An associative database can show us data values as they were
at any past point in time.
We used this diagram to illustrate the associative model's transaction
commit mechanism, whereby all updates are accomplished by adding
items and links to the database.
This mechanism also allows users to see data as it was at any time
in the past by specifiying a date and time at which we wish to view
or query the data.
For example, if we query Google's balance and specified that we
wanted to see its value as at 10:15 on 10-May-2011, it would say
$100,000. If we queried its value as at 18:45 on 11-Jun-2011, it
would say $125,000.
This is because the database's visibility layer screens out any
transaction whose timestamp is later than our chosen date/time.
Any item or link whose transaction is not visible is likewise not
The identical mechanism applies also to changes to the database's
schema, so schema and data will always be in step.