On the first-order expressibility of computing certain answers to conjunctive queries over uncertain databases
Jef Wijsen · 2010
A natural way for capturing uncertainty in the relational data model is by having relations that violate their primary key constraint, that is, relations in which distinct tuples agree on the primary key. A repair (or possible world) of a database is then obtained by selecting a maximal number of tuples without ever selecting two distinct tuples that have the same primary key value. For a Boolean query q, CERTAINTY(q) is the problem that takes as input a database db and asks whether q evaluates to true on every repair of db. We are interested in determining queries q for which CERTAINTY(q) is first-order expressible (and hence in the low complexity class AC0).