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Ontology-based data access (OBDA) is a well established approach to information management that facilitates the access to relational data sources through the mediation of a conceptual domain view, given in terms of an ontology, and the use of a declarative mapping between the data layer and the ontology. We formally introduce here the notion of ontology-based data federation (OBDF) to denote a framework that combines OBDA with a data federation layer where multiple heterogeneous sources are virtually exposed as a single relational database. We discuss opportunities and challenges of OBDF, and propose novel techniques to make query answering in the OBDF setting more efficient. Our techniques are validated through an extensive experimental evaluation based on the Berlin SPARQL Benchmark. This work is an abridged version of [1].