A Proof Procedure Based on Recomputation-free Lemmatization

Several top down proof procedures are presented in the literature for query answering in deductive databases using lemmatization. However, all of these mechanisms suuer from recomputation whenever more general queries than the lemmatized ones appear. We introduce a lemmatization mechanism for top-down evaluation that overcomes the problem by recording with the lemmas which branches of the search space have been examined. Later computation of new answers makes use of this information to divert the search from earlier visited branches. The original rules of the database are rewritten to yield, jointly with the solutions, the explored branches. The unvisited paths (branches) of the search space are inferred from the explored ones. This approach suggests a family of algorithms for the inference of paths of which we describe one in detail.