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Lets discuss about some of the recovery situations in database

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Lets discuss about some of the recovery situations in database

       Online Block Recovery Oracle automatically do this using pmon when a process dies while changing a buffer. Oracle will reconstruct the buffer using the online redo logs and writes it to disk. No action required from your side J

 

       Thread Recovery Oracle automatically do this when an instance crashes while having the database open. Oracle applies all the redo changes in the thread that occurred since the last time the thread was checkpointed. No action required from your side J

 

 

       Media Recovery You need to do this when a data file is restored from backup and the checkpoint count in the data files here are not equal to the check point count in the control file. 

At this moment it is critical to understand Redo vs Undo.

Redo information is recorded so that all commands that took place can be repeated during recovery (it is complete information about redoing operations).

Undo information is recorded so that database can undo changes made by the current transaction but were not committed. The Redo Logs are used to Roll Forward the changes made, both committed and non- committed changes. Then from the Undo segments, the undo information is used to 
rollback the uncommitted changes.

 

Posted on : by Anuj
 
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