Databases Reference
In-Depth Information
values for effective time, creates a version that describes what its
object looks like
from now on
, and also that this transaction, like
every transaction other than the deferred ones, creates an asser-
tion that,
from now on
, claims that the version makes a true
statement.
When the user writes a temporal update, she is also doing
two things. First, she is designating a target span of clock ticks.
Second, she is specifying a change in one or more columns of
business data, a change that she wants applied to every version
or part of a version of the designated object that falls, wholly
or partially, within that effective time target span, a change that
will be visible in current assertion time but not in past assertion
time. She is not, however, necessarily specifying a change that
will be applied to every clock tick in that target span, because a
temporal update transaction does not require that the object
it designates be represented in every clock tick within its target
span—only that it be represented in at least one of those clock
ticks.
A temporal delete is like a temporal update except that it
specifies that every version or part of a version of the designated
object that falls, wholly or partially, within that target span will
be, in current assertion time, removed from that target effective
timespan. Like a temporal update, a temporal delete does not
require that its designated object occupy (be represented in) all
of the clock ticks of the target span, only that it occupy at least
one of them.
It follows that it is possible for more than one episode to be
affected by an update or a delete. All episodes that fall within
the target span of an update or delete transaction are affected.
This includes parts of episodes as well as entire episodes. Given
a target timespan, it is possible for one episode to begin outside
that span and either extend into it or extend past the end of it,
and also for one episode to begin within that span and either
end within that span or extend past the end of it. By the same
token, within either or both of those partially included target
episodes, the start or end of the target span may or may not line
up with the start or end of a specific version. In other words, one
version may start outside a target span but extend into or even
through it, and another version may begin within a target span
and either end within it or extend past the end of it.
The details of how these transactions work will be discussed
in the rest of this chapter. But we can already see that the mental
image of designating a target span of clock ticks and then issuing
a transaction whose scope is limited to that target span, is intui-
tively clear. But it is one thing to provide a clear mental image—
