Chase Termination: A Constraints Rewriting Approach

Several database areas such as data exchange and integra-tion share the problem of fixing database instance viola-tions with respect to a set of constraints. The chase al-gorithm solves such violations by inserting tuples and set-ting the value of nulls. Unfortunately, the chase algorithmmay not terminate and the problem of deciding whether thechase process terminates is undecidable. Recently there hasbeen an increasing interest in the identification of sufficientstructural properties of constraints which guarantee that thechase algorithm terminates [8, 10, 14, 15].In this paper we propose an original technique which al-lows to improve current conditions detecting chase termi-nation. Our proposal consists in rewriting the original setof constraints Σ into an ‘equivalent’ set Σ and verifyingthe structural properties for chase termination on Σ. Therewriting of constraints allows to recognize larger classes ofconstraints for which chase termination is guaranteed. Inparticular, we show that if Σ satisfies chase termination con-ditions T , then the rewritten set Σ satisfies T as well, butthe vice versa is not true, that is there are significant classesof constraints for which Σ satisfies T and Σ does not.