Databases Reference
In-Depth Information
object
version
assertion
Figure 9.4 Assertions Are About Versions Are About Objects.
(January to October in our example) that the effective-time
tagged row represents that object as it is/was during the other
indicated period of time (January to July).
So an effective time tag qualifies the representation of an
object, while an assertion time tag qualifies the effective-time
qualified representation of an object. Effective time containment
turns a row representing an object into a version. Assertion time
containment turns a row representing a version into an assertion
of a version, i.e. into a temporally delimited truth claim.
1
This is
illustrated in
Figure 9.4
.
Temporal integrity constraints govern the effective time
relationships among bi-temporal rows. But, as we pointed out
earlier, these effective time relationships apply only within
shared assertion time
. For example, when one version is asserted
from January 2012 to April 2014, and another version of the same
object is asserted from March 2012 to 12/31/9999, then the
effective time periods of those two versions must not [intersect]
from March 2012 to April 2014 in assertion time. But from Janu-
ary 2012 to March 2012, they neither [intersect] nor do not
[intersect]. During those two periods of assertion time, the com-
parison doesn't apply. During those times, those two versions are
what philosophers call “incommensurable”.
In the following discussion of temporal integrity constraints,
we will assume that all the rows involved exist in shared asser-
tion time.
Note that it is effective time that exists with assertion time,
and not vice versa. If the semantic containment were reversed,
1
And if there were no versioning, and non-temporal statements were contained
directly in assertion time, i.e. non-temporal rows were given an assertion time tag but
not an effective time tag, then assertion time containment would turn non-temporal
statements directly into temporally delimited truth claims.
