Valid-time Indeterminancy

I n valid-time indeterminacy, it is known that an event stored in a temporal database did in fact occur, but it is not known ezactly when the event occurred. W e present an eztension of the tuple-timestamped temporal data model, called the the possible chronons data model, t o support valid-time indetrrtninncy 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. W e eztend the TQuel query languaye with constructs that specify the user's credibility in the underlying valid-time data and the user's plausibility i n the relationships among that data. W e outline a formal tuple calculus semantics, and show that this semantics reduces t o the determinate semantics on determinate data. 1 Overview A valid-time database recorcls the history of a i l enterprise [Jensen et al. It associates with each event. a times-tamp indicating when that. event. occurred. Oft.en a user knows only approximately when an everkt Iiappened. Fur iilstailce, she may know that it happened " between 2 PM and 4 I'M, " " 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 ezactly 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 tiniest,anips in the granularity of a microseconcl happened sometime during that day, but during which microsecond is unknown. should be able to control, via query language constructs, the amount of indeterminacy present in derived information; and the query evaluator should accommodate valid-time indetermi-nacy in its processing. Query evaluation efficiency should remain high iq the presence of valid-time indeterminacy, and it should not be affected at all i n its absence. This docunient describes the possible chronons data model. The model adds valid-time indeterminacy to TQuel [Snodgrass 19871. TQuel is a strict superset of Quel, the query language for Ingres [Stonebraker et al. 1976). TQuel has a complete, formal semantics which we extend to support valid-time inde-terminacy. …