On a problem of Fagin concerning multivalued dependencies in relational databases

Abstract

Multivalued dependencies (MVDs) are an important class of relational constraints that is fundamental to relational database design. Reflexivity axiom, complementation rule, and pseudo-transitivity rule form a minimal set of inference rules for the implication of MVDs. The complementation rule plays a distinctive role as it takes into account the underlying relation schema R which the MVDs are defined on. The R-axiom ∅ ↠ R is much weaker than the complementation rule, but is sufficient to form a minimal set of inference rules together with augmentation and pseudo-difference rule. Fagin has asked whether it is possible to reduce the power of the complementation rule and drop the augmentation rule at the same time and still obtain a complete set. It was argued that there is a trade-off between complementation rule and augmentation rule, and one can only dispense with one of these rules at the same time. It is shown in this paper that an affirmative answer to Fagin's problem can nevertheless be achieved. In fact, it is proven that R-axiom together with a weaker form of the reflexivity axiom, pseudo-transitivity rule and exactly one of union, intersection or difference rule form such desirable minimal sets. The positive solution to this problem gives further insight into the difference between the notions of functional and multivalued dependencies.