Efficient processing of XML twig patterns with parent child edges: a look-ahead approach

With the growing importance of semi-structure data in information exchange, much research has been done to provide an effective mechanism to match a twig query in an XML database. A number of algorithms have been proposed recently to process a twig query holistically. Those algorithms are quite efficient for quires with only ancestor-descendant edges. But for queries with mixed ancestor-descendant and parent-child edges, the previous approaches still may produce large intermediate results, even when the input and output size are more manageable. To overcome this limitation, in this paper, we propose a novel holistic twig join algorithm, namely <i>TwigStackList</i>. Our main technique is to look-ahead read some elements in input data steams and cache limited number of them to <i>lists</i> in the main memory. The number of elements in any list is bounded by the length of the longest path in the XML document. We show that <i>TwigStackList</i> is I/O optimal for queries with only ancestor-descendant relationships below branching nodes. Further, even when queries contain parent-child relationship below branching nodes, the set of intermediate results in <i>TwigStackList</i> is guaranteed to be a subset of that in previous algorithms. We complement our experimental results on a range of real and synthetic data to show the significant superiority of <i>TwigStackList</i> over previous algorithms for queries with <i>parent</i>-<i>child</i> relationships.