Databases Reference
In-Depth Information
this default to be overridden. Temporal transactions may be sub-
mitted, and physical rows created in response to them, prior to
the date on which those rows will begin to be asserted.
To put it the other way around, an Asserted Versioning tem-
poral transaction may be submitted with an assertion begin date
in the future, so that the row the transaction creates will have a
row creation date earlier than its assertion begin date. The row
will be physically part of the table, but it won't be asserted. It
won't be anything we show to the world, anything we are yet
willing to claim makes a true statement. It will be a row which
is physically in the same table as the rows which make up the
currently asserted production data in that table. But semanti-
cally, it will be distinct from those rows.
We will say that transactions like these are
deferred trans-
actions
, and that what they place in the database are
deferred
assertions
. Unlike rows in conventional tables, deferred
assertions do not represent true statements. They do not have
a truth value at all, because we do not yet attribute a truth
value to them. By the same token, as described in earlier
chapters, Asserted Versioning rows which are withdrawn into
past assertion time also do not represent true statements. They
do not have a truth value at all, because we no longer attribute
a truth value to them. They are a record of what we once
claimed was true, just as deferred assertions are a record of
what we may eventually claim is true.
For the most part, we need not concern ourselves with these
logical subtleties. But neither should we ignore them completely,
because they will help us understand this important functional-
ity of Asserted Versioning which distinguishes it from the stan-
dard temporal model and from all other computer science
work on bi-temporal data that we are aware of. So before we
get on with the task of understanding what deferred assertions
are and how to manage them, we should look a little more
closely at the logical and semantic foundation on which the
distinction between assertions and statements is based.
The Semantics of Deferred Assertion Time
Data describes objects. Conventional tables represent types
of objects. Rows in those tables represent instances of those
types, and describe those instances.
We create and maintain the data in these tables. Those who
access this data assume that we believe that the data is correct
and that each row makes a true statement about the object it
