Conquer: efficient management of inconsistent databases

Although integrity constraints have long been used to main-tain data consistency, there are situations in which they may not be enforced or satised. In this paper, we present Con-Quer, a system for ecient and scalable answering of SQL queries on databases that may violate a set of constraints. ConQuer permits users to postulate a set of key constraints together with their queries. The system rewrites the queries to retrieve all (and only) data that is consistent with re-spect to the constraints. The rewriting is into SQL, so the rewritten queries can be eciently optimized and executed by commercial database systems. We study the overhead of resolving inconsistencies dy-namically (at query time). In particular, we present a set of performance experiments that compare the eciency of the rewriting strategies used by ConQuer. The experiments use queries taken from the TPC-H workload. We show that the overhead is not onerous, and the consistent query answers can often be computed within twice the time required to obtain the answers to the original (non-rewritten) query. 1