Databases Reference
In-Depth Information
Creating an Episode
Figure 10.2
depicts the history of policy P861 along a frag-
ment of the calendar timeline, the fragment extending from
January 2010 through January 2014. Let's recall that our examples
all use, unless otherwise indicated, a clock that ticks once a month.
And so, using our page-width-preserving notation, “Jan10” stands
for January 1
st
, 2010, “May12” for May 1
st
, 2012, and so on. In the
text, we will write, more concisely, “January 2010” and “May
2012”, not bothering to indicate that the clock always starts the tick
on the first moment of the first day of eachmonth and lasts through
the last moment of the last day of that month.
WeemphasizehereapointthatweoriginallymadeinChapter3.
A clock tick is a logical concept, not a physical one. It is the smallest
unit of time that can intervene between two adjacent versions of the
same object. Thus, if one version begins on the first of the month,
then given the granularity for clock ticks used in the examples in this
book, the next one cannot begin until the first of the next month.
A clock tick gap, in physical time, is whatever the duration of a
clock tick happens to be. But the duration of a clock tick, in phys-
ical time, is not the issue. As far as recorded data is concerned,
there is no duration. There are either clock ticks that happen
between two rows of interest to us, or clock ticks that don't.
Unless otherwise indicated, these diagrams show the history
of our policy as we currently believe it to be. In other words, it
depicts the history of our policy in current assertion time.
Here is a temporal insert transaction, which takes place on
August 2011.
INSERT INTO Policy [P861, C882, PPO, $30] Jan 2011, Mar 2011
This is a retroactive transaction because it is attempting to
insert a representation of the policy into an effective time period
that is already in the past. This transaction is directing us to
insert its data into January 2011 and February 2011. Another
way to express this target time period in English, using this
example, would be “Starting on January 2011 and continuing
up to but not including March 2011 . ....”
Episode A
Episode B
1
2
3
4
Jan
2010
Jan
2012
Jan
2013
Jan
2014
Jan
2011
Figure 10.2 Creating an Episode: Before the Transaction.
