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With the cutting edge technology advance in wireless and mobile computers, the query processing in a mobile environment involves join processing among different sites which include static servers and mobile computers. Because of the need for energy saving and also the presence of asymmetric features in a mobile computing environment, the conventional query processing for a distributed database cannot be directly applied to a mobile computing system. In this paper, we first explore three asymmetric features of a mobile environment. Then, in light of these features, we devise query processing methods for both join and query processing. Intuitively, employing semijoin operations in a mobile computing environment is able to further reduce both the amount of data transmission and energy consumption. A semijoin which is initiated by a mobile computer (respectively, the server) and is beneficial to reduce the cost of a join operation is termed a mobile-initiated or MI (respectively, server-initiated or SI) profitable semijoin. According to those asymmetric features of a mobile computing system, we examine three different join methods and devise some specific criteria to identify MI/SI profitable semijoins. For query processing, which refers to the processing of multijoin queries, we develop three query processing schemes. In particular, we formulate the query processing in a mobile computing system as a two-phase query processing procedure that can determine a join sequence and interleave that join sequence with SI profitable semijoins to reduce both the amount of data transmission and energy consumption. Performance of these join and query methods is comparatively analyzed and sensitivity analysis on several parameters is conducted. Furthermore, we develop a systematic procedure to derive the characteristic functions of MI and SI profitable semijoins. It is noted that, given some system parameters, those characteristic functions are very important in determining which join method is the most appropriate one to employ in that configuration. It is shown by our simulation results that, by exploiting the three asymmetric features, these characteristic functions are very powerful in reducing both the amounts of energy consumption and data transmission incurred and can lead to the design of an efficient and effective query processing procedure for a mobile computing environment.
Index Terms- Mobile database, mobile computing, query processing, join method.

M. Chen and W. Peng, "Query Processing in a Mobile Computing Environment: Exploiting the Features of Asymmetry," in IEEE Transactions on Knowledge & Data Engineering, vol. 17, no. , pp. 982-996, 2005.
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