Databases Reference
In-Depth Information
is this: transactions are the records of events, the footprints which
events leave on the sands of time.
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Event Temporal Data
Methods for managing event data are most appropriately
used to manage changes to metric values of relationships among
persistent objects, values such as counts, quantities and
amounts. Persistent objects are the things that change, things
like policies, clients, organizational structures, and so on. As per-
sistent objects, they have three important features: (i) they exist
over time; (ii) they can change over time; and (iii) each is distin-
guishable from other objects of the same type. In addition, they
should be recognizable as the same object when we encounter
them at different times (although sometimes the quality of our
data is not good enough to guarantee this).
Events are the occasions on which changes happen to persis-
tent objects. As events, they have two important features: (i) they
occur at a point in time, or sometimes last for a limited period of
time; and (ii) in either case, they do not change. An event happens,
and then it's over. Once it's over, that's it; it is frozen in time.
For example, the receipt of a shipment of product alters the on-
hand balance of that product at a store. The completion of anMBA
degree alters the level of education of an employee. The assign-
ment of an employee to the position of store manager alters the
relationship between the employee and the company. Of course,
the transactions which record these events may have been written
up incorrectly. In that case, adjustments to the datamust bemade.
But those adjustments do not reflect changes in the original
events; they just correct mistakes made in recording them.
A Special Relationship: Balance Tables
The event transactions that most businesses are interested in
are those that affect relationships that have quantitative measures.
A payment is received. This is an event, and a transaction records
it. It alters the relationship between the payer and payee by the
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In this topic, and in IT in general,
transaction
has two uses. The first designates a row
of data that represents an event. For example, a customer purchase is an event,
represented by a row in a sales table; the receipt of a shipment is an event, represented
by a row in a receipts table. In this sense, transactions are what are collected in the fact
tables of fact-dimension data marts. The second designates any insert, update or
delete applied to a database. For example, it is an insert transaction that creates a new
customer record, an update transaction that changes a customer's name, and a delete
transaction that removes a customer from the database. In general, context will make
it clear which sense of the word “transaction” is being used.
