flashback_transaction_query 视图中 undo_sql 的值始终为 null

所以我有一个定义如下的表：

`create table test.test as (select * from dual);

并运行以下更新语句：

update test.test set dummy = '1' where dummy = 'X';
commit;

我想找到上面事务的undo_sql，所以我运行了下面的语句来查找事务id

SQL> select dummy, versions_xid, versions_startscn, versions_endscn from test.test
versions between scn minvalue and maxvalue ;

D VERSIONS_XID     VERSIONS_STARTSCN VERSIONS_ENDSCN
- ---------------- ----------------- ---------------
1 0A000500B00A0000           2802291
X                                            2802291

并运行以下语句以找到 undo_sql 语句

SQL> select undo_sql from flashback_transaction_query 
where xid = (select  versions_xid from test.test versions 
between scn minvalue and maxvalue where versions_xid is not null);

UNDO_SQL
--------------------------------------------------------------------------------

如您所见，我什么也没有，如果我运行以下语句，我也什么也没有。

SQL> select undo_sql from flashback_transaction_query where undo_sql is not null;

no row selected

看起来 undo_sql 总是为空；

有什么我没有配置的吗？这是来自 v$system_parameter 的值

SQL> select name, value from v$system_parameter where name like '%undo%';

NAME                 VALUE
----------------     ----------------
undo_management      AUTO
undo_tablespace      UNDOTBS1
undo_retention       18000