Complexity Analysis of Generalized and Fractional Hypertree Decompositions

Gottlob, Georg; Lanzinger, Matthias; Pichler, Reinhard; Razgon, Igor

Abstract

Hypertree decompositions (HDs), as well as the more powerful generalized hypertree decompositions (GHDs), and the yet more general fractional hypertree decompositions (FHDs) are hypergraph decomposition methods successfully used for answering conjunctive queries and for solving constraint satisfaction problems. Every hypergraph H has a width relative to each of these methods: its hypertree width hw(H), its generalized hypertree width ghw(H), and its fractional hypertree width fhw(H), respectively. It is known that hw(H)≤ k can be checked in polynomial time for fixed k, while checking ghw(H)≤ k is NP-complete for k ≥ 3. The complexity of checking fhw(H)≤ k for a fixed k has been open for over a decade.
We settle this open problem by showing that checking fhw(H)≤ k is NP-complete, even for k=2. The same construction allows us to prove also the NP-completeness of checking ghw(H)≤ k for k=2. After that, we identify meaningful restrictions that make checking for bounded ghw or fhw tractable or allow for an efficient approximation of the fhw.

Citation

Gottlob, G., Lanzinger, M., Pichler, R., & Razgon, I. (2021). Complexity Analysis of Generalized and Fractional Hypertree Decompositions. Journal of the ACM, 68(5), 1-50. https://doi.org/10.1145/3457374

• Journal Article Type: Article
• Acceptance Date: Mar 1, 2021
• Online Publication Date: Sep 13, 2021
• Publication Date: Oct 31, 2021
• Deposit Date: Sep 27, 2024
• Journal: Journal of the ACM
• Print ISSN: 0004-5411
• Electronic ISSN: 1557-735X
• Publisher: Association for Computing Machinery (ACM)
• Peer Reviewed: Peer Reviewed
• Volume: 68
• Issue: 5
• Article Number: 38
• Pages: 1-50
• DOI: https://doi.org/10.1145/3457374
• Public URL: https://durham-repository.worktribe.com/output/2879986