Databases Reference
In-Depth Information
what we used to believe
(i) what we used to believe
things used to be like
what we currently believe
what we will believe
what things
used to be like
(iv) what we currently
believe things used to be
like
(v) what we currently
believe things are like now
(vii) what we will believe
things used to be like
what things
are like
(ii) what we used to believe
things are like now
(viii) what we will believe
things are like now
what things
will be like
(iii) what we used to
believe things will be like
(vi) what we currently
believe things will be like
(ix) what we will believe
things will be like
Figure 12.1 Facts, Beliefs and Time.
are true statements, and beliefs that those statements are true
statements.
Using the terminology of beliefs, we may say that the rows in
tables in relational databases may relate data to time in any of
nine ways. So where “thing” means, more precisely, “persistent
object”, we can organize these nine relationships of rows to time
as shown in
Figure 12.1.
In Asserted Versioning, beliefs are what we assert by means of
rows in our tables, and facts are what those rows describe about
the objects they represent. Columns, in
Figure 12.1
, from left to
right, represent past, present and future beliefs. Rows, in that
same illustration, from top to bottom, represent past, present
and future facts. Temporalized beliefs are represented by rows
with assertion time periods. Temporalized facts are represented
by rows with effective time periods, i.e. by versions.
2
But temporal transactions cannot insert, update or delete all
nine types of rows. Specifically, temporal transactions cannot
insert, update or delete rows making statements about what we
used to believe, statements of type (i), (ii) or (iii).
It's important to understand why this is so. Temporal trans-
actions create new rows in temporal tables. But these rows rep-
resent beliefs, and we can't
now
make a statement about what
we
used to
believe. On the other hand we can, of course, now
make a statement about what used to be true. To understand
what the two temporal dimensions of bi-temporal data really
mean, we need to understand why distinctions like these ones
are valid—why, in this case, we
can
make statements about
how things used to be, but
cannot
make statements about what
we used to think about them.
2
Of course, since we cannot know the future, we cannot state with certainty either
what the facts will be, or what we will believe. Instead, “what things will be like”
should be taken as shorthand for “what things may turn out to be like”, and “what we
will believe” should be taken as shorthand for “what we may come to believe”.
