About Managing Undo Data

About Managing Undo Data
Although by default Oracle Database manages undo data and the undo tablespace
automatically, if your installation uses Oracle Flashback features, then you may have
to perform some undo management tasks to ensure the success of these operations.
Oracle Flashback operations resulting in snapshot too old errors indicate that you
must intervene to ensure that sufficient undo data is retained to support these
operations.
The following methods better support Oracle Flashback operations:
■ Set the minimum undo retention period for the autoextending tablespace to be as
long as the longest expected Oracle Flashback operation.
You achieve this goal by setting the UNDO_RETENTION initialization parameter. See
Oracle Database Administrator's Guide for details.
■ Change the undo tablespace to a fixed size.
For an autoextending undo tablespace, Oracle Database always automatically
tunes the undo retention period to be slightly longer than the longest-running
active query. However, this autotuned retention period may be insufficient to
accommodate Oracle Flashback operations. If the undo tablespace has
autoextending disabled, or has a fixed size, then Oracle Database uses a different
method for tuning the undo retention period to better accommodate Oracle
Flashback operations.
To change the undo tablespace to a fixed size, you must choose a tablespace size
that is sufficiently large. If you choose an undo tablespace size that is too small,
then the following errors could occur:
- DML could fail because there is not enough space to accommodate undo data
for new transactions.
- Long-running queries could fail with a snapshot too old error, which means
that there was insufficient undo data for read consistency.
EM Express includes an Undo Advisor to help you determine the minimum size
for the fixed size of the undo tablespace. See "Computing the Minimum Undo
Tablespace Size Using the Undo Advisor" on page 6-21.
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