Path-hop: efficiently indexing large graphs for reachability queries

Graph reachability is a fundamental research problem that finds its use in many applications such as geographic navigation, bioinformatics, web ontologies and XML databases, etc. Given two vertices, u and v, in a directed graph, a reachability query asks if there is a directed path from u to v. Over the last two decades, many indexing schemes have been proposed to support reachability queries on large graphs. Typically, those schemes based on chain or tree covers work well when the graph is sparse. For dense graphs, they still have fast query time but require large storage for their indices. In contrast, the 2-Hop cover and its variations/extensions produce compact indices even for dense graphs but have slower query time than those chain/tree covers. In this paper, we propose a new indexing scheme, called Path-Hop, which is even more space-efficient than those schemes based on 2-Hop cover and yet has query processing speed comparable to those chain/tree covers. We conduct extensive experiments to illustrate the effectiveness of our approach relative to other state-of-the-art methods.