First-Order Rewritability in Consistent Query Answering with Respect to Multiple Keys

Abstract

We study consistent query answering with respect to key dependencies. Given a (possibly inconsistent) database instance and a set of key dependencies, a repair is an inclusion-maximal subinstance that satisfies all key dependencies. Consistent query answering for a Boolean query is the following problem: given a database instance as input, is the query true in every repair? In [Koutris and Wijsen, ICDT 2019], it was shown that for every self-join-free Boolean conjunctive query and set of key dependencies containing exactly one key dependency per relation name (also called the primary key), this problem is in FO, L-complete, or coNP-complete, and it is decidable which of the three cases applies. In this paper, we consider the more general case where a relation name can be associated with more than one key dependency. It is shown that in this more general setting, it remains decidable whether or not the above problem is in FO, for self-join-free Boolean conjunctive queries. Moreover, it is possible to effectively construct a first-order query that solves the problem whenever such a query exists.