Abstract: Join operation of data stream system plays an important role and whose executive method is dissimilar with the join operation of traditional DBMS.Join load shedding of data stream systems is different from traditional load shedding strategy for networks and highly dependent on applications.Therefore many join load shedding strategies for data stream systems are studied.After describing the models of data streams，data stream systems and the join operation，the system constraints of load shedding and the output goals are presented.The classification standards for join load shedding are presented，too.Then the key mechanisms of existing representative strategies are analyzed，their characteristic and application area are computed.Finally，the important features of good load shedding strategies are summarized，and the future trends are introduced.

Key words: data stream systems, load shedding, join, sliding window

摘要： 连接运算在数据流系统中扮演了关键角色，其执行方式与传统DBMS的连接运算不同，流连接降载策略与传统的网络降载也不相同，已开发许多连接降载策略。论文在介绍流连接运算、数据流及数据流系统的模型后，对连接降载时的系统约束、输出质量目标进行了正式阐述。提出连接降载策略的分类方法，并着重分析了目前一些较为重要的连接降载策略，指出其特征和应用范围。最后总结了好的连接降载策略应具有的特点及未来研究的发展趋势。

关键词: 数据流系统, 降载, 连接, 滑动窗口