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Unformatted text preview: CS345 Notes for Lecture 10/21/96
Proof L/S Algorithm Works: Only-If Suppose Q1 Q2. Let D be any of the canonical
If Q1(D) contains the frozen head of Q1, then
since Q1 Q2, so does Q2(D).
Therefore, the L/S test will be positive for all
canonical DB's. Proof of L/S: If Assume that for every canonical DB, the L/S test
is positive. Let D be some DB, and suppose Q1(D)
contains tuple t. We must show Q2(D) contains t.
Let be the substitution for the variables of
Q1 that yields t.
I.e., for every positive subgoal G of Q1,
(G) is in D, and for no negative subgoal
F of Q1 is (F ) in D. Also, (H1) = t
(Hi is the head of Qi).
Partition the variables of Q1 according to the
value assigns them.
This partition yields a basic canonical DB, C .
Let be the 1-1 correspondence from the
symbols of C to the symbols of D such that
if maps variable X to constant a of D,
and canonical DB C uses constant b for the
block of the partition that contains X , then
(b) = a.
It must be that Q1(C ) contains the frozen H1.
For each positive subgoal of Q1 is
mapped by to a member of D, and ?1
maps that tuple to a member of C (the
one formed from that positive subgoal).
And each negative subgoal of Q1 is
mapped by to something not in D, and
therefore, ?1 cannot map this negative subgoal to a member of C (because
1 (C ) is a subset of D).
Now consider C 0, the extended canonical DB
that is formed from C by taking all tuples of
D formed from the symbols that are in the
range of and applying ?1 to them.
Thus, C 0 \looks like" that part of D consisting of symbols that were involved in
the demonstration that t is in Q1(D).
Since Q1(C ) contains its own frozen head, we
had to conduct the more extensive test where
we looked at the supersets of C .
Evidently, these were all positive.
Since C 0 was constructed from D, it must be
that Q1(C 0) also contains Q1's frozen head.
Thus, Q2(C 0) also contains the frozen head of
If we apply to the constants involved in
the demonstration that Q2(C 0) contains the
frozen head of Q1, we have a demonstration
that Q2(D) contains t.
Remember that D is identical to (C 0)
on the symbols that are in the range of
Thus, t is in Q2(D), completing the proof. CQ's With Arithmetic Suppose we allow subgoals with <, 6=, and other
Negated subgoals also permitted.
We must assume database constants can be
Technique is a generalization of the L/S algorithm, but it is due to Tony Klug (an interesting story).
We shall work the case where < is a total
order; other assumptions lead to other algorithms, and we shall later give an all-purpose
technique using a di erent approach.
2 Example: Consider the rules:
C2: p(X,Z) :- a(X,Y) & a(Y,Z) & X<Y
p(A,C) :- a(A,B) & a(B,C) & A<C Both ask for paths of length 2. But Q1 requires that the rst node be numerically less
than the second, while Q2 requires that the
rst node be numerically less than the third. Klug/Levy/Sagiv Test As before, construct a family of canonical
databases by considering all partitions of the variables of Q1 (assuming we are testing Q1 Q2).
However, now we need to consider also the
order of the values we assign to each partition.
And if there is negation, we also need to consider extended canonical DB's; without negation, the basic canonical DB's are su cient.
Example: To test C1 C2 (the two CQ's of
the previous example) we again need to consider
the partitions of fX; Y; Z g. But now, order of the
values counts too.
The number of di erent basic canonical
databases is 13.
For partition fX gfY gfZ g we have 3! = 6
possible orders of the blocks.
For the three partitions that group two
variables and leave the other separate we
have 2 di erent orders.
For the partition that groups all three,
there is one order.
In this example, the containment test fails.
We have only to nd one of the 13 cases to
For instance, consider X = Z = 0 and Y =
1. The canonical database D for this case is
fa(0; 1); a(1; 0)g, and since X < Y , the body
of C1 is true.
3 Thus, C1(D) includes p(0; 0), the frozen head
However, no assignment of values to A, B,
and C makes all three subgoals of C2 true,
when D is the database.
Thus, p(0; 0) is not in C2(D), and D is a counterexample to C1 C2. 4 ...
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