On the decidability and finite controllability of query processing in databases with incomplete information

Abstract

In this paper we study queries over relational databases with integrity constraints (ICs). The main problem we analyze is OWA query answering, i.e., query answering over a database with ICs under open-world assumption. The kinds of ICs that we consider are functional dependencies (in particular key dependencies) and inclusion dependencies; the query languages we consider are conjunctive queries (CQs), union of conjunctive queries (UCQs), CQs and UCQs with negation and/or inequality. We present a set of results about the decidability and finite controllability of OWA query answering under ICs. In particular: (i) we identify the decidability/undecidability frontier for OWA query answering under different combinations of the ICs allowed and the query language allowed; (ii) we study OWA query answering both over finite databases and over unrestricted databases, and identify the cases in which such a problem is finitely controllable, i.e., when OWA query answering over finite databases coincides with OWA query answering over unrestricted databases. Moreover, we are able to easily turn the above results into new results about implication of ICs and query containment under ICs, due to the deep relationship between OWA query answering and these two classical problems in database theory. In particular, we close two long-standing open problems in query containment, since we prove finite controllability of containment of conjunctive queries both under arbitrary inclusion dependencies and under key and foreign key dependencies. Besides their theoretical interest, we believe that the results of our investigation are very relevant in many research areas which have recently dealt with databases under an incomplete information assumption: e.g., view-based information access, ontology-based information systems, data integration, data exchange, and peer-to-peer information systems.