>> What's wrong with 64-bit oids (except extra 4bytes)?
> Portability, mostly.
Oh, there's one other small problem: breaking the on-the-wire protocol.
We send OIDs as column datatype identifiers, so an 8-byte-OID backend
would not interoperate with clients that didn't also think OID is 8
bytes. Aside from client/server compatibility issues, that raises the
portability ante a good deal --- not only your server machine has to
have 'long long' support, but so do all your application environments.
regards, tom lane