eng
Schloss Dagstuhl – Leibniz-Zentrum für Informatik
Leibniz International Proceedings in Informatics
1868-8969
2023-03-17
15:1
15:19
10.4230/LIPIcs.ICDT.2023.15
article
Approximation and Semantic Tree-Width of Conjunctive Regular Path Queries
Figueira, Diego
1
https://orcid.org/0000-0003-0114-2257
Morvan, Rémi
1
https://orcid.org/0000-0002-1418-3405
Univ. Bordeaux, CNRS, Bordeaux INP, LaBRI, UMR5800, F-33400 Talence, France
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'.
https://drops.dagstuhl.de/storage/00lipics/lipics-vol255-icdt2023/LIPIcs.ICDT.2023.15/LIPIcs.ICDT.2023.15.pdf
graph databases
conjunctive regular path queries
semantic optimization
tree-width
containment
approximation