If that concatenation is too long you can ram anything thru a hash to get a constant length smoothly distributed key. Its also fun to use "weak" hashes for this because it trolls wanna be security types who don't understand the application.

Given the above, some important things concatenated and hashed works. Can always add an application ID or importer ID or process ID or a timestamp of high enough resolution.

Obviously needs are different if you're trying to create a bank user database vs deduplicating sampled engineering data.

So something that's unique is NOW() (assuming low enough sample rate LOL) and something that never changes is a laser serial number, concatenate those and ram thru a hash to make it small and fit.

Once you get into application ids / timestamps / whatever you're no longer really using a natural key in my mind, you're just making your own algorithm for a surrogate key.

That sounds very similar to UUID version 1 except that you replaced MAC address with some other identifier and added a hashing step.

No, for a good primary key, the "unique" parts can be due to concatenation, but the whole key (and thus all the elements) needs to be unchanging. [0]

[0] Modern databases can actually deal with changeable PKs, though with a potentially serious performance hit, but in many use cases where data travels outside the database for some kind of interaction where the results need to get reentered into the database, this is still a problem, since you can't (in any general way) cascade updates to things (which may not even be online systems, e.g., paper records feeding human processes) outside of the DB.

We can all invent algorithms for nice unique values when we include the caveat "apart from the edge cases". It's those edge cases that bugger everything up.