Currently many SPARQL endpoints are freely available and accessible without any costs to users: Everyone can submit SPARQL queries to SPARQL endpoints via a standardized protocol, where the queries are processed on the datasets of the SPARQL endpoints and the query results are sent back to the user in a standardized format. As these distributed execution environments for semantic big data (as i...

A constantly increasing number of data providers publish their data on the Web in the RDF format as Linked Data. SPARQL is the standard query language for retrieving and manipulating RDF data. However, the majority of SPARQL implementations requires the data to be available in advance (in main memory or in a repository), not exploiting thereby the real-time and dynamic nature of Linked Data. In...

Despite much work within the last decade on foundational properties of SPARQL – the standard query language for RDF data – rather little is known about the exact limits of tractability for this language. In particular, this is the case for SPARQL queries that contain the OPTIONALoperator, even though it is one of the most intensively studied features of SPARQL. The aim of our work is to provide...

A growing number of applications require combining SPARQL queries with generic graph search on RDF data. However, the lack of procedural capabilities in SPARQL makes it inappropriate for graph analytics. Moreover, RDF engines focus on SPARQL query evaluation whereas graph management frameworks perform only generic graph computations. In this work, we bridge the gap by introducing SPARTex, an RD...

The SPARQL query language for RDF provides Semantic Web developers with a powerful tool to extract information from large datasets. This report describes a transformation from SPARQL into the relational algebra, an abstract intermediate language for the expression and analysis of queries. This makes existing work on query planning and optimization available to SPARQL implementors. A further tra...

The W3C SPARQL working group is defining the new SPARQL 1.1query language. The current working draft of SPARQL 1.1 focuses mainly on thedescription of the language. In this paper, we provide a formalization of the syntaxand semantics of the SPARQL 1.1 federation extension, an important fragment ofthe language that has not yet received much attention. Besides, we propose opti-<lb...

The W3C SPARQL working group is defining the new SPARQL 1.1 query language. The current working draft of SPARQL 1.1 focuses mainly on the description of the language. In this paper, we provide a formalization of the syntax and semantics of the SPARQL 1.1 federation extension, an important fragment of the language that has not yet received much attention. Besides, we propose optimization techniq...

The Triple Pattern Fragments (TPF) interface demonstrates how it is possible to publish Linked Data at low-cost while preserving data availability. But, data providers hosting TPF servers are not able to analyze the SPARQL queries they execute because they only receive and evaluate subqueries with one triple pattern. Understanding the executed SPARQL queries is important for data providers for ...

The research on optimization of top-k SPARQL query would largely benefit from the establishment of a benchmark that allows comparing different approaches. For such a benchmark to be meaningful, at least two requirements should hold: 1) the benchmark should resemble reality as much as possible, and 2) it should stress the features of the topk SPARQL queries both from a syntactic and performance ...