Energy Eecient Query Optimization

Recently, a number of small yet powerful mobile computers have appeared on the market. Endowed with large disk capacity and network hardware, such systems are capable of holding large amounts of data, as well as of exchanging information with other hosts on a distributed system. These features suggest the need for the design and implementation of a distributed database system for mobile computers. Such mobile database systems present a host of technical challenges. One of them is dealing with the limited battery capacity of current mobile machines. We investigate the possibility of increasing the eeective battery life of these computers by selecting energy-eecient query plans during query optimization. In this paper we review the capabilities of mobile computers and describe the power management issues in such systems. We examine the case of database systems on stand-alone mobile computers, and conclude that only a small modiication is needed in standard query optimizers. However, the situation is far diierent when the mobile client interacts with a database server. In such a case, the appropriate optimization criterion involves optimization along two diierent axes, namely energy spent by the client, and overall server throughput. Optimization criteria that address the above point do not satisfy the basic assumption of the dynamic programming algorithm which lies at the center of most commercial op-timizers. This forces the consideration of dynamic programming algorithms with partial orders. We present two novel two-dimensional dynamic programming search algorithms for query optimization in mobile databases. We also discuss a new approximate optimization algorithm for this problem.