Datalog is a subset of prolog, which is not Turing complete. But, by giving up the ability to compute anything, you get a fully declarative language (the order

Datalog is a subset of prolog, which is not Turing complete. But, by giving up the ability to compute anything, you get a fully declarative language (the order of clauses doesn't matter) whose computations always terminate. In contrast, when programming in prolog you are often thinking about the operational aspects of the underlying engine in order to get more efficiency. You try to order clauses advantageously and introduce extra-logical commands like "cuts" to guide the engine, so prolog code often looks more declarative on the surface than it is in reality. As a result, I've heard Datalog referred to as "the good parts of Prolog," and I suppose that for problems it can handle I do like that Datalog has fewer pragmatic compromises to its purity.