Scalable parallel graph algorithms with matrix–vector multiplication evaluated with queries

Graph problems are significantly harder to solve with large graphs residing on disk compared to main memory only. In this work, we study how to solve four important graph problems: reachability from a source vertex, single source shortest path, weakly connected components, and PageRank. It is well known that the aforementioned algorithms can be expressed as an iteration of matrix–vector multiplications under different semi-rings. Based on this mathematical foundation, we show how to express the computation with standard relational queries and then we study how to efficiently evaluate them in parallel in a shared-nothing architecture. We identify a common algorithmic pattern that unifies the four graph algorithms, considering a common mathematical foundation based on sparse matrix–vector multiplication. The net gain is that our SQL-based approach enables solving “big data” graph problems on parallel database systems, debunking common wisdom that they are cumbersome and slow. Using large social networks and hyper-link real data sets, we present performance comparisons between a columnar DBMS, an open-source array DBMS, and Spark’s GraphX.