Considering indexes and algorithms to answer XPath queries over XML data, we propose an index structure and a related algorithm, both adapted to the comparison model, where elements can be accessed non-sequentially. The indexing scheme uses classical labelling techniques, but structurally represents the ancestor-descendant relationships of nodes of each type, in order to allow exponential searches. The algorithm performs XPath location steps along the descendant axis, and it generates few intermediate results. The complexity of the algorithm is proved worst-case optimal in an adaptive comparison model where the index is given, and where the instances are grouped by the number of comparisons needed to check their answer.
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