Chapter 24 - Efficient Structural Joins on Indexed XML Documents

Abstract

This chapter proposes efficient structural join algorithms in the presence of tag indices. Queries on XML documents typically combine selections on element contents, and, via path expressions, the structural relationships between tagged elements. Structural joins are used to find all pairs of elements satisfying the primitive structural relationships specified in the query, namely, parent–child and ancestor descendant relationships. Efficient support for structural joins is thus the key to efficient implementations of XML queries. The recently proposed node numbering schemes enable the capturing of the XML document structure using traditional indices. The problem of managing and querying XML documents efficiently pose interesting challenges for database researchers. XML documents can have a rather complex internal structure; in fact, an XML document can be viewed as an ordered tree. Tree nodes correspond to document elements (or attributes) while edges represent direct element–subelement relationships. This tree-centric representation is apparent in XML languages like Quilt, XQuery, and XPath, which qualify documents for retrieval both by their structure and the values in their elements.