Databases Reference
In-Depth Information
row/episode must have existed in its table/be currently asserted
and currently effective at the time the child row/version was
added to its table, and must continue to exist/be currently
asserted and currently effective in the parent table for as long as
the child row/version exists/is currently asserted and currently
effective in its table and continues to point to that same parent.
TFKs: A Data Part and a Function Part
As a data element, a TFK is a column in an asserted version
table whose job is to relate child managed objects to parent
managed objects. Of course, the same may be said of FKs. The
difference is that the parent managed object of a FK is a non-
temporal row, while the parent managed object of a TFK is a
group of possibly many rows. A TRI child table is an asserted
version table that contains a TFK. A TRI parent table is an
asserted version table referenced by a TFK. The FK reference is
a data value, and is unambiguous; but the TFK reference, as a
data value, is not unambiguous.
So as a data element, all a TFK can do is designate the object
on which the object represented by its own row is existence
dependent. There may be any number of versions representing
that object in the parent table, and those versions may be
grouped into any number of episodes scattered along the asser-
tion and effective time timelines. So
as a data value
, a TFK refer-
ence is incomplete.
For example, a TFK data value in a Policy table references all
the episodes in a Client table which represent the client on
which that policy is existence dependent, that being the client
whose
oid
matches the data value in the TFK. To complete the
reference, we need to identify, from among those episodes, the
one episode which was in effect when the policy version went
into effect, and will remain in effect as long as that policy version
remains in effect.
What is needed to complete the reference is a function. We
will name this function
f
TRI. It has the following syntax:
fTRI(PTN, TFK, [eff-beg-dt - eff-end-dt])
PTN
is the name of the parent table which this TFK points to.
Given the TFK and effective time period of a version in a TRI
child table, the AVF searches the parent table for an episode
whose versions have that
oid
as part of their primary key, and
whose effective time period fully includes the effective time
period designated by the function. If there is such an episode,
it is the TRI parent episode of that version, and the
f
TRI function
