Query processing for integrated systems

Considers the representation of a global schema of an integrated system as a set of views of the local schemas. A user´s query against the global schema (global query) can then be translated into a set of local queries against the base relations by employing the relevant view definitions. The view definitions are complex; the query modification algorithm is augmented to cover the difficult cases omitted in previous discussions. The authors identify cases in which a query modification in impossible and propose to process these cases by first materializing the views, and then processing the query against the view tables. The translation algorithm may introduce unnecessary subqueries, clauses, and conditions into the local queries. While these redundancies do not affect query correctness, they increase processing costs. Semantic optimizations that eliminate some sources of inefficiency are included