Database name has nothing to do with normal host name resolution; they are two separate worlds. Oracle uses her own name resolution: "sqlnet.ora" is the equivalent of nsswitch.conf(4), and "tnsnames.ora" defines the TNS listener's service name(s), protocol, port, and connection type.
Database name, user and password are then configured in powerdns.conf(4). When linked with Oracle client libraries, pdns(1M) Oracle backend module makes the database connection over "OCI", the Oracle client interface.
Other database backends, like for example SQLite run on the same server so pdns(1M) "gsqlite" backend module opens the database file directly.
The one aspect that concerns me about that is the bootstrap step of resolving the database server name. How is that done? hosts file?