Semantic Transformation Approach with Schema Constraints for XPath Query Axes

XPath queries are essentially composed of a succession of axes defining the navigation from a current context node. Among the XPath query axes family,

child

,

descendant, parent

can be optionally specified using the path notations {/,//,..} which have been commonly used. Axes such as

following-sibling and preceding-sibling

have unique functionalities which provide different required information that cannot be achieved by others. However, XPath query optimization using schema constraints does not yet consider these axes family.

The performance of queries denoting the same result by means of different axes may significantly differ. The difference in performance can be affected by some axes, but this can be avoided. In this paper, we propose a semantic transformation typology and algorithms that transform XPath queries using axes, with no optional path operators, into semantic equivalent XPath queries in the presence of an XML schema. The goal of the transformation is to replace whenever possible the axes that unnecessarily impact upon performance. We show how, by using our semantic transformation, the accessing of the database using such queries can be avoided in order to boost performance. We implement the proposed algorithms and empirically evaluate their efficiency and effectiveness as semantic query optimization devices.