Optimization of parallel query execution plans in XPRS

The authors describe their approach to the optimization of query execution plans in XPRS, a multi-user parallel database machine based on a shared-memory multi-processor and a disk array. The main difficulties in this optimization problem are the compile-time unknown parameters such as available buffer size and number of free processors, and the enormous search space of possible parallel plans. The authors deal with these problems with a novel two phase optimization strategy which dramatically reduces the search space and allows run time parameters without significantly compromising plan optimality. They present their two phase strategy and give experimental evidence from XPRS benchmarks that indicate that it almost always produces optimal plans