Databases Reference
In-Depth Information
foregoing delete on VSP2 does and (b) the fact that no possible update on VSP1 has the same
effect on either S or VSP1 as the foregoing insert on VSP2 does. What's more, no possible
update on VSP2 produces the same
Golden Rule
violation on VSP2 as the foregoing (attempted)
insert does on VSP1.
Clearly there's more going on here than meets the eye. We need to take a closer look.
INFORMATION EQUIVALENCE REVISITED
To review briefly (and abstracting somewhat):
In Example 2, we had a situation in which updates on a view whose definition took the
form
A
INTERSECT (
A
UNION
B
) affected
B
, even though that defining expression is
logically equivalent to just
A
.
In Example 3, we had a situation in which updates on a view whose definition took the
form
A
JOIN
B
again affected
B
, even though that defining expression (in that particular
example) could be “semantically transformed” into just
A
.
So the obvious question in both cases is: Why is
B
affected at all?
These questions, and others like them, are very vexing. But now let me remind you of that
quote from Enrico Bombieri: “When things get too complicated, it sometimes makes sense to
stop and wonder: Have I asked the right question?” Maybe “Why is
B
affected at all?”
isn't
the
right question to ask. Maybe the right question to ask is: What exactly does it mean for two
views—or two view defining expressions, rather—to be equivalent, anyway?
6
Well, I think the key to this latter question lies in a careful examination of a concept I've
been appealing to throughout this topic: viz., the notion of information equivalence. Here
repeated from Chapter 3 is the definition I originally gave for that concept:
Definition:
Let
DB1
and
DB2
be sets of relvars. Then
DB1
and
DB2
are information
equivalent if and only if the constraints that apply to
DB1
and
DB2
are such that every
proposition that can be represented by
DB1
can be represented by
DB2
and vice versa.
Now, given the foregoing definition, it would appear with respect to Example 2—at least at
first sight—that a design consisting of just VPL1 and one consisting of just VPL2 are indeed
information equivalent, given that the values of those two relvars are certainly equal at any given
time and thus apparently represent the same set of propositions at the time in question.
Similarly, it would appear with respect to Example 3 that a design consisting of just VSP1 and
6
Acknowledgments to David McGoveran once again for setting me on the right track in this section.
