使用无环查询进行重写：注意你的头部
Rewriting with Acyclic Queries: Mind Your Head
AI总结：
本文研究合取查询在无环等受限条件下的重写问题，证明了查询本身无环时存在无环重写，并界定了该问题在 NP-hard 与易处理之间的复杂度边界。
摘要：
本文研究重写问题，即对于给定的合取查询 $Q$ 和一组视图 $\mathcal{V}$，是否存在一个基于 $\mathcal{V}$ 且等价于 $Q$ 的合取查询 $Q'$ 的判定问题，适用于查询、视图和/或所需重写为无环甚至更受限的情况。研究表明，如果 $Q$ 本身是无环的，则只要存在任何重写，就存在无环重写。类似的结论也适用于 free-connex acyclic、hierarchical 和 q-hierarchical 查询。关于重写问题的复杂性，本文界定了重写问题的易处理和（推测的）不易处理变体之间的边界：对于有界元数的模式，无环重写问题是 NP-hard 的，即使 $Q$ 和 $\mathcal{V}$ 中的视图都是无环或 hierarchical 的。然而，如果视图是 free-connex acyclic 的（简言之，其主体是 (i) 无环的，并且 (ii) 如果将其头部作为附加原子加入后仍保持无环），则该问题变得易处理。
英文摘要：
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).
机构、总结、中文摘要均由AI分析而来，可能存在错误，请以论文正文为准。
