Databases Reference
In-Depth Information
The 1980s
Historical Databases
In the 80s, as disk storage costs plummeted, it was inevitable
that someone would think to put the most recent backup files
onto disk where it would be possible to access them without first
restoring them from off-line storage media. After that, the next
step was to realize that there was value, not just in having a par-
ticular set of backup data remain on-line, but also in having the
ability to compare multiple backups of the same data, made at
different points in time.
Each backup is a snapshot of a set of data of interest, and just
as a movie film makes motion apparent while being itself a series
of still images, so too a series of database snapshots can make
change apparent while being itself a series of still images. Thus
was born the concept of a data warehouse, whose originators
were Barry Devlin and Paul Murphy.
1
This concept introduced
temporal data management at the database level (as opposed
to the table, row or column levels), since data warehouses are
entire databases devoted to historical data.
History Tables
On an architecturally smaller scale, IT developers were also
beginning to design and implement several other ways of man-
aging temporal data. One of them was the use of history tables,
and another the use of version tables. In the former case, tempo-
ral data management is implemented at the table level in the
sense that individual tables are the objects devoted to historical
data, usually populated by triggers based on updates to the
corresponding current tables. In the latter case, temporal data
management is also implemented at the table level, but in this
case historical and current data reside in the same table. In some
cases, intrepid developers have even attempted to introduce
temporal data management at the level of individual columns.
In addition, developers were also beginning to create on-line
transaction tables by bringing collections of transactions back
from off-line storage media, transactions that originally had
been moved onto that media as soon as their current accounting
periods were over. The difference between history tables and
version tables, on the one hand, and transaction tables on the
1
See [1988, Devlin & Murphy]. The full citation may be found in the appendix
Bibliographical Essay. The year is the year of publication, and entries in that appendix
are organized by year of publication.
