Discovering Veiled Unsatisfiable XPath Queries

The satisfiability problem of queries is an important determinant in query optimization. The application of a satisfiability test can avoid the submission and the unnecessary evaluation of unsatisfiable queries, and thus save processing time and query costs. If an XPath query does not conform to constraints in a given schema, or constraints from the query itself are inconsistent with each other, the evaluation of the query will return an empty result for any valid XML document, and thus the query is unsatisfiable. Therefore, we propose a schema-based approach to filtering the XPath queries not conforming to the constraints in the schema and the XPath queries with conflicting constraints. We present a complexity analysis of our approach, which proves that our approach is efficient at typical cases. We present an experimental analysis of our prototype, which shows the optimization potential of avoiding the evaluation of unsatisfiable queries.