On the Semantics of ``Now'' in Databases.

@article{DBLP:journals/tods/CliffordDIJS97,
  author    = {James Clifford and
               Curtis E. Dyreson and
               Tom{\'a}s Isakowitz and
               Christian S. Jensen and
               Richard T. Snodgrass},
  title     = {On the Semantics of ``Now'' in Databases},
  journal   = {ACM Trans. Database Syst.},
  volume    = {22},
  number    = {2},
  year      = {1997},
  pages     = {171-214},
  ee        = {http://doi.acm.org/10.1145/249978.249980, db/journals/tods/CliffordDIJS97.html},
  bibsource = {DBLP, http://dblp.uni-trier.de}
}

Abstract

Although "now" is expressed in SQL as CURRENT_TIMESTAMP within queries, this value cannot be stored in the database. However, this notion of an ever-increasing current-time value has been reflected in some temporal data models by inclusion of database-resident variables, such as "now", "until-changed," "x," "@," and "-". Time variables are very desirable, but their use also leads to a new type of database, consisting of tuples with variables, termed a variable database.

This article proposes a framework for defining the semantics of the variable databases of the relational and temporal relational data models. A framework is presented because several reasonable meanings may be given to databases that use some of the specific temporal variables that have appeared in the literature. Using the framework, the article defines a useful semantics for such databases. Because situations occur where the existing time variables are inadequate, two new types of modeling entities that address these shortcomings, timestamps that we call now-relative and now-relative indeterminate, are introduced and defined within the framework. Moreover, the article provides a foundation, using algebraic bind operators, for the querying of variable databases via existing query languages. This transition to variable databases presented here requires minimal change to the query processor. Finally, to underline the practical feasibility of variable databases, we show that database variables can be precisely specified and efficiently implemented in conventional query languages, such as SQL, and in temporal query languages, such as TSQL2.

Copyright © 1997 by the ACM, Inc., used by permission. Permission to make digital or hard copies is granted provided that copies are not made or distributed for profit or direct commercial advantage, and that copies show this notice on the first page or initial screen of a display along with the full citation.

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Referenced by

1. Kristian Torp, Christian S. Jensen, Richard T. Snodgrass: Effective Timestamping in Databases. VLDB J. 8(3-4): 267-288(2000)
2. John F. Roddick, Lina Al-Jadir, Leopoldo E. Bertossi, Marlon Dumas, Florida Estrella, Heidi Gregersen, Kathleen Hornsby, Jens Lufter, Federica Mandreoli, Tomi Männistö, Enric Mayol, Lex Wedemeijer: Evolution and Change in Data Management - Issues and Directions. SIGMOD Record 29(1): 21-25(2000)
3. Torben Bach Pedersen, Christian S. Jensen: Multidimensional Data Modeling for Complex Data. ICDE 1999: 336-345
4. Rasa Bliujute, Simonas Saltenis, Giedrius Slivinskas, Christian S. Jensen: Developing a DataBlade for a New Index. ICDE 1999: 314-323
5. Curtis E. Dyreson, Richard T. Snodgrass: Supporting Valid-Time Indeterminacy. ACM Trans. Database Syst. 23(1): 1-57(1998)
6. Rasa Bliujute, Christian S. Jensen, Simonas Saltenis, Giedrius Slivinskas: R-Tree Based Indexing of Now-Relative Bitemporal Data. VLDB 1998: 345-356
7. Jun Yang, Jennifer Widom: Maintaining Temporal Views over Non-Temporal Information Sources for Data Warehousing. EDBT 1998: 389-403