Tractable query answering and rewriting under description logic constraints

AbstractAnswering queries over an incomplete database w.r.t. a set of constraints is an important computational task with applications in fields as diverse as information integration and metadata management in the semantic Web. Description Logics (DLs) are constraint languages that have been extensively studied with the goal of providing useful modeling constructs while keeping the query answering problem decidable. For many DLs, query answering under constraints can be solved via query rewriting: given a conjunctive query Q and a set of DL constraints T, the query Q can be transformed into a datalog query QT that takes into account the semantic consequences of T; then, to obtain answers to Q w.r.t. T and some (arbitrary) database instance A, one can simply evaluate QT over A using existing (deductive) database technology, without taking T into account. In this paper, we present a novel query rewriting algorithm that handles constraints modeled in the DL ELHIO¬ and use it to show that answering conjunctive queries in this setting is PTime-complete w.r.t. data complexity. Our algorithm deals with various description logics of the EL and DL-Lite families and is worst-case optimal w.r.t. data complexity for all of them