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Abstract : |
Abstract. We propose a query-evaluation algorithm for database specifications consisting of overridable rules (defaults) and non-overridable rules (axioms). Both kinds of rules may contain disjunctions. Preferences among the defaults are given by a partial order which corresponds to a subclass relationship allowing multiple inheritance. We use a bottomup theorem proving procedure. It is applied to the set of (disjunctively) assumed default instances which essentially can again be computed by bottom-up evaluation. Our approach has two main advantages: First, it gives a simple foundation for bottom-up evaluation in the presence of defaults; this can be taken as a basis for different optimizations. Second, in contrast to other deduction algorithms, it is able to handle partially ordered defaults with a skeptical / minimal model semantics. 1, |
