I am trying to identify the appropriate fragment of Datalog containing rules of the type:
B(x,z) is an IDB relation and
c(z,y) is EDB relation.
The special property of this fragment is the each free variable (i.e. a variable not in the head,
z in this case) must appear at least twice. Put differently, the following rule is illeagal:
What is the name of this Datalog fragment and is there any complexity upper and/or lower bounds?