I generally use both - a serial for internal RDBMS storage to coerce writes to be in order, and a secondary GUID that is app-specific and generated. The app wil

I generally use both - a serial for internal RDBMS storage to coerce writes to be in order, and a secondary GUID that is app-specific and generated. The app will never care about the serial, and the RDBMS doesn't care about the GUID other than secondary index look-ups (since they're rarely used for range scans).