Study Of Query Optimization Technology Based On XML Schema

Because of semi-structure and self-description characteristics, Extensible Markup Language (XML) has become more and more popular, and has been a de facto standard for information representation and exchange over the Internet. Because XML data is semi-structure, storage and query processing on XML is more complex and cumbersome than relational databases. XPath and XQuery as common XML Query languages can get the information from XML document and then transfer the results to XML. With the rapid growth of XML data, efficient storage as well as query techniques on large XML dataset has become a hot problem of XML research these days.In the paper, we analyse the problems of XML query as well as the optimization technologies, then introduce the query optimization system which uses XML Schema to optimize XML queries. The system chooses XML as the common data model; use the extended XQuery language as the query language. Rewrite the XQuery to optimize by using the syntax information provided by XML Schema. On one hand, the optimization can convert the fuzzy query to exact query; On the other hand, can delete the reductant predicates. The paper also proposes a new algorithm called TwigList-By-PDT to processe twig query, the algorithm makes full of the type information in XML Schema to optimize the twig query, reduces the scopes of path matching in twig pattern matching and decreases the traverses of useless nodes in XML document.With rapid growth of XML data, the storage of useless XML data increases the space cost in XML query processing. The paper introduces the lazy-loaded mechanism to solve the problem, the method can load parts of XML document to memory according to the query, therefore, avoids unnecessary space cost.