Holistic Twig Joins on Indexed XML Documents

Abstract

XML is emerging as a de facto standard for information exchange over the Internet. Although XML documents could have rather complex internal structures, they can generally modeled as ordered trees. Finding all the occurrences of a twig pattern specified by a selection predicate on multiple elements in an XML document is a core operation for efficient evaluation of XML queries. Holistic twig join algorithms were proposed as an optimal solution when the twig pattern only involves ancestor-descendant relationships. The chapter addresses the problem of efficient processing of holistic twig joins on all/partly indexed XML documents. It proposes an algorithm that utilizes available indices on element sets. While it can be shown analytically that the proposed algorithm is as efficient as the existing state-of-the-art algorithms in terms of worst case I/O and CPU cost, experimental results on various datasets indicate that the proposed index-based algorithm performs significantly better than the existing ones, especially when binary structural joins in the twig pattern have varying join selectivities.