Oracle Undo Reading Notes

Two parameters related to undo:

Undo_tablespace

Undo_management =auto

Oracle recommends that we use Undo to automatically manage after Oracle 9i;

Undo_retention

This parameter is used to ensure that the content in the undo rollback segment is not overwritten. However, this parameter should be considered with the Undo size setting (MaxSize, Unlimit).

Otherwise, it is meaningless.

Setting the Undo_retention parameter has no meaning when the undo TBS size is certain. Oracle does not guarantee that content in undo will not be replicated within the time interval set by the undo retention.

The persistence of content in undo directly affects Oracle's consistent read, flashback feature, long-running query.

Enable Undo-retention:

RETENTION GUARANTEE is specified when you create DATABASE or create undo tablespace in a library.

Alter TABLESPACE settings;

Use ALTER tablespace to disable Undo-retention

Aler tablespace xxx RETENTION noguarantee

Oracle automatically adjusts the undo retention, but does not support LOB fields, and the undo information for LOBs are stored in the segment
itself and not in the undo Tablespace

How to determine the retention policy for the current undo:

Select To_char (begin_time, ' dd-mon-rr hh24:mi ') Begin_time,
To_char (End_time, ' dd-mon-rr hh24:mi ') end_time, tuned_undoretention
From V$undostat order by End_time;

How to set Undo_retention

ALTER SYSTEM SET undo_retention = 2400;

This parameter can be set dynamically and takes effect immediately, but there is no guarantee that Oracle will comply with the requirements set by us, whichever is set with the size of the undo, and whether retention is set guarantee

Oracle Undo Reading Notes