252 research outputs found
Conjunctive Regular Path Queries under Injective Semantics
We introduce injective semantics for Conjunctive Regular Path Queries
(CRPQs), and study their fundamental properties. We identify two such
semantics: atom-injective and query-injective semantics, both defined in terms
of injective homomorphisms. These semantics are natural generalizations of the
well-studied class of RPQs under simple-path semantics to the class of CRPQs.
We study their evaluation and containment problems, providing useful
characterizations for them, and we pinpoint the complexities of these problems.
Perhaps surprisingly, we show that containment for CRPQs becomes undecidable
for atom-injective semantics, and PSPACE-complete for query-injective
semantics, in contrast to the known EXPSPACE-completeness result for the
standard semantics. The techniques used differ significantly from the ones
known for the standard semantics, and new tools tailored to injective semantics
are needed. We complete the picture of complexity by investigating, for each
semantics, the containment problem for the main subclasses of CRPQs, namely
Conjunctive Queries and CRPQs with finite languages.Comment: Accepted in the Proceedings of the 42nd ACM SIGMOD-SIGACT-SIGAI
Symposium on Principles of Database Systems (PODS '23
Eliminating Recursion from Monadic Datalog Programs on Trees
We study the problem of eliminating recursion from monadic datalog programs
on trees with an infinite set of labels. We show that the boundedness problem,
i.e., determining whether a datalog program is equivalent to some nonrecursive
one is undecidable but the decidability is regained if the descendant relation
is disallowed. Under similar restrictions we obtain decidability of the problem
of equivalence to a given nonrecursive program. We investigate the connection
between these two problems in more detail
The Dichotomy of Evaluating Homomorphism-Closed Queries on Probabilistic Graphs
We study the problem of probabilistic query evaluation on probabilistic
graphs, namely, tuple-independent probabilistic databases on signatures of
arity two. Our focus is the class of queries that is closed under
homomorphisms, or equivalently, the infinite unions of conjunctive queries. Our
main result states that all unbounded queries from this class are #P-hard for
probabilistic query evaluation. As bounded queries from this class are
equivalent to a union of conjunctive queries, they are already classified by
the dichotomy of Dalvi and Suciu (2012). Hence, our result and theirs imply a
complete data complexity dichotomy, between polynomial time and #P-hardness,
for evaluating infinite unions of conjunctive queries over probabilistic
graphs. This dichotomy covers in particular all fragments of infinite unions of
conjunctive queries such as negation-free (disjunctive) Datalog, regular path
queries, and a large class of ontology-mediated queries on arity-two
signatures. Our result is shown by reducing from counting the valuations of
positive partitioned 2-DNF formulae for some queries, or from the
source-to-target reliability problem in an undirected graph for other queries,
depending on properties of minimal models. The presented dichotomy result
applies to even a special case of probabilistic query evaluation called
generalized model counting, where fact probabilities must be 0, 0.5, or 1.Comment: 30 pages. Journal version of the ICDT'20 paper
https://drops.dagstuhl.de/opus/volltexte/2020/11939/. Submitted to LMCS. The
previous version (version 2) was the same as the ICDT'20 paper with some
minor formatting tweaks and 7 extra pages of technical appendi
The fine classification of conjunctive queries and parameterized logarithmic space
We perform a fundamental investigation of the complexity of conjunctive query evaluation from the perspective of parameterized complexity. We classify sets of boolean conjunctive queries according to the complexity of this problem. Previous work showed that a set of conjunctive queries is fixed-parameter tractable precisely when the set is equivalent to a set of queries having bounded treewidth. We present a fine classification of query sets up to parameterized logarithmic space reduction. We show that, in the bounded treewidth regime, there are three complexity degrees and that the properties that determine the degree of a query set are bounded pathwidth and bounded tree depth. We also engage in a study of the two higher degrees via logarithmic space machine characterizations and complete problems. Our work yields a significantly richer perspective on the complexity of conjunctive queries and, at the same time, suggests new avenues of research in parameterized complexity
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