We have identified the important components of such policies, characterized their desirable properties, and developed sample policies that satisfy these properties. Based on the framework, we compared the MAC policies commonly imposed in or proposed for multilevel relational databases. The framework could be used to capture and resolve the MAC policy mismatches in the secure interoperation of heterogeneous multilevel relational databases. We have investigated the complexity of trusted interoperation of multilevel relational databases that mismatch in security lattices. Moreover, trusted global interoperation is composable from trusted pairwise interoperation.
Mediated queries can be intolerably inefficient in practice.
We are developing implementation-independent techniques for the optimization of multidatabase queries. Cost information is abstracted in a form that is independent of the data models, DBMS implementations, or the hardware platforms of the component databases. The query optimizer decomposes a global query into local queries and generates a global query evaluation plan. We are also developing trusted query mediation techniques for the interoperation of heterogeneous databases, which preserve the autonomy and security of component databases. Queries will be mediated in such a way that neither the rights nor the security of component databases are compromised.