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why we need redo log files when transaction is commited [message #649680]

Sun, 03 April 2016 22:24

aruntutor
Messages: 10
Registered: June 2014
Location: chennai

Junior Member

Hi ,

PFA

I have came across this statement in oracle doc. Hence would like verify the same with u people.

"Online redo log files are used , when there is a instance failure to recover committed data that has not been written to data files."

Here is my Q.

When a transaction is committed, it means data has been saved to data files.

In such cases, why we need redo log files when the data is already committed ???

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Re: why we need redo log files when transaction is commited [message #649681 is a reply to message #649680]

Sun, 03 April 2016 22:41

BlackSwan
Messages: 26766
Registered: January 2009
Location: SoCal

Senior Member

>When a transaction is committed, it means data has been saved to data files.
above is WRONG & INCORRECT

https://docs.oracle.com/cd/B19306_01/server.102/b14220/transact.htm

Commit Transactions
Committing a transaction means making permanent the changes performed by the SQL statements within the transaction.

Before a transaction that modifies data is committed, the following has occurred:

Oracle has generated undo information. The undo information contains the old data values changed by the SQL statements of the transaction.

Oracle has generated redo log entries in the redo log buffer of the SGA. The redo log record contains the change to the data block and the change to the rollback block. These changes may go to disk before a transaction is committed.

The changes have been made to the database buffers of the SGA. These changes may go to disk before a transaction is committed.

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Re: why we need redo log files when transaction is commited [message #649682 is a reply to message #649681]

Mon, 04 April 2016 00:02

aruntutor
Messages: 10
Registered: June 2014
Location: chennai

Junior Member

Thanks dude . got the logic now. nice explanation.

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