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What does redo log files hold?

NightWing Journeyer
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Hello Experts,

 

I have read articles about redo log files and undo segment. I am wondering a very simple thing. What redo log files hold in it? Does it store sql statements?

 

Lets say that my update statement modify 800 data blocks. One single update statement can modify 800 different data blocks right? yes it can be true. I think these data blocks cannot hold by redo log buffers, right? I mean I exactly know what redo log buffer and redo log file do. And I know the LGWR background process job. But, I am just wondering does it hold the data blocks? It is not supposed to hold data blocks like buffer cache, right?

 

 

My second question is, rollback doesn't effect to redo log buffer right? Because, it doesn't need to effect redo log buffer. Conversely, rollback; statement is logged to the redo log buffer when someone isse it, am I right?

 

As far as I know, rollback directly interact with UNDO TABLESPACE?

 

 

I hope I did express myself clearly.

 

Thanks in advance.

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