Databases Reference
In-Depth Information
What are these rows with future assertion begin dates? To
take a single example, they might be rows for which we have
some of the business data, but not all of it, rows which are in
the process of being made ready “for prime time”. These
rows—which may be assertions about past, present or future
versions—are not yet ready, we will say, to become part of the
production data in the table, not yet ready to become rows that
we are willing to present to the world and of which we are will-
ing to say “We stand behind the statements these rows make.
We claim that the statements they make are (or are likely to
become) true, and that the information these rows provide
meetsthestandardsofre iab ityunderstood(orexp icitly
stated) to apply to all rows in this table”.
So the semantics of the standard temporal model are fully
supported by Asserted Versioning. But Asserted Versioning adds
the semantics of what we call deferred assertions, and which we
have just briefly described. As we will see in later chapters,
deferred assertions are just one kind of internalized pipeline
dataset, and the internalization of pipeline datasets can eliminate
a large part of the ITmaintenance budget by eliminating the need
to manage pipeline datasets as distinct physical objects.
Allen Relationships
Allen relationships describe all possible positional
relationships between two time periods along a common time-
line. This includes the special case of one or both time periods
being a point in time, i.e. being exactly one clock tick in length.
There are 13 Allen relationships in total. Six have a
corresponding inverse relationship, and one does not. Standard
treatments of the Allen relationships may be found in both
[2000, Snodgrass] and [2002, Date, Darwen, Lorentzos]. We have
found it useful to reconstruct the Allen relationships as a binary
taxonomy. Our taxonomy is shown in
Figure 3.4
.
In this diagram, the leaf nodes include a graphic in which
there are two timelines, each represented by a dashed line. All
the leaf nodes but one have an inverse, and that one is
italicized; when two time periods are identical, they do not
have a distinct inverse. Thus, this taxonomy specifies 13 leaf-
node relationships which are, in fact, precisely the 13 Allen
relationships.
The names of the Allen relationships are standard, and have
been since Allen wrote his seminal article in 1983. But those
names, and the names of the higher-level nodes in our own tax-
onomy of
the Allen relationships, are also expressions in
