Semantic Tree-Width and Path-Width of Conjunctive Regular Path Queries

TL;DR

This paper proves the decidability of checking equivalence to queries of bounded tree-width in UC2RPQs, extends previous results, and introduces algorithms for approximation and equivalence testing related to tree-width and path-width.

Contribution

It extends decidability results for UC2RPQs to arbitrary tree-widths, introduces algorithms for maximal under-approximations, and enhances methods for equivalence testing based on tree-width.

Findings

Decidability of query equivalence for any tree-width $k$ in UC2RPQs.

Algorithm complexity drops to $ ext{P}_2^{ ext{NP}}$ for certain regular expressions.

Robust approach for testing equivalence with queries of given path-width.

Abstract

We show that the problem of whether a query is equivalent to a query of tree-width $k$ is decidable, for the class of Unions of Conjunctive Regular Path Queries with two-way navigation (UC2RPQs). A previous result by Barcel\'o, Romero, and Vardi [SIAM Journal on Computing, 2016] has shown decidability for the case $k=1$, and here we extend this result showing that decidability in fact holds for any arbitrary $k\geq 1$. The algorithm is in 2ExpSpace, but for the restricted but practically relevant case where all regular expressions of the query are of the form $a^*$ or $(a_1 + \dotsb + a_n)$ we show that the complexity of the problem drops to $\Pi^P_2$. We also investigate the related problem of approximating a UC2RPQ by queries of small tree-width. We exhibit an algorithm which, for any fixed number $k$, builds the maximal under-approximation of tree-width $k$ of a UC2RPQ. The maximal under-approximation of tree-width $k$ of a query $q$ is a query $q'$ of tree-width $k$ which is contained in $q$ in a maximal and unique way, that is, such that for every query $q''$ of tree-width $k$, if $q''$ is contained in $q$ then $q''$ is also contained in $q'$. Our approach is shown to be robust, in the sense that it allows also to test equivalence with queries of a given path-width, it also covers the previously known result for $k=1$, and it allows to test for equivalence of whether a (one-way) UCRPQ is equivalent to a UCRPQ of a given tree-width (or path-width).