Synonyms
Current date; Current time; Current timestamp; Until changed
Definition
The word now is a noun in the English language that means “at the present time.” This notion appears in databases in three guises. The first use of now is as a function within queries, views, assertions, etc. For instance, in SQL, CURRENT_DATE within queries, etc., returns the current date as an SQL DATE value; CURRENT_TIME and CURRENT_TIMESTAMP are also available. These constructs are nullary functions.
In the context of a transaction that contains more than one occurrence of these functions, the issue of which time value(s) to return when these functions are invoked becomes important. When having these functions return the same (or consistent) value, it becomes a challenge to select this time and to synchronize it with the serialization time of the transaction containing the query.
The second use is as a database variable used extensively in temporal data model proposals, primarily as timestamp values...
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Ben-Zvi J. The time relational model. PhD Dissertation, University of California, Los Angeles; 1982.
Bliujūtė R, Jensen CS, Šaltenis S, Slivinskas G. Light-weight indexing of bitemporal data. In: Proceedings of the 12th International Conference on Scientific and Statistical Database Management; 2000. p. 125–38.
Chamberlin DD, Astraham MM, Eswaran KP, Griffiths PP, Lorie RA, Mehl JW, Reisner P, Wade BW. SEQUEL 2: a unified approach to data definition, manipulation, and control. IBM J Res Dev. 1976;20(6):560–75.
Finger M. Handling database updates in two-dimensional temporal logic. J Appl Non Classical Logics. 1992;2(2):201–24.
Clifford J, Dyreson CE, Isakowitz T, Jensen CS, Snodgrass RT. On the semantics of “now.”. ACM Trans Database Syst. 1997;22(2):171–214.
Clifford J, Tansel AU. On an algebra for historical relational databases: two views. In: Proceedings of the ACM SIGMOD International Conference on Management of Data; 1985. p. 247–65.
Lomet D, Snodgrass RT, Jensen CS. Exploiting the lock manager for timestamping. In: Proceedings of the International Conference on Database Engineering and Applications; 2005. p. 357–68.
Lorentzos NA, Johnson RG. Extending relational algebra to manipulate temporal data. Inf Syst. 1988;13(3):289–96.
Montague R. Formal philosophy: selected papers of Richard Montague. New Haven: Yale University Press; 1974.
Šaltenis S, Jensen CS. Indexing of now-relative spatio-bitemporal data. VLDB J. 2002;11(1):1–16.
Snodgrass RT. The temporal query language TQuel. ACM Trans Database Syst. 1988;12(2):247–98.
Snodgrass RT, editor. The TSQL2 Temporal Query Language. Norwell: Kluwer; 1995.
Torp K, Jensen CS, Snodgrass RT. Modification semantics in now-relative databases. Inf Syst. 2004;29(78):653–83.
Wiederhold G, Jajodia S, Litwin W. Integrating temporal data in a heterogeneous environment. In: Tansel A, Clifford J, Gadia SK, Jajodia S, Segev A, Snodgrass RT, editors. Temporal databases: theory, design, and implementation, Chapter 22. Redwood City: Benjamin/Cummings; 1993. p. 563–79.
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Dyreson, C.E., Jensen, C.S., Snodgrass, R.T. (2018). Now in Temporal Databases. In: Liu, L., Özsu, M.T. (eds) Encyclopedia of Database Systems. Springer, New York, NY. https://doi.org/10.1007/978-1-4614-8265-9_248
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DOI: https://doi.org/10.1007/978-1-4614-8265-9_248
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