Eecient Query Evaluation in Disjunctive Deductive Databases

It is known that bottom-up query evaluation can be extended to work with disjunctive facts, but there seems to be the common assumption that it is much too ineecient for practical applications. In this paper, we improve the extended bottom-up evaluation by making the resolvable literal in a disjunctive fact unique. In many cases, this reduces an exponential behaviour to a polynomial one. We introduce the notion of \disjunction types" formalizing which predicates can appear together in a disjunction. This information is needed to generalize implementation techniques based on the predicate dependency graph, e.g. to determine a sequence for the evaluation of the rules. These two ideas are utilized in a translation of disjunctive rules into Horn clauses with some list-valued arguments. This shows that at least the addition of a few disjunctive rules to an otherwise Horn database does not destroy the possibility of eecient query evaluation.