Databases Reference
In-Depth Information
time within a period of shared assertion time.
1
They represent
what we believe, during that period of assertion time, the life his-
tory of that object was/is/will be like, across those contiguous
periods of effective time.
Consider a row representing an object that is inserted into a
non-temporal table at some point in time, say T
1
. Updates take
place at T
2
,T
3
and T
4
. These updates modify the contents of
the row, but do not create new rows. Eventually, the row may
be deleted from that table at, let's say, T
5
. After it's deleted, of
course, the table contains no indication that the row was ever
present.
If this same object, over this same period of time, is
represented in a version table, it is represented by an effective-
time contiguous series of rows, starting with a row representing
that object in the time period T
1
-T
2
and continuing with rows
representing the object in time periods T
2
-T
3
,T
3
-T
4
, and
T
4
-T
5
. This contiguous set of rows is an episode of the object.
Notice that the deletion did not remove any data from the table;
it merely set T
5
as the end date for the last row in the series.
Now suppose that at T
10
, a row representing that same object
is inserted into the same table, goes through a series of three
modifications at T
11
,T
12
and T
13
, and is eventually deleted at
T
14
. If this same object, over this same period of time, is
represented in a version table (and again, we emphasize that
“version table” refers to a view over an asserted version bi-
temporal table), it is represented by an effective-time contiguous
series of rows, starting with a row representing that object in the
time period T
10
-T
11
and continuing with rows representing
the object in time periods T
11
-T
12
,T
12
-T
13
, and T
13
-T
14
.
This contiguous set of rows is another episode of the same
object.
In this way, episodes mirror the existence of rows in non-
temporal tables. They start and end at the same points in time.
But whereas updates to a row in a non-temporal table simply
overwrite the old data, the corresponding updates in an asserted
version table copy the latest row in the episode, apply the update
to that copy, and insert the copy back into the table as the new
latest row in that episode. In the process, the same point in time
1
All effective time relationships exist within shared assertion time. Because this is so
important to keep in mind, we will often add the qualifier “within shared assertion
time” to statements about effective time relationships. At other times, including the
qualifying phrase seems to interfere with clarity. But whether or not the phrase is
included, the qualification is always there.
