•Consider the query:
R(x,y),R(y,z),R(z,x)
•Any of the variables x,y,z may be
equal: results
in the following subgraphs:
H1 = R(x,y)R(y,z)R(z,x)
E=6-3=3
H2 = R(x,x)R(x,z)R(z,x)
E=6-2=4
H3 = R(x,x)R(x,x)R(x,x) = R(x,x)
E=2
•Hence Pr(Q) = Pr(H3) = cR/n2