On acyclic conjunctive queries and constant delay enumeration

Abstract

We study the enumeration complexity of the natural extension of acyclic conjunctive queries with disequalities. In this language, a number of NP-complete problems can be expressed. We first improve a previous result of Papadimitriou and Yannakakis by proving that such queries can be computed in time c.|M.|φ(M)| where M is the structure, φ(M) is the result set of the query and c is a simple exponential in the size of the formula φ. A consequence of our method is that, in the general case, tuples of such queries can be enumerated with a linear delay between two tuples. We then introduce a large subclass of acyclic formulas called CCQ≠ and prove that the tuples of a CCQ≠ query can be enumerated with a linear time precomputation and a constant delay between consecutive solutions. Moreover, under the hypothesis that the multiplication of two n x n boolean matrices cannot be done in time O(n2), this leads to the following dichotomy for acyclic queries: either such a query is in CCQ≠ or it cannot be enumerated with linear precomputation and constant delay. Furthermore we prove that testing whether an acyclic formula is in CCQ≠ can be performed in polynomial time. Finally, the notion of free-connex treewidth of a structure is defined. We show that for each query of free-connex treewidth bounded by some constant k, enumeration of results can be done with O(|M| k+1) precomputation steps and constant delay. © Springer-Verlag Berlin Heidelberg 2007.