Cost-based Query Optimization for XPath

Abstract: In this article, we address the issues which are related to the cost-based XML query optimization for XPath. Specially, we focus on the issue of how to determine the execution order for a given XPath expression according to the cost models. The main impact factor that dominates the execution order is the size of the intermediate results during the evaluation of queries. The hierarchy encoding scheme and the value-encoding histogram are introduced to support the size estimation of the path expressions. Two cost models are proposed to describe the costs of different types of join operations in the path expressions. A heuristic-based dynamic programming approach is proposed to determine the optimal execution tree. The primary experimental results demonstrate the validity of our approaches