Databases Reference
In-Depth Information
The Second Physical Transaction
The result of applying the second physical transaction
implementing the temporal delete is shown in
Figure 7.17
.
Row 5 was the current version before the temporal transac-
tion began. The first physical transaction withdraws it, and this
second physical transaction replaces it. It replaces it with a ver-
sion identical to the withdrawn version, except for having an
effective end date of December 2010, which is the default value
from the temporal transaction. With this effective end date, we
now currently assert a closed episode, and remove all traces of
P861, in current assertion time, from the time period of [Dec
2010 - 12/31/9999].
This is what the result of a delete against an asserted version
table looks like. It removes the representation of an object from
an indicated period of effective time. In this case, doing so results
in closing an open episode as of the specific effective end date.
We now have enough detail in our Policy table to illustrate
how to read the history of an object from its assertion time
snapshots. From the four snapshots in
Figure 7.17
, and without
consulting the table itself, we can follow the assertion time his-
tory of this policy.
(i) In January 2010, we begin to assert an episode of policy
P861, effective on that same date.
3
At this point in time,
the episode consists of a single version with an effective
time period beginning in January 2010 and extending to
12/31/9999. Because the effective end date is 12/31/9999,
this is an open episode, one which will remain in effect
“until further notice”. Row 1, at this time, is not shaded
because it has not yet been withdrawn.
(ii) In May 2010, we update P861, effective on that same date.
This episode now has two versions, and remains an open
episode. Row 1 is now shaded because it is withdrawn as part
of this update transaction. Row 3 will not be withdrawn, of
course, until August 2010. So from now until August 2010,
what we assert about this episode is rows 2 and 3.
(iii) On August 2010, we update P861 again, effective on that
same date. The episode now has three versions, and
remains an open episode. Row 3 is withdrawn at this time.
From now until December 2010, what we assert about this
episode is rows 2, 4 and 5.
3
Notice that we read the assertion begin date from the left of the snapshot, but read
the effective begin date from the vertical alignment of each rectangle with the calendar
timeline underneath the snapshots. These are two distinct dates which, because our
transactions are using default values, happen in this case to be the same.
