Based on the query expansion techniques in information retrieval systems, structure-based query expansion for XML search engines, which is designed to ease the query for XML data while keeping the power and flexibility of XML query, is introduced in this paper. To enable the structure expansion, a structure thesaurus should be built first, which involves the construction of a weighted graph from XML documents and the linkage-based clustering method to cluster the nodes into several groups. After a query comes, the structure thesaurus is examined, so that for each tag in the original query, the tags in the same group are retrieved. Unrelated tags are filtered and some heuristic rules are applied to replacing the tags in the original query with the related tags and to expanding the structure. It is shown that using structure-based query expansion, the system can return result with high precision and recall.
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