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INTRODUCTION The CUBE BY operator (Gray et al., 1996) is anessential facility for data warehousing and OLAP. Itis a multidimensional extension of the standardGROUP BY operator, computing all possible com-binations of the grouping attributes in the CUBE BYclause. A CUBE BY with N grouping attributes willcompute 2N group-bys. In the real world, a fact tableis often very large and sparse. In such cases, the sizeof a group-by is possibly close to the size of the facttable. So th… 相似文献
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Finding all occurrences of a twig query in an XML database is a core operation for efficient evaluation of XML queries. It is important to effectively handle twig queries with wildcards. In this paper, a novel path-partitioned encoding scheme is proposed for XML documents to capture paths of all elements, and a twig query is modeled as an XPattern extended from tree pattern. After definition, simplification, normalization, verification and initialization of the XPattern, both work sets and a join plan are generated. According to these measures, an effective algorithm to answer for a twig query, called DMTwig, is designed without unnecessary elements and invalid structural joins. The algorithm can adaptively deal with twig queries with branch([]), child edge(/), descendant edge(//), and wildcard(*)synthetically. We show that path-partitioned encoding scheme and XPattern guarantee the I/O and CPU optimality for twig queries. Experiments on representative data set indicate that the proposed solution performs significantly. 相似文献
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