Skolemising Blank Nodes While Preserving Isomorphism

In this paper, we propose and evaluate a scheme to produce canonical labels for blank nodes in RDF graphs. These labels can be used as the basis for a Skolemisation scheme that gets rid of the blank nodes in an RDF graph by mapping them to globally canonical IRIs. Assuming no hash collisions, the scheme guarantees that two Skolemised graphs will be equal if and only if the two input graphs are isomorphic. Although the proposed scheme is exponential in the worst case, we claim that such cases are unlikely to be encountered in practice. To support these claims, we present the results of applying our Skolemisation scheme over a diverse collection of 43.5 million real-world RDF graphs (BTC-2014); we also provide results for some nasty synthetic cases.