Temporal Data Warehouses

Temporal data warehouses are databases that provide structures and mechanisms for representing and managing information that varies over time. Much research has been done in the field of temporal databases over the last few decades.

Temporal databases are databases that allow one to represent and manage information that varies over time. In addition to storing current data, temporal databases allow previous or future data to be stored, as well as the times when the changes in this data occurred or will occur. Thus, temporal databases enable users to know the evolution of information as required for solving complex problems in many application domains, for example environmental, land management, financial, and healthcare applications.

Time can be represented in a database in various ways. The vast majority of approaches in the field of temporal databases. Assume a discrete model of time where the instants in the time line are isomorphic (i.e., structurally identical) to the natural numbers. The time line is then represented by a sequence of no decomposable, consecutive time intervals of identical duration, called chronons, which correspond to the smallest time unit that the system is able to represent. Depending on application requirements, consecutive chronons can be grouped into larger units called granules, such as seconds, minutes, or days. The granularity represents the time unit used for specifying the duration of a granule.

Temporality Types

They are several ways of interpreting the time frame associated with the facts contained in a temporal database. These interpretations are captured by several temporality types.

Valid time (VT) specifies the period of time in which a fact is true in the modeled reality; for example, it allows one to capture when a specific salary was paid to an employee. The valid time is usually supplied by the user.

Transaction time (TT) indicates the period of time in which a fact is current in the database and may be retrieved. The transaction time of a fact begins at the time when it is inserted or updated and ends when the fact is deleted or updated. The transaction time is generated by the database system.

Valid time and transaction time can be combined to define bitemporal time (BT). This indicates both when a fact is true in reality and when it is current in the database.

In many applications it is necessary to capture the time during which an object exists. This is represented by the lifespan (LS) or existence time; for example, it can be used to represent the duration of a project. The lifespan of an object o may be seen as the valid time of the related fact, “o exists.” Lifespan also applies to relationships, for example, it may be used to capture the time during which an employee has worked for a project. In addition to a lifespan, an object or relationship can also record a transaction time,

indicating the time when it is current in the database.

Note: These are usually called temporal dimensions; however, we use here the term dimension in the multidimensional context.