Querying the Guarded Fragment with Transitivity
We study the problem of answering a union of Boolean conjunctive queries q against a database Δ, and a logical theory ϕ which falls in the guarded fragment with transitive guards (GF + TG). We trace the frontier between decidability and undecidability of the problem under consideration. Surprisingly, we show that query answering under GF2 + TG, i.e., the two-variable fragment of GF + TG, is already undecidable (even without equality), whereas its monadic fragment is decidable; in fact, it is 2exptime-complete in combined complexity and coNP-complete in data complexity. We also show that for a restricted class of queries, query answering under GF+TG is decidable.
KeywordsDescription Logic Conjunctive Query Transitive Relation Ground Atom Query Answering
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