5.1. Introduction

Resource description framework (RDF) is a standard for the conceptual description of knowledge. The RDF data are cherished and exploited by various domains such as life sciences, Semantic Web and social networks. Furthermore, its integration at Web scale compels RDF management engines to deal with complex queries in terms of both size and structure. Popular examples are provided by Google, which exploits the so-called knowledge graph to enhance its search results with semantic information gathered from a wide variety of sources, or by Facebook, which implements the so-called entity graph to fuel its search engine and provide further information extracted, for instance, by Wikipedia. Another example is provided by recent question–answering systems [CAB 12, ZOU 14a] that automatically translate natural language questions in SPARQL queries and successively retrieve answers by considering the available information in the different linked open data sources. In all...