In order to leverage the full potential of the semantic Web it is necessary to transparently query distributed RDF (Resource Description Framework) data sources in the same way as it has been possible with federated databases for ages. However, there are significant differences between the Web of (linked) data and the traditional database approaches. Hence, it is not straightforward to adapt successful database techniques for RDF federation. Reasons are the missing cooperation between SPARQL endpoints and the need for detailed data statistics for estimating the costs of query execution plans.