2026-07-22T07:15:46Z https://drops.dagstuhl.de/oai

oai:drops-oai.dagstuhl.de:17757 2024-03-06T11:00:25Z ddc:004 open_access

Approximation and Semantic Tree-Width of Conjunctive Regular Path Queries Figueira, Diego Morvan, Rémi graph databases conjunctive regular path queries semantic optimization tree-width containment approximation 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 Barceló, Romero, and Vardi [Pablo Barceló et al., 2016] has shown decidability for the case k = 1, and here we show that decidability in fact holds for any arbitrary k > 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 + ... + a_n) we show that the complexity of the problem drops to Π^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'. Schloss Dagstuhl – Leibniz-Zentrum für Informatik Diego Figueira and Rémi Morvan 2023 Is Part Of LIPIcs, Volume 255, 26th International Conference on Database Theory (ICDT 2023) InProceedings Text doc-type:ResearchArticle publishedVersion application/pdf doi:10.4230/LIPIcs.ICDT.2023.15 urn:nbn:de:0030-drops-177575 https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.ICDT.2023.15 eng https://creativecommons.org/licenses/by/4.0/legalcode