Validity-Sensitive Querying of XML Databases Extended Abstract †

We consider the problem of using XPath to query XML documents which are not valid with respect to given DTDs. If a query is formulated under the assumption that documents satisfy a DTD and a given document does not, then the query answer in this document may be different from the expected one. We propose an alternative mode of query evaluation which actively incorporates the schema into the evaluation process: If a document is invalid, all possible valid documents obtained from it by applying the minimum number of repairing operations are considered. Conceptually, the query is evaluated in each such document, and the intersection of all the results is returned to the user. We study the issue of how the computational complexity of query evaluation in our approach depends on the repertoire of the repairing operations and the syntax of the query. We describe experiments that validate our approach.