Report from Polytechnic Univ. Area Background Conventional database systems capture only a single snapshot of the modeled reality. The valid time of a fact is the time when the fact is true in the modeled reality. Transaction time on the other hand refers to the time when a new value is posted to the database by a transaction. The transaction time dimension represents the history of a database activity rather than real world history.
Transaction times are system generated and monotonically increasing.
Since it is impossible to change the past, transaction times cannot be changed and there is no way to correct errors in past tuples. A valid-time database maintains the entire history of an enterprise as best known now, i.e., it stores the current knowledge about the past and future. Any errors discovered in this history, are corrected by modifying the database. Clearly both time dimensions are needed in order to accurately model reality. Due to the possible large amount of data that a bitemporal database may record, it is important to invent efficient access methods for such databases.