Databases Reference
In-Depth Information
But in our experience, which combines several decades of
work in business IT, the greatest cost is the cost of the business
community learning to do without the data they need. In
many cases, it simply never crosses their minds to ask for tem-
poral data that isn't already directly queryable. The core of
the problem is that satisfying these requests is not the part of
the work of computer operators, DBAs and developers that they
get evaluated on. If performance reviews, raises, bonuses and
promotions depend on meeting other criteria, then it is those
other criteria that will be met. Doing a favor for a business
user you like, which is what satisfying this kind of one-off
request often amounts to, takes a decidedly second place.
To paraphrase Samuel Johnson, “The imminent prospect of
being passed over for a promotion wonderfully focuses the
mind”.
2
Queryable Temporal Data: Events and States
Having distinguished queryable data from reconstructable
data, we move on to a partitioning of the former. We think that
the most important distinction among methods of managing
queryable data is the distinction between data about things
and data about events. Things are what
exist;
events are what
happen
. Things are what change; events are the occasions on
which they change.
The issue here is change, and the best way to keep track of it.
One way is to keep a history of things, of the states that objects
take on. As an object changes from one state to the next, we
store the before-image of the current state and update a copy
of that state, not the original. The update represents the new
current state.
Another way to keep track of change is to record the initial
state of an object and then keep a history of the events in
which the object changed. For example, with insurance policies,
we could keep an event-based history of changes to policies by
adding a row to the Policy table each time a new policy is
created, and after that maintaining a transaction table in which
each transaction is an update or delete to the policy. The rele-
vance of transactions to event-based temporal data management
2
The form in which we knew this quotation is exactly as it is written above, with the
word “death” substituted for “being passed over for a promotion”. But in fact, as
reported in Boswell's
Life of Johnson
, what Johnson actually said was: “Depend upon it,
sir, when a man knows he is to be hanged in a fortnight, it concentrates his mind
wonderfully.” The criteria for annual bonuses do the same thing.
