下面以例子說話：

SQL> create table test1.unused_test? as select rownum a,rownum*2 b,rownum*10?c from dba_objects where rownum<=100;

Table created.

看看數據字典：

SQL> select column_name,data_type,column_id,hidden_column,segment_column_id seg_

cid,internal_column_id internal_cid from dba_tab_cols where wner='TEST1' and ta

ble_name='UNUSED_TEST';

COLUMN_NAM DATA_TYPE??????????? COLUMN_ID HIDDEN??? SEG_CID INTERNAL_CID

---------- -------------------- --------- ------ ---------- ------------

A????????? NUMBER?????????????????????? 1 NO????????????? 1??????????? 1

B????????? NUMBER?????????????????????? 2 NO????????????? 2??????????? 2

C????????? NUMBER?????????????????????? 3 NO????????????? 3??????????? 3

SQL> select column_name,data_type,column_id from dba_tab_columns where wner='TE

ST1' and table_name='UNUSED_TEST';

COLUMN_NAM DATA_TYPE??????????? COLUMN_ID

---------- -------------------- ---------

A????????? NUMBER?????????????????????? 1

B????????? NUMBER?????????????????????? 2

C????????? NUMBER?????????????????????? 3

SQL> select object_id from dba_objects where wner='TEST1' and object_name='UNUS

ED_TEST' and object_type='TABLE';

OBJECT_ID

----------

6577

SQL> select col#,segcol#,name,intcol# from col$ where obj#=6577;

COL#??? SEGCOL# NAME????????? INTCOL#

---------- ---------- ---------- ----------

1????????? 1 A?????????????????? 1

2????????? 2 B?????????????????? 2

3????????? 3 C?????????????????? 3

通過DUMP數據庫文件塊可以看到每行有三列(這里不再列出DUMP文件內容)

下面將B列置為unused狀態：

SQL> alter table test1.unused_test set unused (b);

Table altered.

SQL> select column_name,data_type,column_id,hidden_column,segment_column_id seg_

cid,internal_column_id internal_cid from dba_tab_cols where wner='TEST1' and ta

ble_name='UNUSED_TEST';

COLUMN_NAME????????????????? DATA_T COLUMN_ID HIDDEN??? SEG_CID INTERNAL_CID

---------------------------- ------ --------- ------ ---------- ------------

A??????????????????????????? NUMBER???????? 1 NO????????????? 1??????????? 1

SYS_C00002_08011915:24:34$?? NUMBER?????????? YES???????????? 2??????????? 2

C??????????????????????????? NUMBER???????? 2 NO????????????? 3??????????? 3

這里原來的B列，其名字為系統自動生成的一列，命名形式為SYS_CNNNNN_YYMMDDHH24:MI:SS$，NNNNN為原來的COLUMN_ID，前面補0補足成5數。hidden已經變為YES，COLUMN_ID為空。其他兩列A和C的COLUMN_ID順序作了調整。這三列的SEGMENT_COLUMN_ID和INTERNAL_COLUMN_ID沒有變化。

SQL> select column_name,data_type,column_id from dba_tab_columns where wner='TE

ST1' and table_name='UNUSED_TEST';

COLUMN_NAME????????????????? DATA_T COLUMN_ID

---------------------------- ------ ---------

A??????????????????????????? NUMBER???????? 1

C??????????????????????????? NUMBER???????? 2

在DBA_TAB_COLUMNS視圖中，B列已經沒有顯示出來。

SQL> select col#,segcol#,name,intcol# from col$ where obj#=6577;

COL#??? SEGCOL# NAME??????????????????????????? INTCOL#

---------- ---------- ---------------------------- ----------

1????????? 1 A???????????????????????????????????? 1

0????????? 2 SYS_C00002_08011915:24:34$??????????? 2

2????????? 3 C???????????????????????????????????? 3

這里B列的COL#已經變成0.SEGCOL#和INTCOL#列沒有變化，NAME也已經變化

DUMP出來的數據中，每一行仍然有三列。

嘗試插入數據：

SQL> insert into test1.unused_test values (1234,4321,1);

insert into test1.unused_test values (1234,4321,1)

*

ERROR at line 1:

ORA-00913: too many values

SQL> insert into test1.unused_test values (1234,4321);

1 row created.

SQL> select rowid from test1.unused_test where a=1234 and c=4321;

ROWID

------------------

AAABmxAAFAAAAEuAAA

此ROWID對應的rfile#為5，block#為302，row number為0

DUMP出這一塊查看第0行數據，發現在數據塊中每行仍然是3列，第二列也就是原來的B列其值為NULL。

現在我們將C列刪除：

SQL> alter table test1.unused_test drop (c);

Table altered.

SQL> select column_name,data_type,column_id,hidden_column,segment_column_id seg_

cid,internal_column_id internal_cid from dba_tab_cols where wner='TEST1' and ta

ble_name='UNUSED_TEST';

COLUMN_NAME????????????????? DATA_T COLUMN_ID HIDDEN??? SEG_CID INTERNAL_CID

---------------------------- ------ --------- ------ ---------- ------------

A??????????????????????????? NUMBER???????? 1 NO????????????? 1??????????? 1

這里可以看出B列和C列都已經刪除。

SQL> select column_name,data_type,column_id from dba_tab_columns where wner='TE

ST1' and table_name='UNUSED_TEST';

COLUMN_NAME????????????????? DATA_T COLUMN_ID

---------------------------- ------ ---------

A??????????????????????????? NUMBER???????? 1

SQL> select col#,segcol#,name,intcol# from col$ where obj#=6577;

COL#??? SEGCOL# NAME??????????????????????????? INTCOL#

---------- ---------- ---------------------------- ----------

1????????? 1 A???????????????????????????????????? 1

都可以看出B列和C列已經被刪除。從這個實驗就可以看出，在刪除C時會將UNUSED列一并刪除。

DUMP出數據塊可以發展，塊中每一行只有1列。因此SET UNUSED只是修改了數據字典，速度較多。而將COLUMN DROP掉，不僅修改數據字典，而且修改實際的塊數據。如果表比較大，會耗費比較長的時間。

總結

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