Algebras for querying text regions (extended abstract)

There is a significant amount of interest in combining and extending database and information retrieval technologies to manage textual data. The challenge is becoming more relevant due to the increased availability of documents in digital form. Document data has a natural hierarchical structure, which may be made explicit due to the use of markup conventions (as it is the case with SGML). An important aspect of managing structured and semi-structured textual data consists of supporting the efficient retrieval of text components based both on their content and structure. In this paper we study issues related to the expressive power and optimization of a class of algebras that support combining string (or pattern) searches with queries on the hierarchical structure of the text. The region algebra studied is a set-at-a-time algebra for manipulating -tezt regions (substrings of the text) that supports finding out nesting and ordering properties of the text regions. The region algebra is part of the language in use in commercial text retrieval systems, and can be implemented very efficiently. The results in this work are obtained by showing a close relationship between the region algebra and the monadic first order theory of binary trees. We show that queries in the algebra can be optimized, but the optimization can be difficult (Co-NP-Hard in the general case, although there is an important class of queries that can be optimized in polynomial time). On the negative side, we show that the language is incapable of capturing some important properties of the text structure, related to the nesting and ordering of text regions. We conclude by suggesting possible extensions *Research done while the author was at the University of Toronto. Permission to copy without fee all or part of this material is granted provided that the copies are not made or distributed for direct commercial advantage, the ACM copyright notice and the title of the publication and its date appear, and notice ISgiven that copying is by permission of the Association of Computing Machinery.To copy otherwise, or to republish, requires a fee and/or specific permission, PODS ’95 San Jose CA USA (3 1995 ACM 0-89791 -730-8/95/0005.. $3.50 Tova Mile* Department of Computer Science

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