应用VB5中的RDO访问数据库的高级技术

本文针对企业范围的客户机／服务器模式,介绍了用ＶＢ５中的ＲＤＯ访问数据库的高级技术和技巧,包括查询、存储进程、结果集管理及异步查询等。     

数据仓库管理技术的研究

本文研究了数据仓库管理中的一些关键技术,包括数据复制技术、故障恢复、访问控制与安全、数据增长的管理、数据库性能的管理、以及数据仓库的档次与扩充等。     

用Maplnfo开发油田水利地理信息系统

本文简单介绍了用Maplnfo开发油田水利地理信息系统(GIS)的建设情况。着重阐述了系统的设计、结构、功能以及系统开发过程中所采用关键技术。     

实现数据库细粒度访问控制的一种动态查询修改算法

由于隐私保护和基于W cb的安全需求的驱动,数据库细粒度访问控制引起了研究者的广泛关注。细粒度访问控制主要通过查询改写方法实现。然而,在以往的查询改写算法中,没有考虑用户提交的SQL语句的特性和细粒度访问控制策略的特性,从而导致最终执行的SQL中存在大量的冗余,影响了SQL语句执行的性能。在考虑SQL语句和细粒度访问控制策略的特性的前提下,分析了两类冗余,并给出了相应的移除方法。最终提出了一种用于细粒度访问控制实现的查询改写算法。实验证实该算法具有更好的性能。     

Discovering and using entity mappings in federated databases

In the context of a federated database containing similar information in local component databases, we consider the problem of discovering the different local representations which database designers might use to map the same entity. An extension of the normal SQL system catalog to hold additional domain metadata is discussed. This enables common local mappings to be discovered and allows SQL retrieval queries expressed in terms of the logical federated schema to be transformed to match the appropriate local database schema. The result is a practical means of coping with differing mappings in federated databases without having to resort to special higher order languages or a layer of information mediation middleware. Copyright © 1999 John Wiley & Sons, Ltd.     

Performance improvement of TCP in ad hoc networks by mitigating channel contention

In ad hoc networks, the spatial reuse property limits the number of packets which can be spatially transmitted over a path. In standard Transmission Control Protocol (TCP), however, a TCP sender keeps transmitting packets without taking into account this property. This causes heavy contention for the wireless channel, resulting in the performance degradation of TCP flows. Hence, two techniques have been proposed independently in order to reduce the contention. First, a TCP sender utilizes a congestion window limit (CWL), by considering the spatial reuse property. This prevents the TCP sender from transmitting more than CWL number of packets at one time. Second, a delayed ack (DA) strategy is exploited in order to mitigate the contention between the TCP ACK and DATA packets. Recently, although TCP‐DAA (Dynamic Adaptive Acknowledgment) attempts to utilize a CWL‐based DA strategy, TCP‐DAA overlooks a dynamic correlation between these two techniques. This paper, therefore, reveals the dynamic correlation and also proposes a protocol which not only reduces the frequency of the TCP ACK transmissions but also determines a CWL value dynamically, according to network conditions. Simulation studies show that our protocol performs the best in various scenarios, as compared to TCP‐DAA and standard TCP (such as TCP‐NewReno). Copyright © 2009 John Wiley & Sons, Ltd.     

突发包分割机制研究与仿真

对现有的突发包分割机制进行了研究,提出了一种新复合突发封装方法。分析和仿真表明,与原复合封装方法相比,改进后的方法在不增加端到端延时的情况下,将进一步减少高优先级数据的丢失率,保证其服务质量。     

Action selection in complex routinized sequential behaviors.

We report two experiments in which errors and interaction latencies were recorded during routinization of hierarchically structured computer-based tasks. Experiment 1 demonstrates that action selection is slowed at subtask transitions, especially when selecting lower frequency actions. This frequency effect is compounded by concurrent performance of a secondary, attentionally demanding, task. Experiment 2 replicates these results in a more complex task and further demonstrates that the effects are reduced by experience. Several other factors were also found to affect latencies, including the availability of an external disambiguation cue and the temporal distance over which task context needs to be internally maintained. The results support a “dual-systems” account of action selection in which a “routine” system, sensitive to frequency, context, and experience, is selectively modulated by an attentionally demanding “nonroutine” system. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

空间数据库性能对比实验分析

空间数据库是管理空间数据的重要方式,亦是GIS的重要组成部分。介绍了空间数据库的概念和发展历程,分析了影响空间数据库性能的因素,进而探讨了图层加载、要素插入和空间计算等应用性能,设计了三组空间数据库的操作实验,对目前常用的几种空间数据库管理空间数据的性能进行了对比分析。