
Chapter
8:
Intelligent Query Answering
295
This rule is transformable by the join in our query Q since "it does not
change" the projection over the attribute "Supplier." We now have to apply
the join transformation and finally obtain the transformed set of rules:
r
3
= Q(x,y,z) Λ Subpart(v,y) —Q(x,v,z);
r
4
= Q(x,y,z) Λ Q(u,y,w) -^Q(x,y,w)
Rule r
4
is our r rule for a join transformation (it is, in fact, a so-called join
dependency). Notice the computational benefit stemming from the rule trans-
formation here. The resulting query (or "view") could be stored economically
by evaluating only the simplified join as described before and leaving th