A heterogeneous database system cannot be properly characterized
without evaluating the assumptions under which it operates,
including the degree of database heterogeneity the system can deal with,
the extent of cooperation expected from component databases, and so on.
For example, the database federation promoted by GDB (Fasman 1994)
is based on the assumption of a high level of
cooperation between component MBDs regarding the specification
of schema correlations and data sharing;
the fact that the component MBDs of the new version
of GDB
are developed using the same data model (OPM) and implemented with the same
DBMS (Sybase) represents another significant simplifying assumption.
Some assumptions express limitations of the component databases and consequently affect the approach taken for developing the heterogeneous database system. For example, if component MBDs do not provide facilities for processing external queries (see below) then heterogeneous database querying is severely limited.