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arXiv · 2201.05129

Rewriting with Acyclic Queries: Mind Your Head

Gaetano Geck·Jens Keppeler·Thomas Schwentick·Christopher Spinrath
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Abstract

The paper studies the rewriting problem, that is, the decision problem whether, for a given conjunctive query $Q$ and a set $\mathcal{V}$ of views, there is a conjunctive query $Q'$ over $\mathcal{V}$ that is equivalent to $Q$, for cases where the query, the views, and/or the desired rewriting are acyclic or even more restricted. It shows that, if $Q$ itself is acyclic, an acyclic rewriting exists if there is any rewriting. An analogous statement also holds for free-connex acyclic, hierarchical, and q-hierarchical queries. Regarding the complexity of the rewriting problem, the paper identifies a border between tractable and (presumably) intractable variants of the rewriting problem: for schemas of bounded arity, the acyclic rewriting problem is NP-hard, even if both $Q$ and the views in $\mathcal{V}$ are acyclic or hierarchical. However, it becomes tractable if the views are free-connex acyclic (i.e., in a nutshell, their body is (i) acyclic and (ii) remains acyclic if their head is added as an additional atom).

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Gaetano Geck, Jens Keppeler, Thomas Schwentick, Christopher Spinrath. 2022-01-13. Rewriting with Acyclic Queries: Mind Your Head. https://doi.org/10.46298/lmcs-19(4%3A17)2023

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