基于层次组织位置数据库的连续位置相关查询处理

1 引言现有的数据库系统一般假设数据在未被显式修改前是不变的,例如:如果字段salary的值是30.000,那么只有通过事务更新才会改变该字段的值。但对连续变化的对象,如移动对象的位置,应用传统的数据库管理系统来管理会造成两种结果:或者移动对象位置的频繁更新占用大量的系统资源;或者使用移动对象过时的位置信息而导致错误的决策。

Location-Dependent Continuous Query Processing Using Hierarchical Location Databases

With the advances in mobile computing and mobile communication technology, there comes a kind of novel applications in which the locations of moving objects are maintained and processed. In existing literatures, a data model called moving objects sptio-temporal (MOST)1,2] is proposed and a new location record is generated when the distance between the actual location and the database location of a moving object exceeds a pre-defined distance threshold. In a mobile computing environment, a user can issue location-dependent continuous queries (LDCQs). To cater for the large number of moving objects in the system, this paper first gives a hierarchical distributed location database model to store the locations of moving objects. Based on the distribution of the location databases for different moving objects, this paper then proposes a method to determine the processing site for a location-dependent query. When a LDCQ is processed, a set of tuples (O. begin, end) is provided indicating that object O satisfies the condition presented in the LDCQ from time begin to end. In the existing literatures, when there is a location update generation, the related LDCQ is re-processed and the answering tuples are re-transmitted via the wireless channel. This location-update-based LDCQ processing method has its disadvantages: it has much CPU calculation cost and imposes a high overhead in the wireless bandwidth which is very undesirable in a wirelss environment. Based on the maximal speed of a moving object, this paper presents a deferred LDCQ evaluation strategy.