Data exchange: On the complexity of answering queries with inequalities

The complexity of answering Boolean queries and the local-as-view (LAV) setting, which is a special case of the general data exchange problem setting, is described. It is observed that the certain answers for queries being unions of conjunctive queries can be computed in polynomial time. There is a polynomial-time algorithm for computing the certain answers of unions of conjunctive queries with at most one inequality per disjunct. Thus the minimal number of inequalities giving rise to co-NP-hardness of this problem is at least two and six at most.