Modern applications, such as digital libraries, e-commerce, portals, etc., require integrated access to various information sources from traditional RDBMS to the semistructured Web repositories, with fast deployment and low maintenance cost in a rapidly evolving environment. Because of its flexibility, there is an increasing interest in using XML as a middleware model for such applications. XML enables fast wrapping and declarative integration. However, query processing in XML-based integration systems is still penalized by the lack of algebra with adequate optimization properties and the difficulty to understand source query capabilities.
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