Semantic query optimization in the presence of types

Abstract

Both semantic and type-based query optimization rely on the idea that queries may exhibit non-trivial rewritings if the state space of the database is restricted. While these two problems have always been studied as separate problems in previous work, in this paper we present a unifying, logic-based query optimization framework that builds upon the classical chase algorithm and brings both problems together. As a major challenge, our novel setting requires chasing conjunctive queries with union and negation in the presence of dependencies containing negation and disjunction. Tackling this problem, we study the applicability of the chase in this setting, develop novel conditions that guarantee its termination, identify fragments for which minimal query computation (w.r.t. a generic cost function) is always possible, and investigate the complexity of related decision problems.