Efficient Retrieval of Similar Time Sequences Using DFT

We propose an improvement of the known DFT-based indexing technique for fast retrieval of similar time sequences. We use the last few Fourier coefficients in the distance computation without storing them in the index since every coefficient at the end is the complex conjugate of a coefficient at the beginning and as strong as its counterpart. We show analytically that this observation can accelerate the search time of the index by more than a factor of two. This result was confirmed by our experiments, which were carried out on real stock prices and synthetic data

Visualizing the World Wide Web

We discuss some principles that we believe are important in creating useful visualizations of the World Wide Web. They are: layout, abstraction, focus, and interaction. We illustrate these points with examples from the work of our group at the University of Toronto. Introduction The World Wide Web[BLCL + 94] is a large distributed hypermedia network implemented on the global Internet. To give an idea of the size of this network, as of early 1996 one popular index server reports having indexed 21 million Web pages[Cor96]. Not surprisingly, many researchers have proposed visualization as a technology to address the severe problems of disorientation and information overload that are caused by interacting with such a large and chaotic information space. See, for example, [Doe94, And95, AS95, GS95, TL95, HDWB95, MDH95]. In this paper we discuss some principles, based on our experience with information visualization in other domains as well as in the Web domain, that we believe are impor..

Concise descriptions of subsets of structured sets

We study the problem of economical representation of subsets of structured sets, that is, sets equipped with a set cover. Given a structured set U, and a language L whose expressions define subsets of U, the problem of Minimum Description Length in L (L-MDL) is: “given a subset V of U, find a shortest string in L that defines V ”. We show that the simple set cover is enough to model a number of realistic database structures. We focus on two important families: hierarchical and multidimensional organizations. The former is found in the context of semistructured data such as XML, the latter in the context of statistical and OLAP databases. In the case of general OLAP databases, data organization is a mixture of multidimensionality and hierarchy, which can also be viewed naturally as a structured set. We study the complexity of the L-MDL problem in several settings, and provide an efficient algorithm for the hierarchical case. Finally, we illustrate the application of the theory to summarization of large result sets, (multi) query optimization for ROLAP queries, and XML queries. 1

Indexing temporal XML documents

Different models have been proposed recently for representing temporal data, tracking historical information, and recovering the state of the document as of any given time, in XML documents. We address the problem of indexing temporal XML documents. In particular we show that by indexing continuous paths, i.e. paths that are valid continuously during a certain interval in a temporal XML graph, we can dramatically increase query performance. We describe in detail the indexing scheme, denoted TempIndex, and compare its performance against both a system based on a nontemporal path index, and one based on DOM.

Formal Models Of Web Queries

We present a new formal model of query and computation on the Web. We focus on two important aspects that distinguish the access to Web data from the access to a standard database system: the navigational nature of the access and the lack of concurrency control. We show that these two issues have significant effects on the computability of queries. To illustrate the ideas and how they can be used in practice for designing appropriate Web query languages, we consider a particular query language, the Web calculus, an abstraction and extension of the practical Web query language WebSQL. c fl1998 Elsevier Science Ltd. All rights reserved Key words: World Wide Web, Web Queries, Query Languages, Computability, Formal Models 1. INTRODUCTION Tools and techniques for retrieving information from the World Wide Web are rapidly being developed [9, 10, 13, 4, 12, 8]. Most of these works are based on the metaphor of the Web as a database, in order to carry over and adapt familiar query languages s..