【转】dcba的文章:Oracle的SET UNUSED COLUMN操作到底做了什么?
我们先来搞一个测试, 将其中的一个列设为UNUSED: SQL DESC T_FID Name Null? Type - COL1 NUMBER COL2 NUMBER COL3 NUMBER SQL ALTER TABLE T_FID SET UNUSED (COL3); Table altered. SQL alter system checkpoint; System altered. 然后我们去查COL$表中的
我们先来搞一个测试, 将其中的一个列设为UNUSED:
SQL> DESC T_FID
Name Null? Type
———————– ——– ——-
COL1 NUMBER
COL2 NUMBER
COL3 NUMBER
SQL> ALTER TABLE T_FID SET UNUSED (COL3);
Table altered.
SQL> alter system checkpoint;
System altered.
然后我们去查COL$表中的三列(COL#,SEGCOL#,NAME), 看看值是什么?
在这儿为了方便, 我就用AUL/MyDUL
的DESC命令去看了
AUL> desc alex.t_fid
Storage(OBJ#=7641 OBJD=7641 TS=2 FILE=3 BLOCK=1529 CLUSTER=0)
No. SEQ Column Name Type
— — —————————– —————-
1 1 COL1 NUMBER
2 2 COL2 NUMBER
0 3 SYS_C00004_05092817:31:11$ NUMBERAUL> UNLOAD TABLE alex.t_fid;
2005-09-28 17:32:40
Unload OBJD=7641 FILE=3 BLOCK=1529 CLUSTER=0 …
9,8,7
2005-09-28 17:32:40
从这儿可以看到, SET UNUSED时将一个列的名字换一下, 此外还将这个列的显示顺序的值设为0, 原来为3.
虽然Oracle不提供反向操作,但实际上原来的数据还存在, 不提供的原因可能是因为不能保证not null和其他的约束吧.

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