People should have a look as k-sortable unique identifiers (KSUID). Binary they are represented by 20 bytes and their string representation has 27 characters, which is shorter than UUIDs since the use a base62 encoding. They are sortable since the 20 bytes start with a 32 bit UNIX timestamp followed by random 128 bits. They should be very efficient for clustered indexes / B+-Trees.

Note also that as long as you have a single central database you don't need UUIDs. They are only needed if you have several processes creating objects without coordination.