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How does rollback really work?


How does rollback really work?

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lotusnotes
lotusnotes
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When you rollback a transaction, how does sql know what to rollback and indeed what data to replace it with?

A proper technical answer please.



GilaMonster
GilaMonster
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The short answer:

SQL reads back over the transaction log entries for the transaction that needs to be rolled back and generates compensating operations (operations that reverse the effect of each logged change) which it then logs and executes.

Gail Shaw
Microsoft Certified Master: SQL Server, MVP, M.Sc (Comp Sci)
SQL In The Wild: Discussions on DB performance with occasional diversions into recoverability

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