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Monday, September 7, 2009

Temporal Database Concepts

A temporal database is a database with built-in time aspects, e.g. a temporal data model and a temporal version of structured query language. More specifically the temporal aspects usually include valid-time and transaction-time. These attributes go together to form bitemporal data.
* Valid time denotes the time period during which a fact is true with respect to the real world.
* Transaction time is the time period during which a fact is stored in the database.
* Bitemporal data combines both Valid and Transaction Time.

- Temporal DBMS manages time-referenced data, and times are associated with database entities.
- Modeled reality.
- Database entities.
- Fact: any logical statement than can meaningfully be assigned a truth value, i.e., that is either true or false.
- Valid Time (vt).
- Valid time is the collected times when the fact is true.
- Possibly spanning the past, present & future.
- Every fact has a valid time.
- Transaction Time (tt).
- The time that a fact is current in the database.
- Maybe associated with any database entity, not only with facts.
- TT of an entity has a duration: from insertion to deletion.
- Deletion is pure logical operation.
- Time domain may be discrete or continuous.
- Typically assume that time domain is finite and discrete in database.
- Assume that time is totally ordered.
- Uniqueness of “NOW”.
- The current time is ever-increasing.
- All activities is happed at the current time.
- Current time separates the past from the future.
- “NOW” <> “HERE”.
- Time cannot be reused!
- A challenge to temporal database management.

Most applications of database technology are temporal in nature:
- Financial applications : portfolio management, accounting & banking.
- Record-keeping applications : personnel, medical record and inventory management.
- Scheduling applications : airline, car, hotel reservations and project management.
- Scientific applications : weather monitoring.

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