Study Of XML Data Query Optimization Method Based On PAT Algebra

There are a large number of semi-structured XML data on the Internet. Because of the semi-structured feature, it is very difficult to retrieve data with the traditional query optimization of database. The method of depending on algebraic system to transform XML query expression and using algebraic transformation rules to optimize the expression make us optimize the queries for XML data more effectively. The research method has become the focus of XML data query optimization field at present.The Research of XML algebra currently focus on normalizing XML query semantics, and do not consider the factor of query optimization. Because the algebra has obviously procedural thinking, it is hard to further optimize, and solving the problem only by traversing. So the query efficiency is very low and the method does not suit to the large-scale XML data query. After studying and analysizing the current XML data query optimization, we have drawed on the existing XML query optimization, and used the set-oriented PAT algebra system, and then proposed the method of query optimization based on PAT algebra.Firstly, the existing XML query optimization and the system of XML data query optimization have been studied and analysized, and the method of query optimization based on the structural index has been proposed. In the method the query path can be shorten. And then, we have analysized the query equivalents of PAT algebra system, extended PAT algebra with the equivalents, and with the heuristic thinking proposed the system of the deterministic transformation rules based on PAT algebra and the transformation method of the rules based on the structural index.At last, through using the transformation rules of the expanded algebra and combining with the semantic optimization strategies of the paper, we transform the XML query expressions successfully. The redundant operations are removed more reasonably and the query expressions are simplified. Thereby the numbers of algebraic operations are reduced. The query path is shortened effectively depending on using the introduction strategy of index to introduce the structural index into the process of the algebraic transformation of query expressions. Through validating and analysize the example, using the proposed query optimization can improve the efficiency of XML data query more effectively.