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
.