Approximate XML Query Answers
The rapid adoption of XML as the standard for data representation and exchange foreshadows a massive increase in the amounts of XML data collected, maintained, and queried over the Internet or in large corporate data-stores. Inevitably, this will result in the development of on-line decision support systems, where users and analysts interactively explore large XML data sets through a declarative query interface (e.g., XQuery or XSLT). Given the importance of remaining interactive, such on-line systems can employ approximate query answers as an effective mechanism for reducing response time and providing users with early feedback. This approach has been successfully used in relational systems and it becomes even more compelling in the XML world, where the evaluation of complex queries over massive tree-structured data is inherently more expensive.In this paper, we initiate a study of approximate query answering techniques for large XML databases. Our approach is based on a novel, conceptually simple, yet very effective XML-summarization mechanism: TREESKETCH synopses. We demonstrate that, unlike earlier techniques focusing solely on selectivity estimation, our TREESKETCH synopses are much more effective in capturing the complete tree structure of the underlying XML database. We propose novel construction algorithms for building TREESKETCH summaries of limited size, and describe schemes for processing general XML twig queries over a concise TREESKETCH in order to produce very fast, approximate tree-structured query answers. To quantify the quality of such approximate answers, we propose a novel, intuitive error metric that captures the quality of the approximation in terms of both the overall structure of the XML tree and the distribution of document edges. Experimental results on real-life and synthetic data sets verify the effectiveness of our TREESKETCH synopses in producing fast, accurate approximate answers and demonstrate their benefits over previously proposed techniques that focus solely on selectivity estimation. In particular, TREESKETCHes yield faster, more accurate approximate answers and selectivity estimates, and are more efficient to construct. To the best of our knowledge, ours is the first work to address the timely problem of producing fast, approximate tree-structured answers for complex XML queries.