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關于Oracle的Dual表

瀏覽:68日期:2023-11-23 19:15:37

有朋友問到關于Oracle的Dual表問題,并且提到了Tom的一個鏈接:http://asktom.oracle.com/pls/asktom/f?p=100:11:0::::P11_QUESTION_ID:1562813956388 很多人關心的是Dual的內部實現,這Oracle自然是不會披露的,不過我們可以從一些有限的資料獲得關于Dual的印象:

There is internalized code that makes this happen. Code checks that ensure that a table scan of SYS.DUAL only returns one row. Svrmgrl behaviour is incorrect but this is now an obsolete product.

The base issue you should always remember and keep is: DUAL table should always have 1 ROW. Dual is a normal table with one dummy column of varchar2(1).This is basically used from several applications as a pseudo table for getting results from a select statement that use functions like sysdate or other prebuilt or application functions. If DUAL has no rows at all some applications (that use DUAL) may fail with NO_DATA_FOUND exception. If DUAL has more than 1 row then applications (that use DUAL) may fail with TOO_MANY_ROWS exception.

So DUAL should ALWAYS have 1 and only 1 row.

前兩句話最為關鍵,實際上我們也容易猜到,Oracle通過內部代碼來實現對于DUAL的訪問和控制,并且通過Internal Code使得這個表與眾不同。

Tom提到在Close了Database之后,可以看到這個表的內存地址及展現,這說明這個表的結構并不單純:

sql> select * from dual; D-X

sql> alter database close;

Database altered.

sql> select * from dual;

ADDR; INDX;INST_ID D-------- ---------- ---------- -0A4F8858; 0; 1 X

注意不要更改Dual表的內容,否則可能引起數據庫的問題。

如果該表意外刪除,可以通過設置初始化參數replication_dependency_tracking = FALSE,重啟數據庫來重建該表。

標簽: Oracle 數據庫
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