Ontology-based data access: Ontop of databases

Abstract. We present the architecture and technologies underpinning the OBDA system Ontop and taking full advantage of storing data in relational databases. We discuss the theoretical foundations of Ontop: the tree-witness query rewriting, T-mappings and optimisations based on database integrity constraints and SQL features. We analyse the per-formance of Ontop in a series of experiments and demonstrate that, for standard ontologies, queries and data stored in relational databases, On-top is fast, efficient and produces SQL rewritings of high quality.