Does the prefix ("user_") get recorded in the DB (so every string in the column starts with the same "user_"), or does are there constraints and other clever chicanery to save those bytes in every record? Or do modern DB engines even care? Is this premature optimization?
This is more optimal for Postgres while making it slightly more difficult to interop between the db and the language (db driver needs to handle custom types, and you need to inject a custom type converter).
And while there are hacks you can do to make storing uuid-alikes as strings less terrible for db engines, if you want the best performance and smallest space consumption (compressed or not) make sure to use native ID types or convert to BINARY/numeric types.
In my experience, using just uuid as a pkey in Postgres already causes noticeable slowdowns vs the typical bigint. I wouldn't jump into anything other than bigint pkeys unless I'm solving an existing problem and have benchmarks to prove it.
