dual在数据库不同启动状态其字段数

不经意间在《oracle DBA工作日记》看到dual这个系统表在数据库nomount、mount、open状态其字段不一样，验证后记录如下：
--关闭数据库
 

SQL> shutdown immediate
Database closed.
Database dismounted.
ORACLE instance shut down.

--启动数据库到nomount状态,nomount状态有四个字段
 

SQL> startup nomount
ORACLE instance started.
Total System Global Area  776646656 bytes
Fixed Size                  2257272 bytes
Variable Size             478154376 bytes
Database Buffers          289406976 bytes
Redo Buffers                6828032 bytes
SQL> select * from dual;
ADDR                   INDX    INST_ID DUM
---------------- ---------- ---------- ---
000000000A622620          0          1 X

--启动到mount状态与nomount状态有同样的四个字段
 

SQL> alter database mount;
Database altered.
SQL> select * from dual;
ADDR                   INDX    INST_ID DUM
---------------- ---------- ---------- ---
000000000A622620          0          1 X

--启动到open状态,dual只有一个字段
 

SQL> alter database open;
Database altered.
SQL> select * from dual;
DUM
---
X