Databases Reference
In-Depth Information
So the status of that address is that it will remain in effect until
that customer moves, at some unknown date in the future.
In the case of an assertion end date, that date is always set to
12/31/9999 when a row is created. This reflects the assumption
that we would never want to assert that a version is true, while
also knowing that, at some later point in time, we will change
our minds and no longer be willing to make that assertion. While
we adopt this assumption in this topic, and in the software
which is our implementation of these concepts, we note that a
more robust semantics might not include this assumption and
that, consequently, later releases of our Asserted Versioning
Framework may permit non-12/31/9999 assertion end dates to
be specified on temporal transactions.
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An effective end date may be changed, i.e. the time period it
ends may be shortened or lengthened, as long as the change
does not violate temporal integrity constraints, which we will
discuss in Part 3. As for an assertion end date, it may be changed
for one of four reasons.
First, an assertion end date may be changed because we real-
ize that the assertion is incorrect, and we have the correction for
that error. In that case, the incorrect assertion will cease to be
asserted, i.e. it will be withdrawn, as part of the same atomic
unit of work that inserts the correcting assertion, and the same
date will be used both for the assertion end date of the incorrect
assertion and the assertion begin date of
the correcting
assertion.
Second, an assertion end date may be changed because we
realize that an assertion is incorrect even though we do not know
what the correct data is, or else just because, for some reason, we
do not wish to make that assertion any longer. Third, we may
conclude that an assertion about the future is no longer action-
able, probably because both we and our competitors have more
recent and presumably more accurate forecasts. In either case,
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As we briefly allude to, later on, what we assert by means of a row in a table is not so
much that the statement made by the row is true. It is, rather, that the statement is
actionable. An actionable statement, roughly, is one which is good enough to base a
business decision on. Presumably, statements about the past or the present must be
true in order to be actionable. But statements about the future, such as financial
forecasts, lack a known truth value until the future time about which they are
forecasting comes to pass, at which point those statements become either true or false.
Such statements may be actionable when first made, and thus be actionable without
being true. Moreover, they may be actionable when first made but, over time, become
outdated, even prior to becoming true or false. (See, later in this chapter, the section
The Very Concept of Bi-Temporality. See also Chapter 13, the section Posted
Projections: Past Claims About the Future.)
