A Top-Down Proof Procedure for Generalized Data Dependencies


Data dependencies are well known in the context of relational database. They aim to specify constraints that the data must satisfy to model correctly the part of the world under consideration. The implication problem for dependencies is to decide whether a given dependency is logically implied by a given set of dependencies. A proof procedure for the implication problem, called “chase”, has already been studied in the generalized case of tuple-generating and equality-generating dependencies. The chase is a bottom-up procedure: from hypotheses to conclusion, and thus is not goal-directed. It also entails in the case of TGDs the dynamic creation of new constants, which can turn out to be a costly operation. This paper introduces a new proof procedure which is top-down: from conclusion to hypothesis, that is goal-directed. The originality of this procedure is that it does not act as classical theorem proving procedures, which require a special form of expressions, such as clausal form, obtained after Skolemization. We show, with our procedure, that this step is useless, and that the notion of piece allows infering directly on dependencies, thus saving the cost of Skolemizing the dependencies set and, morever, that the inference can be performed without dynamically creating new constants. Although top-down approaches are known to be less efficient in time than bottom-up ones, the notion of piece cuts down irrelevant goals usually generated, leading to a usable top-down method. With the more recent introduction of constrained and ordered dependencies, some interesting perspectives also arise.

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