SF-Tree: An Efficient and Flexible Structure for Estimating Selectivity of Simple Path Expressions with Statistical Accuracy Guarantee

Abstract

Estimating the selectivity of a simple path expression (SPE) is essential for selecting the most efficient evaluation plans for XML queries. To estimate selectivity, we need an efficient and flexible structure to store a summary of the path expressions that are present in an XML document collection. In this paper we propose a new structure called SF-Treeto address the selectivity estimation problem. SF-Tree provides a flexible way for the users to choose among accuracy, space requirement and selectivity retrieval speed. It makes use of signature files to store the SPEs in a tree form to increase the selectivity retrieval speed and the accuracy of the retrieved selectivity. Our analysis shows that the probability that a selectivity estimation error occurs decreases exponentially with respect to the error size.