Counting to k or how SPARQL1.1 Property Paths Can Be Extended to Top-k Path Queries.

While the volume of graph data available on the Web in RDF is steadily growing, SPARQL, as the standard query language for RDF still remains effectively unusable for the basic task of finding paths through the graph between selected nodes. Property Paths, as introduced in SPARQL 1.1 are unfit for this purpose, as they can only be used to test path existence. More expressive features, such as counting distinct paths between two nodes, have been shown highly intractable in the worst case, in particular in graphs with high degree of cyclicity. Still, practical use cases demand a solution for path retrieval even when the total number of paths is prohibitively large. A common approach is to ask not for all, but only for the k shortest paths. In this paper, we extend SPARQL 1.1 property paths in a manner that allows to compute and return the k shortest paths matching a property path expression between two nodes. For RDF graphs in the compact HDT format, we evaluate or algorithm for top k shortest paths showing that a relatively simple approach works (in fact, more efficiently than other, more complex algorithms in the literature) in practical use cases.