If I try to implement RDF based application using existed DB, the application may run somewhat slowly, I think. As the mechanism of RDF is not fitted with current popular Relationship Database. The most efficient way to implement RDF is using FileSystem directly. But it will take much time to implement such file based RDF system as lots of things must be took into consideration, such as sorting algorithm, security of data.

Maybe the best way for me is : First, implement things using JDO on DB; Second, implements things using JDBC on DB; Third, implements things using File; and so on.