Accelerating reachability query processing based on $$\varvec{}$$ reduction

Answering reachability queries is one of the fundamental graph operations. The existing approaches build indexes and answer reachability queries on a directed acyclic graph (DAG) \(G\), which is constructed by coalescing each strongly connected component of the given directed graph \(\mathcal {G}\) into a node of \(G\). Considering that \(G\) can still be large to be processed efficiently, there are studies to further reduce \(G\) to a smaller graph. However, these approaches suffer from either inefficiency in answering reachability queries, or cannot scale to large graphs. In this paper, we study DAG reduction to accelerate reachability query processing, which reduces the size of \(G\) by computing transitive reduction (TR) followed by computing equivalence reduction (ER). For TR, we propose a bottom-up algorithm, namely buTR, which removes from \(G\) all redundant edges to get the unique smallest DAG \(G^{t}\) satisfying that \(G^{t}\) has the same transitive closure as that of \(G\). For ER, we propose a divide-and-conquer algorithm, namely linear-ER. Given the result \(G^{t}\) of TR, linear-ER gets a smaller DAG \(G^{\varepsilon }\) in linear time based on equivalence relationship between nodes in \(G\). Our DAG reduction approaches (TR and ER) significantly improve the cost of time and space and can be scaled to large graphs. Based on the result of DAG reduction, we further propose a graph decomposition-based algorithm to efficiently answer reachability queries. We confirm the efficiency of our approaches by extensive experimental studies for TR, ER, and reachability query processing using 20 real datasets. The complete source code is available for download at https://pan.baidu.com/s/1skHBXXN.