Yes, you're right that one cannot express Datalog semantics (and also transitive closure semantics) with just one "query", as queries cannot be recursive. If y

Yes, you're right that one cannot express Datalog semantics (and also transitive closure semantics) with just one "query", as queries cannot be recursive. If you view each rule as a query, however, looping over rules does capture Datalog semantics. Furthermore, by optimizing over rules using the relational algebra, one can derive algorithms "equivalent" to traditional graph algorithms. (I don't think you would disagree with me; just want to clarify for other people who might be reading.)