•Note: a conjunctive query = a
hypergraph
•A tree decomposition of a
conjunctive query (or hypergraph) with variables (nodes) V is a
tree T, and a set Bt µ V for each node t in T such that:
–For every x 2 V,
the set {t | x 2 Bt} is connected
–Every hyperedge of the query
(hypergraph) is contained in some Bt