中文摘要：

XML已经成为网络上信息描述和信息交换的标准．由于网络上信息的本质特性和XML数据内在的灵活性，很多用XML编码的数据都是半结构化的，随着XML应用得越来越广泛，人们提出了多种XML数据的存储模型与此同时，XML的查询优化也是数据库领域研究的一个重要课题．综合论述了XML数据查询优化技术的现状。指出了XML查询优化的特点和研究的关键性问题，描述了查询优化技术各个方面的重要研究成果以及存在的问题，进一步展望了未来的研究方向，并在此基础上提出了对XML查询优化方法的一些观点。

英文摘要：

XML has become the de-facto standard for data representation and exchange on the World-Wide Web. Due to the nature of information on the Web and the inherent flexibility of XML, it is expected that much of the data encoded in XML will be semi-structured. Data on the internet is increasingly presented in XML format which enables researches on various kinds of XML storage model. Meanwhile, XML query optimization has become a hot research topic in database field. This paper gives an overview of the current status of technology for XML query optimization. The features of XML query optimization and key problems of research are also discussed deeply. Main aspects of current work on XML query optimization include XML algebra, cost model, complex path selectivity estimation, statistics information, and so on. Finally, this paper prospects future research directions and presents some viewpoints of XML query optimization.