TY - JOUR
T1 - Modification semantics in now-relative databases
AU - Torp, Kristian
AU - Jensen, Christian S.
AU - Snodgrass, Richard T.
N1 - Funding Information:
This research was supported in part by the Danish Technical Research Council through grant 9700780, by a grant from the Nykredit corporation, and by grants IRI-9632569, IIS-9817798, IIS-0100436 and EIA-0080123 from the U.S. National Science Foundation.
PY - 2004/12
Y1 - 2004/12
N2 - Most real-world databases record time-varying information. In such databases, the notion of "the current time," or NOW, occurs naturally and prominently. For example, when capturing the past states of a relation using begin and end time columns, tuples that are part of the current state have some past time as their begin time and NOW as their end time. While the semantics of such variable databases has been described in detail and is well understood, the modification of variable databases remains unexplored. This paper defines the semantics of modifications involving the variable NOW. More specifically, the problems with modifications in the presence of NOW are explored, illustrating that the main problems are with modifications of tuples that reach into the future. The paper defines the semantics of modifications - including insertions, deletions, and updates - of databases without NOW, with NOW, and with values of the type NOW+Δ, where Δ is a non-variable time duration. To accommodate these semantics, three new timestamp values are introduced. Finally, implementation is explored. We show how to represent the variable NOW with columns of standard SQL data types and give a mapping from SQL on NOW-relative data to standard SQL on these columns. The paper thereby completes the semantics, the querying, and the modification of now-relative databases.
AB - Most real-world databases record time-varying information. In such databases, the notion of "the current time," or NOW, occurs naturally and prominently. For example, when capturing the past states of a relation using begin and end time columns, tuples that are part of the current state have some past time as their begin time and NOW as their end time. While the semantics of such variable databases has been described in detail and is well understood, the modification of variable databases remains unexplored. This paper defines the semantics of modifications involving the variable NOW. More specifically, the problems with modifications in the presence of NOW are explored, illustrating that the main problems are with modifications of tuples that reach into the future. The paper defines the semantics of modifications - including insertions, deletions, and updates - of databases without NOW, with NOW, and with values of the type NOW+Δ, where Δ is a non-variable time duration. To accommodate these semantics, three new timestamp values are introduced. Finally, implementation is explored. We show how to represent the variable NOW with columns of standard SQL data types and give a mapping from SQL on NOW-relative data to standard SQL on these columns. The paper thereby completes the semantics, the querying, and the modification of now-relative databases.
KW - Now
KW - Now-relative information
KW - SQL
KW - Temporal data
KW - Temporal query language
KW - Updates
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U2 - 10.1016/S0306-4379(03)00047-4
DO - 10.1016/S0306-4379(03)00047-4
M3 - Article
AN - SCOPUS:3342894932
SN - 0306-4379
VL - 29
SP - 653
EP - 683
JO - Information Systems
JF - Information Systems
IS - 8
ER -