Indexing XML data is essential for XML query optimization. Most of the existing approaches that combine a labeling scheme with a path index use labeling schemes that reflect the structure of the indexed data. In addition, the labeling rules do not depend on the combined path indexes. By designing a labeling scheme that does not reflect the structure of XML data, since it is available in the accompanied path index; and by aligning the data nodes ’ labels with the path index nodes’ labels, we can support the join process more efficiently. We propose a novel index structure called LTIX (Level-based Tree Index for XML databases). This index structure is based on Level-based Labeling Scheme (LLS) that not only minimizes the number of joins and matches required to evaluate twig queries, if it is used with path indexes, but also facilitates effective query optimization through early pruning of the space search. Experimental tests show the performanc
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