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Shrinking rollback segments for backups

In one of your responses, you mentioned "Shrink all rollback segments to their optimal size before beginning the export operation." I would like to know how would shrinking rollback segments help the export backup.

I mentioned this advice in the following question: https://searchoracle.techtarget.com/answer/Help-with-ORA-01555-during-export

If you read that question and answer, you will see that I was specifically responding to the user getting ORA-1555 and ORA-22924 errors when performing the export. The ORA-1555 error is a Snapshot Too Old message. The export operation requires some level of consistency. The level of consistency is determined by the CONSISTENT parameter. Consistency in the Oracle database is resolved with the rollback (or undo) segments. The ORA-1555 errors occur when a transaction that has committed is overwritten in the rollback segment, but your transaction needs that information for read consistency. Once that information is lost, the current transaction throws the ORA-1555 error.

The most common methods to resolve that error are to use a larger optimal value on your rollback segments, or add more rollback segments. The reason that shrinking before you begin your import can help you from receiving ORA-1555 errors is that space is freed up in the rollback segments' tablespace which means that old transaction information *may* not be as easily overwritten. It does not guarantee that the ORA-1555 errors will not occur.

If you search Metalink for ORA-1555 errors, you should find that all three recommendations that I made in response to this specific question are found as suggested solutions to the problem. Shrinking rollback segments does not help export processes, but it can help export processes that experience ORA-1555 errors.


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