Satisfiability-Test, Rewriting and Refinement of Users’ XPath Queries According to XML Schema Definitions

Abstract

Writing correct and precise XPath queries needs much effort from users: the user must be familiar with the complex structure of the queried XML documents and has to compose queries, which must be syntactically and semantically correct and precise. Incorrect queries select no data and thus lead to highly inefficient processing of queries. Unprecise queries might select more data than what the user really wants and thus might lead to unnecessarily high processing and transportation costs. Therefore, we propose a schema-based approach to the satisfiability test and to the refinement of users’ XPath queries. Our schema-based approach checks whether or not an XPath query conforms to the constraints given in the schema, rewrites and refines the XPath query according to the information of the schema. If an XPath query does not conform to the constraints given in the schema, its results will be every time an empty node set, which is a hint for semantic errors in the XPath query. Our rewriting approach for XPath queries replaces wildcards with specific node tests, replaces recursive axes with non-recursive axes, eliminates reverse axes, and redundant location steps. Thus, our rewriting approach generates a query, which contains more information, and can be more easily refined by the user in comparison to the original query. Our performance analysis shows the optimization potential of avoiding the evaluation of unsatisfiable XPath queries and of processing rewritten and refined XPath queries.