XSKETCH synopses for XML data graphs

Abstract

Effective support for XML query languages is becoming increasingly important with the emergence of new applications that access large volumes of XML data. All existing proposals for querying XML (e.g., XQuery) rely on a pattern-specification language that allows (1) path navigation and branching through the label structure of the XML data graph, and (2) predicates on the values of specific path/branch nodes, in order to reach the desired data elements. Clearly, optimizing such queries requires approximating the result cardinality of the referenced paths and hence hinges on the existence of concise synopsis structures that enable accurate compile-time selectivity estimates for complex path expressions over the base XML data. In this article, we introduce a novel approach to building and using statistical summaries of large XML data graphs for effective path-expression selectivity estimation. Our proposed graph-synopsis model (termed XSketch) exploits localized graph stability and value-distribution summaries (e.g., histograms) to accurately approximate (in limited space) the path and branching distribution, as well as the complex correlation patterns that can exist between and across path structure and element values in the data graph. To the best of our knowledge, ours is the first work to address this timely problem in the most general setting of graph-structured XML data with values, and complex (branching) path expressions.