{フラッシュバック問合せ}undo保存の保証を設定

https://docs.oracle.com/cd/F19136_01/admin/managing-undo.html#GUID-FE48AEC0-64AE-4431-8A29-CD4EA15E6548

alter tablespace undo retention guarantee;
alter tablespace undo retention noguarantee;

select tablespace_name,retention from dba_tablespaces;