Valid time (VT), a concept originated by Richard T. Snodgrass and his doctoral student, is used in temporal databases. [1] It denotes the time period during which a database fact was, is, or will be valid in the modeled reality.
In a database table valid time is often represented by two extra table-columns StartVT and EndVT. The time interval is closed at its lower bound and open at its upper bound.
Example:
| Date | What happened in the real world | Database Action | What the database shows |
|---|---|---|---|
| April 3, 1975 | John is born | Nothing | There is no person called John Doe |
| April 4, 1975 | John's father officially reports John's birth | Inserted:Person(John Doe, Smallville) | John Doe lives in Smallville |
| August 26, 1994 | After graduation, John moves to Bigtown, but forgets to register his new address | Nothing | John Doe lives in Smallville |
| December 26, 1994 | Nothing | Nothing | John Doe lives in Smallville |
| December 27, 1994 | John registers his new address | Updated:Person(John Doe, Bigtown) | John Doe lives in Bigtown |
| April 1, 2001 | John dies | Deleted:Person(John Doe) | There is no person called John Doe |
Valid time is the time for which a fact is true in the real world. In the example above, the Person table gets two extra fields, Valid-From and Valid-To, specifying when a person's address was valid in the real world. On April 4, 1975 John's father proudly registered his son's birth. An official will then insert a new entry to the database stating that John lives in Smallville from April, 3rd. Notice that although the data was inserted on the 4th, the database states that the information is valid since the 3rd. The official does not yet know if or when John will ever move to a better place so in the database the Valid-To is filled with infinity (∞). Resulting in this entry in the database:
| Person(John Doe, Smallville, 3-Apr-1975, ∞). |
December 27, 1994 John reports his new address in Bigtown where he has been living since August 26, 1994. The Bigtown official does not change the address of the current entry of John Doe in the database. He adds a new one:
| Person (John Doe, Big Town, 26-Aug-1994, ∞). |
The original entry Person (John Doe, Smallville, 3-Apr-1975, ∞) is then updated (not removed!). Since it is now known that John stopped living in Smallville on August 26, 1994 the Valid-To entry can be filled in. The database now contains two entries for John Doe
| Person(John Doe, Smallville, 3-Apr-1975, 26-Aug-1994). |
| Person(John Doe, Bigtown, 26-Aug-1994, ∞). |
When John dies the database is once more updated. The current entry will be updated stating that John does not live in the Bigtown any longer. No new entry is being added because officials never report heaven as a new address. The database now looks like this
| Person(John Doe, Smallville, 3-Apr-1975, 26-Aug-1994). |
| Person(John Doe, Bigtown, 26-Aug-1994, 1-Apr-2001). |
valid time and transaction time are two different apspects in designing a database