    <p>Many object-oriented systems allow applications to operate on objects
uniformly, whether they are in memory or on disk. These systems create
the illusion that all objects are in memory all the time.  The advantage
to object-oriented programmers who simply want object storage and
retrieval is clear. They need never be aware of whether an object is in
memory or not.  The application simply uses objects, and the database
system moves them between disk and memory transparently.  All of the
operations on 