dual在資料庫不同啟動狀態其欄位數

不經意間在《oracle DBA工作日記》看到dual這個系統表在資料庫nomount、mount、open狀態其欄位不一樣，驗證後記錄如下：
--關閉資料庫
 

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

--啟動資料庫到nomount狀態,nomount狀態有四個欄位
 

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
 

--啟動到mount狀態與nomount狀態有同樣的四個欄位
 

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

--啟動到open狀態,dual只有一個欄位
 

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