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).
翻译:论文研究了重写问题,即,对于某一组合查询(QQ$)和一套观点($mathcal{V}),对于查询、意见和(或)所希望的重写本身是周期性的、甚至更受限制的情况,是否有一个折叠查询(Q$)等于美元的问题,即决定问题,对于某一组合查询(Q$)和一套观点($mathcal{V}),是否有一个折叠查询(QQ$)等于$$$(Mathcal{V})的折叠查询(Q),对于折叠问题的复杂性,本文确定了可移动和(估计)棘手的重写问题变体之间的界线:对于交错的精度,循环重写问题是难以解决的,即使有重写(QQ$)和折叠式重写($\mathcal{V}的折叠件是循环的或等级问题。然而,如果观点是自由循环的,(chollical),(如果是循环的,则在循环中),(collical-viii)还是一个循环的。 (cal-he) (cal) (cal) (colviii) (colviii)。