Abstract.One of the prime goals of the LOD2 project is improving the performance and scalability of RDF storage solutions so that the increasing amount of Linked Open Data (LOD) can be efficiently managed. Virtuoso has been chosen as the basic RDF store for the LOD2 project, and during the project it has been significantly improved by incorporating advanced relational database techniques from MonetDB and Vectorwise, turning it into a compressed column store with vectored execution. This has reduced the performance gap ("RDF tax") between Virtuoso's SQL and SPARQL query performance in a way that still respects the "schema-last" nature of RDF. However, by lacking schema information, RDF database systems such as Virtuoso still cannot use advanced relational storage optimizations such as table partitioning or clustered indexes and have to execute SPARQL queries with many selfjoins to a triple table, which leads to more join effort than needed in SQL systems. In this chapter, we first discuss the new column store techniques applied to Virtuoso, the enhancements in its cluster parallel version, and show its performance using the popular BSBM benchmark at the unsurpassed scale of 150 billion triples. We finally describe ongoing work in deriving an "emergent" relational schema from RDF data, which can help to close the performance gap between relational-based and RDF-based storage solutions.
General ObjectivesOne of the objectives of the LOD2 EU project is to boost the performance and the scalability of RDF storage solutions so that it can, efficiently manage huge datasets of Linked Open Data (LOD). However, it has been noted that given similar data management tasks, relational database technology significantly outperformed RDF data stores. One controlled scenario in which the two technologies can be compared is the BSBM benchmark [2], which exists equivalent relational and RDF variants. As illustrated in Fig. 1, while the SQL systems can process by up to 40-175K QMpH, the Triple stores can only reach 1-10K QMpH, showing a factor of 15-40 of performances difference.