Query optimization for ontology-based information integration

In recent years, there has been an explosion of publicly available RDF and OWL data sources. In order to effectively and quickly answer queries in such an environment, we present an approach to identifying the potentially relevant Semantic Web data sources using query rewritings and a term index. We demonstrate that such an approach must carefully handle query goals that lack constants; otherwise the algorithm may identify many sources that do not contribute to eventual answers. This is because the term index only indicates if URIs are present in a document, and specific answers to a subgoal cannot be calculated until the source is physically accessed - an expensive operation given disk/network latency. We present an algorithm that, given a set of query rewritings that accounts for ontology heterogeneity, incrementally selects and processes sources in order to maintain selectivity. Once sources are selected, we use an OWL reasoner to answer queries over these sources and their corresponding ontologies. We present the results of experiments using both a synthetic data set and a subset of the real-world Billion Triple Challenge data.