Tuning the UNDO_RETENTION parameter in Oracle

Tuning the UNDO_RETENTION parameter in Oracle is essential for optimizing the performance of transactions and ensuring efficient space management in the undo tablespace. The UNDO_RETENTION parameter specifies the time (in seconds) that Oracle retains old undo data before it can be overwritten. Proper tuning of this parameter helps in managing long-running queries and maintaining read consistency.

Steps for Tuning UNDO_RETENTION Parameter

1. Understand the Requirements

• Transaction Duration: Determine the longest-running query or transaction in your database.
• Flashback Requirements: If using Flashback Query, set the UNDO_RETENTION parameter to cover the period you need for flashback operations.

2. Monitor Current Undo Usage

• View Undo Statistics: Use the V$UNDOSTAT view to monitor undo usage and transaction durations.

  SELECT BEGIN_TIME, END_TIME, UNDOBLKS, TXNCOUNT, MAXQUERYLEN, SSOLDERRCNT FROM V$UNDOSTAT;

3. Set the UNDO_RETENTION Parameter

• Initial Configuration: Set the UNDO_RETENTION parameter based on the longest query duration and flashback requirements.

  ALTER SYSTEM SET UNDO_RETENTION = 3600;  -- Set retention to 1 hour

4. Configure Undo Tablespace with AUTOEXTEND  (-- Be careful while making it autoextend)

• Enable Autoextend: Ensure the undo tablespace can autoextend to accommodate the undo retention period without running out of space.

  ALTER DATABASE DATAFILE '/path/to/undo01.dbf' AUTOEXTEND ON NEXT 10M MAXSIZE UNLIMITED;

5. Monitor and Adjust

• Regular Monitoring: Continuously monitor the undo tablespace usage and adjust the UNDO_RETENTION parameter as necessary.

  SELECT TABLESPACE_NAME, BYTES, MAXBYTES, AUTOEXTENSIBLE
  FROM DBA_DATA_FILES WHERE TABLESPACE_NAME = 'UNDOTBS1';
  

Example: Tuning UNDO_RETENTION

1. Check Current Undo Statistics

   SELECT BEGIN_TIME, END_TIME, UNDOBLKS, TXNCOUNT, MAXQUERYLEN, SSOLDERRCNT
   FROM V$UNDOSTAT;
   

2. Determine Longest Query Duration

   • Identify the longest-running query to set an appropriate undo retention period.

   SELECT MAX(MAXQUERYLEN) AS LONGEST_QUERY_DURATION
   FROM V$UNDOSTAT;
   

3. Set the UNDO_RETENTION Parameter

   • Assume the longest query duration is 1800 seconds (30 minutes).

   ALTER SYSTEM SET UNDO_RETENTION = 3600;  -- Set retention to 1 hour for safety

4. Enable Autoextend on Undo Tablespace

   • Ensure the undo tablespace can handle the increased retention period.

   ALTER DATABASE DATAFILE '/u01/oradata/ORCL/undotbs01.dbf' AUTOEXTEND ON NEXT 10M MAXSIZE UNLIMITED;

5. Monitor Undo Tablespace Usage

   • Regularly check the undo tablespace usage and adjust if necessary.

   SELECT TABLESPACE_NAME, FILE_NAME, BYTES, AUTOEXTENSIBLE
   FROM DBA_DATA_FILES
   WHERE TABLESPACE_NAME = 'UNDOTBS1';
   

Key Considerations

• Space Management: Ensure that the undo tablespace has sufficient space to accommodate the UNDO_RETENTION setting, especially if using AUTOEXTEND.
• Undo Tablespace Performance: Monitor the performance of the undo tablespace to avoid performance degradation due to excessive undo usage.
• Retention Guarantee: If precise control over undo retention is required, enable the RETENTION GUARANTEE option for the undo tablespace.

  ALTER TABLESPACE UNDOTBS1 RETENTION GUARANTEE;

Summary

Tuning the UNDO_RETENTION parameter involves understanding the duration of the longest-running transactions, setting an appropriate retention period, enabling autoextend for the undo tablespace, and continuously monitoring and adjusting the configuration. Proper tuning ensures that undo data is retained for an adequate period, supporting long-running queries and flashback operations while efficiently managing space in the undo tablespace.