On parallel execution of multiple pipelined hash joins

Abstract

In this paper we study parallel execution of multiple pipelined hash joins. Specifically, we deal with two issues, processor allocation and the use of hash filters, to improve parallel execution of hash joins. We first present a scheme to transform a bushy execution tree to an allocation tree, where each node denotes a pipeline. Then, processors are allocated to the nodes in the allocation tree based on the concept of synchronous execution time such that inner relations (i.e., hash tables) in a pipeline can be made available approximately the same time. In addition, the approach of hash filtering is investigated to further improve the overall performance. Performance studies are conducted via simulation to demonstrate the importance of processor allocation and to evaluate various schemes using hash filters. Simulation results indicate that processor allocation based on the allocation tree significantly outperforms that based on the original bushy tree, and that the effect of hash filtering becomes prominent as the number of relations in a query increases.