Databases Reference
In-Depth Information
So why can't we? Surely we make statements about what we
used to believe all the time. For example, we can now state that
we used to believe that Bernie Madoff was an honest man. If we
can make such statements in ordinary conversation, why can't
we make them as transactions that will update a database?
The reason is that in a database, as we said, a belief is
expressed by the presence of a row in a table. No row, no belief.
So if we write a transaction today that creates a row stating that
we believed something yesterday, we are creating a row that
states that we believed something at a time when there was no
row to represent that belief. Given that the beliefs we are talking
about are beliefs that certain statements about persistent objects
are true, and given that those statements are the statements
made by rows in tables, it would be a logical contradiction to
state that we had such a belief at a point or period in time during
which there was no row to represent that belief.
3
This leaves us six combinations of beliefs and what they are
about that we can, without logical contradiction, modify by means
of a temporal transaction. Asserted Versioning recognizes all six
combinations. But the standard temporal model does not permit
data to be located in future belief time, and so it does not recognize
combinations (vii), (viii) or (ix) as meaningful. It does not attempt
to develop a data management framework within which we can
make statements about what we may in the future believe.
Future beliefs, and their representation in temporal tables as
not yet asserted rows, are precisely what make the difference
between the assertion time dimension of Asserted Versioning
and the transaction time dimension of the standard temporal
model. Without it, the two temporal dimensions of Asserted
Versioning are semantically equivalent to the two temporal
dimensions of the standard temporal model. Without it, asser-
tion time is equivalent to transaction time.
But is it valid to locate data in future belief time? After all, as we
noted in a footnote a short while ago, we can be certain about
what we once believed and about what we currently believe, but
we cannot be certain about what we will believe. On the other
hand, a lack of certainty is not the same thing as a logical contra-
diction. There is nothing logically invalid about making
statements about what we think was, is or may come to be true.
By the same token, there is nothing logically invalid about making
3
In fact, we offer this as a statement of what we will call the
temporalized extension
of
the Closed World Assumption (CWA). All too briefly: the CWA is about the relationship
of a collection of statements to the world. Its temporalized extension is about the
relationship of beliefs (assertions, claims, etc.) to each of those statements.
