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In this paper, we present an optimization framework for querying XML data by caching frequent query patterns. In this framework, frequent query patterns are mined online from user queries, and these query results are cached for future use. The mining process of frequent query patterns is launched automatically when user queries meet predefined requirements. To deal with queries that are similar to but not exactly same as the cached ones, a novel technique named query rewriting is adopted. This technique is able to handle four kinds of similar queries namely exact matching, exact containment, semantic matching and semantic containment. A cache replacement scheme that utilizes both the query pattern support and query pattern accessing time is employed to perform a fine-grained region purging. Experiments are carried out on the XMARK datasets. The results show that the proposed methods are both effective and efficient in improving the performance of XML queries.