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Intemporal databases,transaction time is the time when some data has beenloaded into adatabase. The time when a transaction is valid can be called thetransaction time-period. It is a technical timeline controlled by aintegration layer (for example adata warehouse).[1] More formally, it is thepoint-in-time during which a fact stored in the database is considered to be true.
The period is an interval based onload times (calledload datetime indata vault[1][2]), also calledinscription timestamp.[1] Other names of the interval isassertion timeline[3]), state timeline[3]) ortechnical timeline.[3]SQL:2011 has support for transaction time through so-calledsystem-versioned tables.[4][5][6][7]
For many reasons, transaction time (when data arrives from asource system) is almost always different fromvalid 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] Inbitemporal data models, valid-time and transaction time can be represented two-dimensionally in aCartesian coordinate system. When data is delivered from the integration layer and is to be presented in a presentation layer (often in adimensional model orwide 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-columnsstart_tt andend_tt. The time interval isclosed[ at itslower bound andopen) at itsupper bound.[8] When the ending transaction time is unknown, it may be considered asuntil_changed. Academic researchers and somerelational database management systems (RDBMS) have representeduntil_changed with the largest timestamp supported or the keywordforever. This convention is a technicalworkaround, and not technically precise.
The term transaction time was coined byRichard T. Snodgrass and his doctoral student Ilsoo Ahn (1986).[9]
As of December 2011, ISO/IEC 9075, Database LanguageSQL:2011 Part 2: SQL/Foundation included clauses in table definitions to define "system-versioned tables" (that is, transaction-time tables).
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