PARALLEL EXECUTION OF HASH JOINS IN PARALLELDATABASESHui

In this paper we explore two important issues, processor allocation and the use of hash lters, to improve the parallel execution of hash joins. To exploit the opportunity of pipelining for hash join execution, a scheme to transform a bushy execution tree to an allocation tree is rst devised. In an allocation tree, each node denotes a pipeline. Then, using the concept of synchronous execution time, processors are allocated to the nodes in the allocation tree in such a way that inner relations in a pipeline can be made available approximately the same time. Also, the approach of hash ltering is investigated to further improve the parallel execution of hash joins. Extensive performance studies are conducted via simulation to demonstrate the importance of processor allocation and to evaluate various schemes using hash lters. It is experimentally shown that processor allocation is in general the dominant factor to performance, and the eeect of hash ltering becomes more prominent as the number of relations in a query increases.