| undo_retention [message #61829] |
Thu, 03 June 2004 06:57  |
surendar
Messages: 3
Registered: February 2002
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Junior Member |
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Will manual rollback management in Oracle920
use undo_retention period ?
Just send me some links or docs which is related
to it .
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| Re: undo_retention [message #61832 is a reply to message #61829] |
Thu, 03 June 2004 08:57  |
Mark
Messages: 284
Registered: July 1998
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Senior Member |
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Check the documentation ...
Undo Retention Control
Long-running queries sometimes fail because undo information required for consistent read operations is no longer available. This happens when committed undo blocks are overwritten by active transactions.
Automatic undo management provides a way to explicitly control when undo space can be reused; that is, how long undo information is retained. A database administrator can specify a retention period by using the parameter UNDO_RETENTION. For example, if UNDO_RETENTION is set to 30 minutes, then all committed undo information in the system is retained for at least 30 minutes. This ensures that all queries running for 30 minutes or less, under usual circumstances, do not encounter the OER error, "snapshot too old."
You can either set UNDO_RETENTION at startup or change it dynamically with the ALTER SYSTEM statement. The following example sets retention to 20 minutes:
ALTER SYSTEM SET UNDO_RETENTION = 1200;
If you do not set the UNDO_RETENTION parameter, then Oracle uses a small default value that should be adequate for most OLTP systems, where queries are not usually not very long.
In general, it is a good idea not to set retention to a value very close to what the undo tablespace can support, because that may result in excessive movement of space between undo segments. A 20% buffer of undo space is recommended
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