Oracle 参数 UNDO_RETENTION 官方解释，作用，如何配置最优化建议

For both AUTOEXTEND undo tablespaces and fixed-size undo tablespaces, the system retains undo for at least the time specified in this parameter, and automatically tunes the undo retention period to satisfy the undo requirements of the queries.

Automatic tuning of undo retention is not supported for LOBs. The RETENTION value for LOB columns is set to the value of the UNDO_RETENTION parameter.

The UNDO_RETENTION parameter is honored only if the current undo tablespace has enough space. If an active transaction requires undo space and the undo tablespace does not have available space, then the system starts reusing unexpired undo space. This action can potentially cause some queries to fail with a “snapshot too old” message.

The amount of time for which undo is retained for the Oracle Database for the current undo tablespace can be obtained by querying the TUNED_UNDORETENTION column of the V$UNDOSTAT dynamic performance view.

Recommendations for Modifying UNDO_RETENTION

Oracle generally recommends that you leave UNDO_RETENTION set to its default value. Modifying this parameter is recommended only in the following situations:

• You may want to increase the value of UNDO_RETENTION when using flashback features, such as Oracle Flashback Query, which require undo to be retained for longer than the longest running query in the system.

• In Oracle Active Data Guard environments, you may want to increase the value of UNDO_RETENTION on the primary instance in order to accommodate undo retention requirements on the standby instances. This allows the primary instance to retain undo for a longer period of time to serve queries on the standby instances. For more information, see Oracle Database
  Administrator’s Guide and Oracle Data Guard Concepts
  and Administration.

Modifying UNDO_RETENTION in a CDB

You can set the value of UNDO_RETENTION in the CDB root (CDB$ROOT) and in individual PDBs. The CDB root and PDBs can have the same value or different values. Note that you can modify this parameter in a PDB only if the PDB is in local undo mode; you cannot modify this parameter if the PDB is in shared undo mode.

Starting with Oracle Database release 19c, version 19.9, the value of UNDO_RETENTION is not inherited in a CDB. Therefore, if you change the value for UNDO_RETENTION in the CDB root, the PDBs do not inherit the new value. If you want to change the value for UNDO_RETENTION at the PDB level, you must explicitly do so. For example, if your CDB contains two PDBs (PDB1 and PDB2), you can set UNDO_RETENTION to 2000 in the CDB root and both PDBs as follows:

CopyALTER SESSION SET CONTAINER = CDB$ROOT;
ALTER SYSTEM SET UNDO_RETENTION=2000 SCOPE=BOTH;

ALTER SESSION SET CONTAINER = PDB1;
ALTER SYSTEM SET UNDO_RETENTION=2000 SCOPE=BOTH;

ALTER SESSION SET CONTAINER = PDB2;
ALTER SYSTEM SET UNDO_RETENTION=2000 SCOPE=BOTH;