Efficient path index and extraction of structural information for XML data

Abstract

Due to its flexibility, XML is becoming the de facto standard for exchanging and querying documents over the Web. The key distinction between data in XML and data in traditional models is that XML is not rigidly structured. Because of the self-describing feature of XML data, XML can naturally model irregularities that cannot be modeled by relational or object-oriented data. Thus, new structures such as path indexes that improve the XML query performance are needed. Also, to describe the XML data structure, some languages are proposed. Many XML query languages such as XQuery and XPath use label paths to traverse the irregularly structured XML data. Without a structural summary and efficient indexes, query processing can be quite inefficient due to an exhaustive traversal on XML data. To overcome the inefficiency, several path indexes have been proposed in the research community. Traditional indexes generally record all label paths from the root element in XML data and are constructed with the use of data only. Such path indexes may result in performance degradation due to large sizes and exhaustive navigations for partial matching path queries which start with the self-or-descendent axis("//"). To improve the query performance, we propose an adaptive path index for XML data (termed APEX). APEX does not keep all paths starting from the root and utilizes frequently used paths on query workloads. APEX also has a nice property that it can be updated incrementally according to the changes of query workloads. Experimental results with synthetic and real-life data sets clearly confirm that APEX improves the query processing cost typically 2 to 69 times compared with the traditional indexes, with the performance gap increasing with the irregularity of XML data. To specify the constraint on the XML data structure, many languages such as Document Type Definition (DTD), Document Content Description (DCD), and XML Schema have been proposed. The schema described by these str...