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  • 标题:How Many Variables Are Needed to Express an Existential Positive Query?
  • 本地全文:下载
  • 作者:Simone Bova ; Hubie Chen
  • 期刊名称:LIPIcs : Leibniz International Proceedings in Informatics
  • 电子版ISSN:1868-8969
  • 出版年度:2017
  • 卷号:68
  • 页码:9:1-9:16
  • DOI:10.4230/LIPIcs.ICDT.2017.9
  • 出版社:Schloss Dagstuhl -- Leibniz-Zentrum fuer Informatik
  • 摘要:The number of variables used by a first-order query is a fundamental measure which has been studied in numerous contexts, and which is known to be highly relevant to the task of query evaluation. In this article, we study this measure in the context of existential positive queries. Building on previous work, we present a combinatorial quantity defined on existential positive queries; we show that this quantity not only characterizes the minimum number of variables needed to express a given existential positive query by another existential positive query, but also that it characterizes the minimum number of variables needed to express a given existential positive query, over all first-order queries. Put differently and loosely, we show that for any existential positive query, no variables can ever be saved by moving out of existential positive logic to first-order logic. One component of this theorem's proof is the construction of a winning strategy for a certain Ehrenfeucht-Fraiissé type game.
  • 关键词:existential positive queries; finite-variable logics; first-order logic; query optimization
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