In an ideal distributed database system, the sites would share a common
global schema (although some relations may be stored only at some sites), all
sites would run the same distributed database-management software, and the
sites would be aware of each other’s existence. If a distributed database is built
from scratch, it would indeed be possible to achieve the above goals. However,
in reality a distributed database has to be constructed by linking together multiple
already-existing database systems, each with its own schema and possibly
running different database-management software. Such systems are sometimes
called multidatabase systems or heterogeneous distributed database systems.
We discuss these systems in Section 19.8, wherewe show how to achieve a degree
of global control despite the heterogeneity of the component systems