We show that all--instances termination of chase is undecidable. More precisely, there is no algorithm deciding, for a given set T consisting of Tuple Generating Dependencies (a.k.a. Datalog∃ program), whether the T-chase on D will terminate for every finite database instance D. Our method applies to Oblivious Chase, Semi-Oblivious Chase and -- after a slight modification -- also for Standard Chase. This means that we give a (negative) solution to the all--instances termination problem for all version of chase that are usually considered. The arity we need for our undecidability proof is three. We also show that the problem is EXPSPACE-hard for binary signatures, but decidability for this case is left open. Both the proofs -- for ternary and binary signatures -- are easy. Once you know them.
|Publication status||Published - 2014|