| Reg. Rollback data [message #59828] |
Sun, 28 December 2003 21:18  |
sachin kumar gupta
Messages: 157
Registered: March 2003
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Senior Member |
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Hi all,
Just let me clear one simple doubt. When the CHKPT occurs then the all changes from redolog file r applied to datafiles since last CHKPT. This generates the undo/rollback data in undo tablespace. I want to know that whether rollback data is also generated for committed transactions.
TIA,
Sachin K. Gupta
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| Re: Reg. Rollback data [message #59829 is a reply to message #59828] |
Sun, 28 December 2003 22:43  |
Frank Naude
Messages: 4580
Registered: April 1998
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Senior Member |
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Hi,
Undo blocks (rollback data) are generated for all database changes - whether committed or not. If Oracle doesn't keep rollback data for committed transactions, it cannot provide read-consistency. This will lead to nasty side effects (such as dirty reads and lockouts) like you get with some other popular DB systems.
Best regards.
Frank
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| Re: Reg. Rollback data [message #59839 is a reply to message #59828] |
Mon, 29 December 2003 04:32 |
Thiru
Messages: 1089
Registered: May 2002
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Senior Member |
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Sachin,
a correction.
"When the CHKPT occurs then the all changes from redolog file r applied to datafiles since last CHKPT"
Checkpoint DOESNT cause all changes from redolog files to be applied to the data files. SMON performs this during the roll forward phase of Instance recovery. Checkpoint process writes all the modified buffers from the buffer cache on to the datafiles and updates the datafile/controlfile headers(in the Full checkpoint) or just the controlfile headers(in the incremental checkpoints).
-Thiru
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