This paper describes the overall design and architecture of the Timber XML database system currently being implemented at the University of Michigan. The system is based upon a bulk algebra for manipulating trees, and natively stores XML. New access methods have been developed to evaluate queries in the XML context, and new cost estimation and query optimization techniques have also been developed. We present performance numbers to support some of our design decisions. We believe that the key intellectual contribution of this system is a comprehensive set-at-a-time query processing ability in a native XML store, with all the standard components of relational query processing, including algebraic rewriting and a cost-based optimizer.

We are addressing the ecient processing of continuous XML streams, in which the server broadcasts XML data to multiple clients concurrently through a multicast data stream, while each client is fully responsible for processing the stream.

Most current information management systems can be classified into text retrieval systems, relational/object database systems, or semistructured/XML database systems. However, in practice, many applications data sets involve a combination of free text, structured data, and semistructured data. Hence, integration of different types of information management systems has been, and continues to be, an active research topic. In this paper, we present a short survey of prior work on integrating and inter-operating between text, structured, and semistructured database systems. We classify existing literature based on the kinds of systems being integrated and the approach to integration. Based on this classification, we identify the challenges and the key themes underlying existing work in this area.

XML permits repeated and missing sub-elements, and missing attributes. We discuss the consequent implications on grouping, both with respect to specification and with respect to implementation. The techniques described here have been implemented in the TIMBER native XML database system being developed at the University of Michigan.

An extent join to compute path expressions containing parent-children and ancestordescendent operations and two path expression optimization rules, path-shortening and path-complementing, are presented in this paper. Path-shortening reduces the number of joins by shortening the path while path-complementing optimizes the path execution by using an equivalent complementary path expression to compute the original one. Experimental results show that the algorithms proposed are more efficient than traditional algorithms.

Keeping views fresh by maintaining the consistency between materialized views and their base data in the presence of base updates is a critical prob-lem for many applications, including data warehousing and data integra-tion. While heavily studied for traditional databases, the maintenance of XML views remains largely unexplored. Maintaining XML views is com-plex due to the richness of the XML data model and the powerful capabili-ties of XML query languages, such as XQuery. This dissertation proposes a comprehensive solution for the general problem of maintaining materialized XQuery views. Our solution is the first to enable the maintenance of a large class of XQuery views including XPath expressions, FLWOR expressions, and Element Constructors. These views may contain arbitrary result construction and arbitrary grouping and join operations. Our solution also supports the unique order requirements of XQuery including source document order and query order.

Abstract. The research community is addressing a number of issues in response to an increased reliance of organisations on data warehousing. Most work addresses aspects related to the internal operation of a data warehouse server, such as selection of views to materialise, maintenance of aggregate views and performance of OLAP queries. Issues related to data warehouse maintenance, i.e. how changes to autonomous sources should be detected and propagated to a warehouse, have been addressed in a fragmented manner. We have shown earlier that a number of maintenance policies based on source characteristics and timing are relevant and meaningful to single source views. In this report we detail how this work has been extended for multiple sources. We focus on exploring policies for data integration from heterogeneous sources. As the number of policies is very large, we first analyse their behaviour intuitively with respect to broader source and policy characteristics. Further, we extend the single source cost model to these policies and incorporate it into a Policy Analyser for Multiple sources (PAM). We use this to analyse the effect of source characteristics and join alternatives on various policies. We have developed a Testbed for Maintenance of Integrated Data (TMID). We report on experiments conducted to validate the policies that are recommended by the tool, and confirm our initial analysis. Finally, we distil a set of heuristics for the selection of multi-source policies based on quality of service and other requirements. 1.

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We present algebraic equivalences that allow to unnest nested algebraic expressions for order-preserving algebraic operators. We illustrate how these equivalences can be applied successfully to unnest nested queries given in the XQuery language. Measurements illustrate the performance gains possible our approach.

XML permits repeated and missing sub-elements, and missing attributes. We discuss the consequent implications on grouping, both with respect to specification and with respect to implementation. The techniques described here have been implemented in the TIMBER native XML database system being developed at the University of Michigan.