Mixing Bottom-Up and Top-Down XPath Query Evaluation

Available XPath evaluators basically follow one of two strategies to evaluate an XPath query on hierarchical XML data: either they evaluate it top-down or they evaluate it bottom-up. In this paper, we present an approach that allows evaluating an XPath query in arbitrary directions, including a mixture of bottom-up and top-down direction. For each location step, it can be decided whether to evaluate it top-down or bottom-up, such that we can start e.g. with a location step of low selectivity and evaluate all child-axis steps top-down at the same time. As our experiments have shown, this approach allows for a very efficient XPath evaluation which is 15 times faster than the JDK1.6 XPath query evaluation (JAXP) and which is several times faster than MonetDB if the file size is ≤ 30 MB or the query to be evaluated contains at least one location step that has a low selectivity. Furthermore, our approach is applicable to most compressed XML formats too, which may prevent swapping when a large XML document does not fit into main memory but its compressed representation does.