Databases Reference
In-Depth Information
Maintaining Asserted Version Tables:
The Basic Scenario
In response to a temporal transaction, the AVF generates one
or more physical transactions and at the same time enforces
temporal entity integrity and temporal referential integrity.
In this way, it encapsulates bi-temporal complexity, and pre-
serves for the user the image of a single transaction affecting a
single physical representation of a single object.
Let's now see how temporal transactions aremapped to physical
transactions in this situation we call the basic scenario. To avoid
unnecessary complications in this initial look at how asserted ver-
sion tables are updated, wewill ignore temporal referential integrity
issues, and leave an explanation of how theywork to a later chapter.
A Temporal Insert Transaction
Figure 7.2
shows the mapping for a temporal insert transac-
tion. In the example shown in
Figure 7.3
, the transaction specifies
no bi-temporal parameters and is therefore a basic transaction.
Assertion begin and end dates delimit the assertion time period
for a row in an asserted version table. For the next several chapters,
we will assume that all temporal transactions accept the default
value for the assertion begin date, that default value being the date
current when the version is created. As long as this is the case, our
assertion time periods will behave like what the standard temporal
model calls transaction time periods. This means that an assertion
begin date will function like a row creation date. Not only are both
assigned the date current when the physical transaction is applied,
but also once created, neither date can be changed.
1
Temporal Insert
Physical Transaction(s)
Insert an object into a
designated timespan.
Assert a version.
Reset affected versions.
Figure 7.2 Basic Scenario, Insert Transaction: Temporal to Physical Mapping.
1
In fact, a future assertion begin date can be changed. But in this basic scenario, we are
limiting ourselves to temporal transactions which use the current date as the assertion
begin date. And neither past nor current assertion begin dates can be changed because
once we begin to claim that something is so, we can't “take it back”. If we did, we would
lose the information that once upon a time, we did make such claims. And it is an explicit
objective of bi-temporal data management to preserve such information.
