depending on the domain you are in, which is probably what many apps do and do fine with it.
I would guess that pg's built in uuidv4-generating implementation might re-try on colision under-the-hood, but i haven't checked to see, and it may also just count on the improbability and raise an error if it happens!
So if you are using it as a PK value, only ever insert and if there is a duplicate, blow up loudly.
Performance wise, that isn’t particularly impactful IMO.