Approximating fractional hypertree width

Abstract

Fractional hypertree width is a hypergraph measure similar to tree width and hypertree width. Its algorithmic importance comes from the fact that, as shown in previous work [14], constraint satisfaction problems (CSP) and various problems in database theory are polynomial-time solvable if the input contains a bounded-width fractional hypertree decomposition of the hypergraph of the constraints. In this paper, we show that for every w ≥ 1, there is a polynomial-time algorithm that, given a hypergraph H with fractional hypertree width at most w, computes a fractional hypertree decomposition of width O(w3) for H. This means that polynomial-time algorithms relying on bounded-width fractional hypertree decompositions no longer need to be given a decomposition explicitly in the input, since an appropriate decomposition can be computed in polynomial time. Therefore, if H is a class of hypergraphs with bounded fractional hypertree width, then CSP restricted to instances whose structure is in H is polynomial-time solvable. This makes bounded fractional hypertree width the most general known hypergraph property that makes CSP, Boolean Conjuctive Queries, and Conjunctive Query Containment polynomial-time solvable.