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In recent years, the Web more and more evolved from a Web of Documents to a Web of Data. This development started a few years ago, when the Linked Data principles [83] were formulated with the vision to create a globally connected data space. The goal to integrate semantically similar data by establishing links between related resources is especially pursued in the Linking Open Data initiative, a project that aims at connecting distributed RDF data on the Web. Currently, the Linked Open Data cloud comprises more than 200 datasets that are interlinked by RDF links, spanning various domains ranging from Life Sciences over Media to Cross Domain data. With the uptake of Linked Data in recent years, the topic of integrated querying over multiple distributed data sources has attracted significant attention. In order to join information provided by different sources, efficient query processing strategies are required …