> They are awful as keys – being strings, comparisons are dramatically slower than with integers. And even if your database has a UUID type, it’s still worse because the identifier doesn’t fit into a machine word.

I’m just a bit confused, a UUID is made up of hexadecimal digits, so why would it be stored as a string? It’s also 128 bits long, so it should fit into two words, excluding whatever overhead the DBMS puts on the data type, which is really their problem to worry about.