Databases Reference
In-Depth Information
ability of the database to let us reinstate a policy after a period of
time during which it was not in effect. In more general terms, it
allows us to record the reappearance of an object after a period
of non-effectivity.
Effective time versioning builds on temporal gap versioning.
And it does so by providing limited support for bi-temporality.
With temporal gap versioning, the two dates—the version begin
and end dates—are what they say they are; they are version
dates. They say when the version became effective and if and
when it stopped being in effect. But effective time versioning
has no way to make corrections to existing versions other than
by overwriting the erroneous data on those versions. And this
is a shortcoming common to all best practice forms of
versioning.
When temporal gaps between adjacent versions of the same
object must be supported, data designers usually use effective
time versioning, not merely temporal gap versioning. One reason
is that effective time versioning only requires one more column
on the table, a row create date. And the maintenance of that col-
umn is trivial; whenever a row is physically created, the current
date is put into its row create date column.
Both temporal gap versioning and effective time versioning
also allow us to recognize the reappearance of an object as the
same object we once kept track of, but one for which we no lon-
ger have a currently effective version.
In this chapter, we are not yet concerned with bi-temporality.
But any form of versioning which supports the as-was vs. as-is
distinction is bi-temporal. Effective time versioning starts us on
the road to bi-temporality because it includes both (i) dates
which designate a period of time that applies to the object itself,
and also (ii) one date which describes a point in time that
applies to the data about that object, i.e. to the row itself. But
there is a lot more to bi-temporality than we have seen so far,
and a lot of bi-temporal support which is lacking in effective
time versioning. We need to move beyond existing best
practices.
Glossary References
Glossary entries whose definitions form strong inter-
dependencies are grouped together in the following list. The
same glossary entries may be grouped together in different ways
at the end of different chapters, each grouping reflecting the
semantic perspective of each chapter. There will usually be
