Complexity and a Method of Extracting a Database Schema over Semistructured Documents

概要

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Semistructured data comprises irregular structure and has no a-priori database schema, therefore we encounter several problems such as inefficient data retrieval and wasteful data storage. To cope with such problems, some schema extraction algorithms over semistructured data have been proposed, in which data is modeled as an unordered tree. However, the order of elements is indispensable for document data, therefore we consider extracting all optimal database schema over an ordered tree. We consider an optimization problem to extract a smallest database schema such that the density of each class is no less than a given threshold. where the density of a class represents a similarity between the type of the class and those of the objects in the class We first prove that the corresponding decision problem is strongly NP-complete, and show that another version of the problem is strongly NP-hard and belongs to Δ_2P. Then we showthat for any r <3/2, there is no polynomial-time r-approximation algorithm that solves the optimization problem unless P = NP. Finally, we propose a kind of class called bounded class that can be constructed efficiently then show a polynomial-time algorithm for constructing a database schema by using bounded classes.
社団法人電子情報通信学会の論文
2002-06-01