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Modification semantics in now-relative databases

机译：现在相对数据库中的修改语义

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摘要

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.
机译：大多数实际数据库记录时变信息。在这样的数据库中，“当前时间”或NOW的概念自然而突出地出现。例如，当使用开始时间和结束时间列捕获关系的过去状态时，作为当前状态一部分的元组将过去时间作为开始时间，将NOW作为结束时间。尽管已经详细描述了这种可变数据库的语义并很好地理解了它，但是对可变数据库的修改仍待探索。本文定义了涉及变量NOW的修改的语义。更具体地说，探讨了在NOW存在下进行修改的问题，这说明主要问题在于对元组的修改，这种修改将在未来出现。本文定义了不具有NOW，具有NOW且类型值为NOW +Δ的数据库的修改（包括插入，删除和更新）的语义，其中Δ为不可改变的持续时间。为了适应这些语义，引入了三个新的时间戳值。最后，探讨了实现。我们展示了如何用标准SQL数据类型的列表示变量NOW，并给出从NOW相对数据上的SQL到这些列上的标准SQL的映射。因此，本文完成了现在相对数据库的语义，查询和修改。

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