UNDO

How to Resolve ORA-01555 UNDO_RETENTION

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ORA-01555 UNDO_RETENTION Let’s see the error ORA-01555 UNDO_RETENTION first: Description ORA-01555: snapshot too old: rollback segment number string with name “string” too small Cause rollback records needed by a reader for consistent read are overwritten by other writers Action If in Automatic Undo Management mode, increase undo_retention setting. Otherwise, use larger rollback segments Before we […]
Initialization Parameter

UNDO_RETENTION, How and Why

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UNDO_RETENTION At times, you may find out or feel that UNDO_RETENTION is useless at all. Raising the number of seconds cannot resolve ORA-01555. This is true under some specific conditions. Here I quote the requisites from the official document below: Fix-sized Undo Tablespace The UNDO_RETENTION parameter is ignored for a fixed size undo tablespace. The […]
UNDO

How to Resolve SMON: about to recover undo segment xx SMON: mark undo segment xx as available

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Repeated messages were found in the alert log after instance startup:…SMON: about to recover undo segment 32SMON: mark undo segment 32 as availableSMON: about to recover undo segment 32SMON: mark undo segment 32 as available…It looked like that SMON was rolling back the database with undo segment. But somehow, it met problems when recovering undo segment […]
UNDO

How to Check UNDO Tablespace Usage

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The first purpose of using UNDO is for storing all uncommitted data in case we need to undo uncommitted data in rollback or after system failures. As you know, UNDO could be growing very fast during system busy hours. If you ignore undo tablespace usage, some transactions may fail. In this post, we will discuss […]