Tractable Hypergraph Properties for Constraint Satisfaction and Conjunctive Queries
An important question in the study of constraint satisfaction problems (CSP) is understanding how the graph or hypergraph describing the incidence structure of the constraints influences the complexity of the problem. For binary CSP instances (that is, where each constraint involves only two variabl...
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Veröffentlicht in: | Journal of the ACM 2013-11, Vol.60 (6), p.1-51 |
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Sprache: | eng |
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Zusammenfassung: | An important question in the study of constraint satisfaction problems (CSP) is understanding how the graph or hypergraph describing the incidence structure of the constraints influences the complexity of the problem. For binary CSP instances (that is, where each constraint involves only two variables), the situation is well understood: the complexity of the problem essentially depends on the treewidth of the graph of the constraints [Grohe 2007; Marx 2010b]. However, this is not the correct answer if constraints with unbounded number of variables are allowed, and in particular, for CSP instances arising from query evaluation problems in database theory. Formally, if
H
is a class of hypergraphs, then let CSP(
H
) be CSP restricted to instances whose hypergraph is in
H
. Our goal is to characterize those classes of hypergraphs for which CSP(
H
) is polynomial-time solvable or fixed-parameter tractable, parameterized by the number of variables. Note that in the applications related to database query evaluation, we usually assume that the number of variables is much smaller than the size of the instance, thus parameterization by the number of variables is a meaningful question.
The most general known property of
H
that makes CSP(
H
) polynomial-time solvable is bounded fractional hypertree width. Here we introduce a new hypergraph measure called
submodular width
, and show that bounded submodular width of
H
(which is a strictly more general property than bounded fractional hypertree width) implies that CSP(
H
) is fixed-parameter tractable. In a matching hardness result, we show that if
H
has unbounded submodular width, then CSP(
H
) is not fixed-parameter tractable (and hence not polynomial-time solvable), unless the Exponential Time Hypothesis (ETH) fails. The algorithmic result uses tree decompositions in a novel way: instead of using a single decomposition depending on the hypergraph, the instance is split into a set of instances (all on the same set of variables as the original instance), and then the new instances are solved by choosing a different tree decomposition for each of them. The reason why this strategy works is that the splitting can be done in such a way that the new instances are “uniform” with respect to the number extensions of partial solutions, and therefore the number of partial solutions can be described by a submodular function. For the hardness result, we prove via a series of combinatorial results that if a hypergraph
H
has large s |
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ISSN: | 0004-5411 1557-735X |
DOI: | 10.1145/2535926 |