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The advent of the Internet and the Web and their subsequent ubiquity have brought forth opportunities to connect information sources across all types of boundaries (local, regional, organizational, etc.). Examples of such information sources include databases, XML documents, and other unstructured sources. Uniformly querying those information sources has been extensively investigated. A major challenge relates to query optimization. Indeed, querying multiple information sources scattered on thedoi:10.1023/b:dapd.0000018574.71588.06 fatcat:qmdqsvtmhzhbpk5atffbwa2msu