Tree-width and functional dependencies in databases

Conjunctive query (CQ) evaluation on relational databases is NP-complete in general. Several restrictions, like bounded tree-width and bounded hypertree-width, allow polynomial time evaluations. We extend the framework in the presence of functional dependencies. Our extended CQ evaluation problem has a concise equivalent formulation in terms of the homomorphism problem (HOM) for non-relational structures. We introduce the notions of closure tree-width and hyperclosure tree-width for arbitrary structures, and we prove that HOM (and hence CQ) restricted to bounded (hyper)closure tree-width becomes tractable. There are classes of structures with bounded closure tree-width but unbounded tree-width. Similar statements hold for hyperclosure tree-width and hypertree-width, and for hyperclosure treewidth and closure tree-width. It follows from a result by Gottlob, Mikl?os, and Schwentick that for fixed k 3, deciding whether a given structure has hyperclosure tree-width at most k, is NP-compl...

• Type: Conference
• Year: 2008
• Where: PODS
• Authors: Isolde Adler
• Keywords: Closure Tree-width, Database, K-bounded Closure Tree-width, PODS 2008, Unbounded Tree-width