it is discovered on 2-Feb-2001 that he was in fact in Beachy during those dates. To record this fact, the existing entry about John living in Bigtown must be split into two separate records, and a new record inserted recording his residence in Beachy. The database would then appear as follows: Person(John Doe, Smallville, 3-Apr-1975, 26-Aug-1994). Person(John Doe, Bigtown, 26-Aug-1994, 1-Jun-1995). Person(John Doe, Beachy, 1-Jun-1995, 3-Sep-2000). Person(John Doe, Bigtown, 3-Sep-2000, 1-Apr-2001). However, this leaves no record that the database ever claimed that he lived in Bigtown during 1-Jun-1995 to 3-Sep-2000. This might be important to know for auditing reasons, or to use as evidence in the official's tax investigation. Transaction time allows capturing this changing knowledge in the database, since entries are never directly modified or deleted. Instead, each entry records when it was entered and when it was superseded (or logically deleted). The database contents then look like this: Name, City, Valid From, Valid Till, Entered, Superseded Person(John Doe, Smallville, 3-Apr-1975, ∞, 4-Apr-1975, 27-Dec-1994). Person(John Doe, Smallville, 3-Apr-1975, 26-Aug-1994, 27-Dec-1994, ∞ ). Person(John Doe, Bigtown, 26-Aug-1994, ∞, 27-Dec-1994, 2-Feb-2001 ). Person(John Doe, Bigtown, 26-Aug-1994, 1-Jun-1995, 2-Feb-2001, ∞ ). Person(John Doe, Beachy, 1-Jun-1995, 3-Sep-2000, 2-Feb-2001, ∞ ). Person(John Doe, Bigtown, 3-Sep-2000, ∞, 2-Feb-2001, 1-Apr-2001 ). Person(John Doe, Bigtown, 3-Sep-2000, 1-Apr-2001, 1-Apr-2001, ∞ ). The database records not only what happened in the real world, but also what was officially recorded at different times. Using three axes: valid time, decision time, and transaction time Decision time is an alternative to the transaction time period for recording the time at which a database entry may be accepted as correct. This enables queries that show the officially recognized facts at a given time, even if there was a delay in committing those facts to the database. Support for decision time preserves the entire history and prevents the loss of information during updates. Decision time periods can only occur in the past or up to the transaction time. As in a transaction time table, records are never deleted. Only new records can be inserted, and existing ones updated by setting their decision end time to show that they are no longer current. To enable decision time, two more fields are added to a database table: Decision From and Decision To. Decision From is the time a decision was made, and Decision-To is the time that the decision was superseded (which may be infinity if