摘要 : Emerging distributed query-processing systems support flexible execution strategies in which earch query can be run using a combination of data shipping and query shipping. As in any distributed environment, these systems can obta... 展开 Emerging distributed query-processing systems support flexible execution strategies in which earch query can be run using a combination of data shipping and query shipping. As in any distributed environment, these systems can obtain tremendous performance and availability benefits by employing dynamic data caching. When flexible execution and dynamic caching are combined, however, a circular dependency arises: Caching occurs as a by-product of query operator placement, but query operator placement decisions are base don (cached) data location. The practical impact of this dependency is that query optimization decisions that appear valid on a per-query basis can actually cause suboptimal performance for all queries in the long run. 收起