First-Order Expressibility Results for Queries over Inconsistent DL-Lite Knowledge Bases

An important problem which arises in ontology-based data access is how to handle inconsistencies. This problem is especially relevant in an information integration setting where the data comes from multiple sources and one generally lacks the ability to modify the data so as to remove the inconsistencies. In the database community, the related problem of querying databases which violate integrity constraints has been extensively studied under the name of consistent query answering. The standard approach is based on the notion of a repair, which is a database which satisfies the integrity constraints and is as similar as possible to the original database. Consistent answers are defined as those answers which hold in all repairs. A similar strategy can be used for description logics by replacing the integrity constraints with the ontology. Recent work on consistent query answering in description logics has shown this problem to be co-NP-hard in data complexity, even for instance queries and the simplest DL-Lite dialect. In light of this negative result, we consider the problem of identifying cases where consistent query answering is feasible, and in particular, can be done using query rewriting, with the aim of better understanding the cases in which query rewriting can be profitably used. In this paper, we make some first steps towards this goal. Specifically, we formulate general conditions which can be used to prove that a consistent rewriting does or does not exist for a given DL-Lite TBox and instance query