Applying Segmented Right-Deep Trees to Pipelining Multiple Hash Joins

The pipelined execution of multijoin queries in a multiprocessor-based database system is explored in this paper. Using hash-based joins, multiple joins can be pipelined so that the early results from a join, before the whole join is completed, are sent to the next join for processing. The execut ion of a query is usually denoted by a query execution tree. To improve the execution of pipelined hash joins, an innovative approach on query execution tree selection is proposed to exploit segmented rightdeep trees, which are bushy trees of right-deep subtrees. We first derive an analytical mode1 for the execution of a pipeline segment, and then, in light of the model, develop heuristic schemes to determine the query execution plan based on a segmented rightdeep tree so that the query can be efficiently executed. As shown by our simulation, the proposed approach, without incurring additional overhead on plan execution, possesses more flexibility in query plan generation, and can lead to query plans of better performance than those achievable by the previous schemes using right-deep trees.