Databases Reference
In-Depth Information
transactions that violate the semantic constraints that give bi-
temporal data its meaning. It is the AVF that gives the user a
declarative means of expressing her intentions with respect to
the transactions she submits.
In the Asserted Versioning temporal model, the two bi-
temporal dimensions are effective time and assertion time. If
assertion time were completely equivalent to the standard tem-
poral model's transaction time, then every row added to an
asserted version table would use the date the transaction was
physically applied as its assertion begin date. Important addi-
tional functionality is possible, however, if we permit rows to
be added with assertion begin dates in the future. This is func-
tionality not supported by the standard temporal model. But it
comes at the price of additional complexity, both in its seman-
tics and in its implementation.
Fortunately, it is possible to segregate this additional func-
tionality, which is based on what we call
deferred transactions
and
deferred assertions
, and to discuss Asserted Versioning as
though both its temporal dimensions are strictly analogous to
the temporal dimensions of the standard temporal model. This
makes the discussion easier to follow, and so this is the approach
we will adopt. Deferred assertions, then, will not be discussed
until Chapter 12.
Effective Time Within Assertion Time
A row in a conventional table makes a statement. Such a row,
in a conventional Policy table, is shown in
Figure 9.1
.
This row makes the following statement: “I represent a policy
which has an object identifier of P861, a client of C882, a type of
HMO and a copay of $15.” The statement makes no explicit ref-
erence to time. But we all understand that it means “I represent
a policy which exists at the current moment, and which at the
current moment has an object identifier of .....”.
This same row, with an effective time period attached, is
shown in
Figure 9.2
.
It makes the following statement: “I represent a policy which
has an object identifier of P861 and which, from January 2010 to
oid
client
type
copay
P861
C882
HMO
$15
Figure 9.1 A Non-Temporal Row.
