Run-Time Optimizatins of Join Queries for Distributed Databases over the Internet

摘要

A probe-based run-tiem optimization technique is developed and demonstrated in the context of an Internet-based distributed database environment. More and more common are database systems which are distributed across servers communicating iva the Internet where a query at a given site might require data from remote sites. Optimizing the response time of such queries is a challenging task due to the unpredictability of server performance and network traffic at the time of data shipment; this may resutl in the selection of an expensive query plan using a static query optimizer. We constructed an experimental setup consisting of two servers runnign the same DBMS connected via the Internet. Concentrating on join queries, we demonstrate how a static query optimizer might choose an expensiver plan by mistake. This is due to the lack of a priori knowledge of the run-time environment, inaccurate statistical assumptions in size estimation, and neglecting the cost of remote method invocation. These shortcomings are addressed collectively by proposing a run-tiem probing mechanism. An implementation of our run-time probing mechanism. An implementation of our run-time optimization technique for join queries was constructed i nthe Java language and incorporated into the experimental setup. The results demonstrate the superiority of run-time probe-based optimization over a static optimization.