Schema-Agnostic Query Rewriting for OWL QL.
Stefan Bischof, Markus Krötzsch, Axel Polleres, Sebastian Rudolph · WU Research · 2015
Ontology-based query answering (OBQA) has long been an important topic in applied and foundational research, and in particular in the area of description logics (DLs). Query answering has been studied for every major DL, but the most prominent use of DLs in query answering is based on the DLs of the DL-Lite family. In particular, these have widely been used in ontology-based data access (OBDA), e.g., to integrate disparate data sources or to provide views over legacy databases [3,14,4,9]. DL-LiteR is also the basis of the W3C OWL 2 Web Ontology Language profile OWL QL, which was specifically designed for OBDA applications [12]. On the other hand, research on query languages has led to a range of expressive features beyond basic pattern matching, e.g., by supporting navigational constructs or other forms of recursion. These developments have also affected practical query languages. SPARQL 1.1, the recent revision of the W3C SPARQL standard, introduces significant extensions to the capabilities of the popular RDF query language [7]. Even at the very core of the query language, we can find many notable new features, including property paths, value creation (BIND), inline data (VALUES), negation, and extended filtering capabilities. In addition, SPARQL 1.1 now supports query answering over OWL ontologies, taking full advantage of ontological information in the data [6,5]. Thus, with the arrival of SPARQL 1.1, every aspect of OBQA is supported by W3C technologies. In practice, however, SPARQL and OWL QL are rarely integrated. Most works on OBDA address the problem of answering conjunctive queries (CQs), which correspond to SELECT-PROJECT-JOIN queries in SQL, and (to some degree) to Basic Graph Patterns in SPARQL. The most common approach for OBDA is query rewriting, where a given CQ is rewritten into a (set of) CQs that fully incorporate the schema information of the ontology. The answers to the rewritten queries (obtained without considering the ontology) are guaranteed to agree with the answers of the original queries (over the ontology). This approach separates the ontology (used for query rewriting) from the rest of the data (used for query answering), and it is typical that the latter is stored in a relational database. Correspondingly, the rewritten queries are often transformed into SQL for query answering. SPARQL and RDF do not play a role in this. In a recent paper [1], we took a fresh look on the problem of OBQA query rewriting with SPARQL 1.1 as our target query language. The additional expressive power of SPARQL 1.1 allows us to introduce a new paradigm of schema-agnostic query rewriting, where the ontological schema is not needed for rewriting queries. Rather, the ontology is stored together with the data in a single RDF database. This is how many ontologies are managed today, and it corresponds to the W3C view on OWL and RDF, which does