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Federations of RDF data sources provide great potential when queried foranswers and insights that cannot be obtained from one data source alone. Achallenge for planning the execution of queries over such a federation is thatthe federation may be heterogeneous in terms of the types of data accessinterfaces provided by the federation members. This challenge has not receivedmuch attention in the literature. This paper provides a solid formal foundationfor future approaches that aim to address this challenge. Our main conceptualcontribution is a formal language for representing query execution plans;additionally, we identify a fragment of this language that can be used tocapture the result of selecting relevant data sources for different parts of agiven query. As technical contributions, we show that this fragment is moreexpressive than what is supported by existing source selection approaches,which effectively highlights an inherent limitation of these approaches.Moreover, we show that the source selection problem is NP-hard and in$\Sigma_2^\mathrm{P}$, and we provide a comprehensive set of rewriting rulesthat can be used as a basis for query optimization.

FedQPL: A Language for Logical Query Plans over Heterogeneous Federations of RDF Data Sources (Extended Version)