Equivalence of Datalog queries is undecidable

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Abstract

Datalog is a powerful query language for relational databases [10]. We consider the problems of determining containment, equivalence, and satisfiability of Datalog queries.

We show that containment and equivalence are recursively unsolvable. This should be contrasted with the work of Aho, Sagiv, and Ullman on relational queries [1]. Satisfiability is easily decidable for Datalog quer ies. We also consider Datalogf which allows function symbols. Here, satisfiability is recursively unsolvable.

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A preliminary version of this paper is contained in “Decidability and Expressiveness Aspects of Logic Queries,“ in the ACM Symp. on Principles of Database Systems, 1987