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In temporal databases, transaction time is the time when some data has been loaded into a database. The time when a transaction is valid can be called the transaction time-period. It is a technical timeline controlled by a integration layer (for example a data warehouse).[1] More formally, it is the point-in-time during which a fact stored in the database is considered to be true.
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The period is an interval based on load times (called load datetime in data vault[1][2]), also called inscription timestamp.[1] Other names of the interval is assertion timeline[3]), state timeline[3]) or technical timeline.[3] SQL:2011 has support for transaction time through so-called system-versioned tables.[4][5][6][7]
For many reasons, transaction time (when data arrives from a source system) is almost always different from valid time (when the event happened in the real world). For a data warehouse to unambiguously report what actually happened in the past it must be able to combine these two timelines.[1] In bitemporal data models, valid-time and transaction time can be represented two-dimensionally in a Cartesian coordinate system. When data is delivered from the integration layer and is to be presented in a presentation layer (often in a dimensional model or wide table) it is often desirable to have the data on only one timeline.
In a database table, the transaction time is often represented as an interval allowing the system to "remove" entries by using two table-columns start_tt and end_tt. The time interval is closed [ at its lower bound and open ) at its upper bound.[8] When the ending transaction time is unknown, it may be considered as until_changed. Academic researchers and some relational database management systems (RDBMS) have represented until_changed with the largest timestamp supported or the keyword forever. This convention is a technical workaround, and not technically precise.
The term transaction time was coined by Richard T. Snodgrass and his doctoral student Ilsoo Ahn (1986).[9]
As of December 2011, ISO/IEC 9075, Database Language SQL:2011 Part 2: SQL/Foundation included clauses in table definitions to define "system-versioned tables" (that is, transaction-time tables).
- Valid time, when an event in a database happened in the real world
- Decision time, when a decision was made about interpretation of history in a database
- Using transaction time
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