Recursion in Datalog

Path( X, Y ) :- Edge( X, Y )

Path( X, Y ) :- Path( X, Z ), Path( Z, Y ).

Semantics: evaluate the rules until a fixedpoint:

Iteration #0: Edge: {(a,b), (a,c), (b,d), (c,d), (d,e)}

Path: {}

Iteration #1: Path: {(a,b), (a,c), (b,d), (c,d), (d,e)}

Iteration #2: Path gets the new tuples:

(a,d), (b,e), (c,e)

Iteration #3: Path gets the new tuple:

(a,e)

Iteration #4: Nothing changes -> We stop.

Note: number of iterations depends on the data. Cannot be

anticipated by only looking at the query!