In this paper, we deal with the problem of effective search and query answering in heterogeneous web document bases containing documents in XML format of which the schemas are available. We propose a new solution for the structural approximation of the submitted queries which, in a preliminary schema matching process, is able to automatically identify the similarities between the involved schemas and to use them in the query processing phase, when a query written on a source schema is automatically rewritten in order to be compatible with the other useful XML documents. The proposed approach has been implemented in a web service and can deliver middleware rewriting services in any open-architecture XML repository system offering advanced search capabilities.