Selectivity Estimation for XML Twigs

Twig queries represent the building blocks of declarative query languages over XML data. A twig query describes a complex traversal of the document graph and generates a set of element tuples based on the intertwined evaluation (i.e., join) of multiple path expressions. Estimating the result cardinality of twig queries or, equivalently, the number of tuples in such a structural (path-based) join, is a fundamental problem that arises in the optimization of declarative queries over XML. It is crucial, therefore, to develop concise synopsis structures that summarize the document graph and enable such selectivity estimates within the time and space constraints of the optimizer. In this paper, we propose novel summarization and estimation techniques for estimating the selectivity of twig queries with complex XPath expressions over tree-structured data. Our approach is based on the XSKETCH model, augmented with new types of distribution information for capturing complex correlation patterns...

• Type: Conference
• Year: 2004
• Where: ICDE
• Authors: Neoklis Polyzotis, Minos N. Garofalakis, Yannis E. Ioannidis
• Keywords: Database, Declarative Queries, ICDE 2004, Selectivity Estimates, Twig Queries