So, I started looking into this and I think we should probably defer it to another PR.

In the case where the full OID can be faster and/or simpler than the partial OID, it feels like this shifts the responsibility for handling the optimization from `src/odb.c` and into the back end. Of course, since it also eliminates some backend entry points, it would probably be a wash for the backend implementor and it would save some code.

Anyhow, unless you really hate this `exists_prefix` entry point, I'd probably rather keep it in a separate PR.