Also, you can store them as a BIGINT, which is awesome. So much smaller than even a binary-encoded UUID. IIRC the spec reserves the right to use the sign bit, s

Also, you can store them as a BIGINT, which is awesome. So much smaller than even a binary-encoded UUID. IIRC the spec reserves the right to use the sign bit, so if you’re concerned, use BIGINT UNSIGNED (natively in MySQL, or via extension in Postgres). I wish more people cared about the underlying tech of their storage layer – UUIDv4 as a string is basically the worst-case scenario for a PK, especially for MySQL / InnoDB.