In which situation may the UNDO_RETENTION parameter be ignored, even if it is set to a value?

An autoextensible data file lets the tablespace grow to accommodate retention requirements, making Oracle more likely to honor UNDO_RETENTION rather than ignore it.

The number of undo tablespaces in the database does not affect whether UNDO_RETENTION is honored; only one undo tablespace is active at a time and it independently respects the retention setting.

With a fixed-size undo tablespace and retention guarantee disabled, Oracle treats UNDO_RETENTION as advisory only; if space runs out, the database overwrites the oldest undo segments regardless of whether their retention period has expired, effectively ignoring the parameter.

An autoextensible undo tablespace can expand to preserve undo data for the configured retention period, so Oracle is generally able to honor UNDO_RETENTION in this scenario rather than ignore it.