Half-remembered university courses from >= 10 years ago, I'm afraid. I now think I was probably thinking of ASP, not Datalog. See for example the "Disjunctive logic programs" section of https://www.cs.uni-potsdam.de/~torsten/Lehre/ASP/Folien/asp-.... Since the syntax is very Prolog/Datalog-like, I probably got mixed up. My bad, sorry. The first few sources on Datalog I looked at only talked about Horn clauses, not ones with more heads.

> My understanding is that without definite clauses (and "p,q:-r" is not definite) there are no fixpoint semantics and without fixpoint semantics there is no guarantee of program termination.

Skimming https://en.wikipedia.org/wiki/Answer_set_programming and https://en.wikipedia.org/wiki/Stable_model_semantics I get the impression that even disjunctive logic programs appear to always terminate, though the complexity might be daunting.