19
Conjunctive Queries
•
Now consider
Q =
R(a,x),R(y,b)
•
•
Two graphs:
H
1
= R(a,x)R(y,b)
E = 4-2=2
H
2
= R(a,b)
E = 2
•
One can prove:
Pr(Q) = Pr(H
1
) + Pr(H
2
) = (c + c
2
)/n
2