Can Filters do Magic for Deductive Databases?

Abstract

The magic sets method [Bancilhon et al 86, Been & Ramakrishnan 87] is one of the most popular methods of recursive query processing for deductive databases. The magic sets method and its variants may be regraded as computing filters for restricting bottom-up evaluation of rules and then applying these filters to various rules. There is a tradeoff between the simplicity of the filters and their effectiveness. The magic sets method sacrifices the simplicity of the filters for their effectiveness. [Sippu & S-Soininen 88] describes a method which always keeps the filter computation much simpler than the processing of the query at the expense of filters which can be much less tight than the magic predicates. [Sagiv 90] describes a method of envelopes which has the advantage that the size of an envelope is much smaller than the size of magic predicates. In this paper, we approach this tradeoff from a structural perspective. We show that under certain conditions, which depend on the structure of the rules, it is possible to use filters which are much less in size than magic predicates, while preserving the effectiveness of magic predicates. Thus, our filters are smaller than magic predicates, while the restriction imposed by them is the same as that imposed by the magic predicates.