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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...
Recommended Reading
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Dyreson, C.E., Jensen, C.S., Snodgrass, R.T. (2016). Now in Temporal Databases. In: Liu, L., Özsu, M. (eds) Encyclopedia of Database Systems. Springer, New York, NY. https://doi.org/10.1007/978-1-4899-7993-3_248-2
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DOI: https://doi.org/10.1007/978-1-4899-7993-3_248-2
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