Skew Handling in the DBS3 Parallel Database System

The gains of parallel query execution can be limited because of high start-up time, interference between execution entities, and poor load balancing. In this paper, we present a solution which reduces these limitations in DBS3, a shared-memory parallel database system. This solution combines static data partitioning and dynamic processor allocation to adapt to the execution context. It makes DBS3 almost insensitive to data skew and allows decoupling the degree of parallelism from the degree of data partitioning. To address the problem of load balancing in the presence of data skew, we analyze three important factors that influence the behavior of our parallel execution model: skew factor, degree of parallelism and degree of partitioning. We report on experiments varying these three parameters with the DBS3 prototype on a 72-node KSR1 multiprocessor. The results demonstrate high performance gains, even with highly skewed data.