Temporary Undo in Oracle Database 12c Release 1 (12.1)

Writing to the undo tablespace requires the database to be open in read-write mode, so global temporary tables can't be used in read-only databases and physical standby databases.

Enable/Disable Temporary Undo
-- Session level
ALTER SESSION SET TEMP_UNDO_ENABLED = TRUE;
ALTER SESSION SET TEMP_UNDO_ENABLED = FALSE;