DL-Lite without UNA

Description logics (DLs) have recently been used to provide access to large amounts of data through a high-level conceptual interface, which is of relevance to both data integration and ontology-based data access. The fundamental inference service in this case is answering queries by taking into account the axioms in the TBox and the data stored in the ABox. The key property for such an approach to be viable in practice is the efficiency of query evaluation. To address these needs a series of description logics has been proposed and investigated in [6–8, 16] (the so-called DL-Lite family) and in [1, 2]. The significance of the DL-Lite logics is testified by the fact that they form the basis of OWL 2 QL, one of the three profiles of the web ontology language OWL 2. According to the current version of the official W3C profiles document, the purpose of OWL 2 QL is to be the language of choice for applications that use very large amounts of data and where query answering is the most important reasoning task. A very important difference between description logics and OWL is the status of the unique name assumption (UNA), which is commonly made in DL but not adopted in OWL. Instead, the OWL syntax provides explicit means for stating which object names are supposed to denote the same individual and which of them should be interpreted differently (in OWL, these constructs are called sameAs and differentFrom). Until recently, it had been assumed that the DL-Lite logics are interpreted under the UNA. The role of the UNA (for DL-LiteA) is discussed in [4, 5], where it is shown that if the UNA is dropped, instance checking becomes NLogSpacehard for data complexity. The aim of this paper is to investigate in depth the impact of dropping the UNA on both the combined and data complexity of reasoning in the DL-Lite family and extensions, classified according to the following features: