A database fact is stored in a database at some point in time, and
after it is stored, it may be retrieved. The transaction time
of a database fact is the time when the fact is stored in the
database. Transaction times are consistent with the serialization
order of the transactions. Transaction time values cannot be after the
current time. Also, as it is impossible to change the past,
transaction times cannot be changed. Transaction times may be
implemented using transaction commit times.