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Normal tuple-generating dependencies (NTGDs) are TGDs enriched with default negation, a.k.a. negation as failure. Query answering under NTGDs, where negation is interpreted according to the stable model semantics, is an intriguing new problem that gave rise to flourishing research activity in the database and KR communities. So far, all the existing works that investigate this problem, except for one recent paper that adopts an operational semantics based on the chase, follow the so-called logic programming (LP) approach. According to the LP approach, the existentially quantified variables are first eliminated via Skolemization, which leads to a normal logic program, and then the standard stable model semantics for normal logic programs is applied. However, as we discuss in the paper, Skolemization is not appropriate in the presence of default negation since it fails to capture the intended meaning of NTGDs, while the operational semantics mentioned above fails to overcome the limitations of the LP approach. This reveals the need to adopt an alternative approach to stable model semantics that is directly applicable to NTGDs with existentially quantified variables. We propose such an approach based on a recent characterization of stable models in terms of second-order logic, which indeed overcomes the limitations of the LP approach. We then perform an in-depth complexity analysis of query answering under prominent classes of NTGDs based on the main decidability paradigms for TGDs, namely weak-acyclicity, guardedness and stickiness. Interestingly, weakly-acyclic NTGDs give rise to robust and highly expressive query languages that allow us to solve in a declarative way problems in the second level of the polynomial hierarchy.

Stable Model Semantics for Tuple-Generating Dependencies Revisited