Splitting Spanner Atoms: A Tool for Acyclic Core Spanners

Authors Dominik D. Freydenberger , Sam M. Thompson

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Dominik D. Freydenberger
  • Loughborough University, UK
Sam M. Thompson
  • Loughborough University, UK


The authors would like to thank Justin Brackemann, and the anonymous reviewers for all their helpful comments and suggestions.

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Dominik D. Freydenberger and Sam M. Thompson. Splitting Spanner Atoms: A Tool for Acyclic Core Spanners. In 25th International Conference on Database Theory (ICDT 2022). Leibniz International Proceedings in Informatics (LIPIcs), Volume 220, pp. 10:1-10:18, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2022)


This paper investigates regex CQs with string equalities (SERCQs), a subclass of core spanners. As shown by Freydenberger, Kimelfeld, and Peterfreund (PODS 2018), these queries are intractable, even if restricted to acyclic queries. This previous result defines acyclicity by treating regex formulas as atoms. In contrast to this, we propose an alternative definition by converting SERCQs into FC-CQs - conjunctive queries in FC, a logic that is based on word equations. We introduce a way to decompose word equations of unbounded arity into a conjunction of binary word equations. If the result of the decomposition is acyclic, then evaluation and enumeration of results become tractable. The main result of this work is an algorithm that decides in polynomial time whether an FC-CQ can be decomposed into an acyclic FC-CQ. We also give an efficient conversion from synchronized SERCQs to FC-CQs with regular constraints. As a consequence, tractability results for acyclic relational CQs directly translate to a large class of SERCQs.

Subject Classification

ACM Subject Classification
  • Theory of computation → Complexity theory and logic
  • Document spanners
  • information extraction
  • conjunctive queries


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