Valid-time Indeterminacy 2 Motivating Example

In valid-time indeterminacy, it is known that an event stored in a temporal database did in fact occur, but it is not known exactly when the event occurred. We present an extension of the tuple-timestamped temporal data model, called the the possible chronons data model, to support valid-time indeterminacy In the possible chronons data model, each event is represented with a set of possible chronons, delimiting when the event might have occurred, and a probability distribution over that set. We extend the TQuel query language with constructs that specify the user's credibility in the underlying valid-time data and the user's plausibility in the relationships among that data. We outline a formal tuple calculus semantics, and show that this semantics reduces to the determinate semantics on determinate data. 1 Overview A valid-time database records the history of an enterprise Jensen et al. 1992]. It associates with each event a times-tamp indicating when that event occurred. Often a user knows only approximately when an event happened. For instance, she may know that it happened \between 2 PM and 4 PM," \on Friday," \sometime last week," or \around the middle of the month." These are examples of valid-time indeterminacy. Information that is valid-time indeterminate can be characterized as \don't know when" information, or more precisely, \don't know exactly when" information. This kind of information has various sources, including the following. granularity | In perhaps most cases, the granularity of the database does not match the precision to which an event time is known. For example, an event time known to within one day and recorded on a system with timestamps in the granularity of a microsecond happened sometime during that day, but during which microsecond is unknown. dating techniques | Many dating techniques are inherently imprecise, such as radioactive and Carbon-14 dating. future planning | Projected completion dates are often inexactly speciied, i.e., the project will complete three to six months from now. unknown or imprecise event times | In general, event times could be unknown or imprecise. For example, assume that we do not know when an individual was born. The individual's date of birth could be recorded in the database as either unknown (they were born between now and the beginning of time) or imprecise (they were born between now and 150 years ago). Temporal database management systems should provide support for valid-time indeterminacy. In particular, users should be able to …