Interoperability of heterogeneous databases with schematic discrepancies

It is widely accepted that relational language capabilities are insufficient to prove interoperability of databases even if they are all relational. In particular, view definitions cannot reconcile schematic discrepancy, in which one database's data (values) correspond to metadata of another database. Two new features are necessary: higher order expressions are needed where variables can range over data and metadata, and these expressions can be used to define a unified view over the original databases; higher order view definitions are necessary where the number of relations/attributes defined are dependent on the state of the database, in contrast to the traditional view definitions which specify a fixed set of relations for all states of the database.<>