If it's indexed in a unique index, as a primary key certainly would be, the DB is of course already checking for collision on insert, so there's no reason for a

If it's indexed in a unique index, as a primary key certainly would be, the DB is of course already checking for collision on insert, so there's no reason for application code to do it. (If it did, it would be an indexed lookup rather than a table scan). 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!