Home Database Mysql Tutorial 【转】dcba的文章:Oracle的SET UNUSED COLUMN操作到底做了什么?

【转】dcba的文章:Oracle的SET UNUSED COLUMN操作到底做了什么?

Jun 07, 2016 pm 03:33 PM
oracle set article

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$    NUMBER

AUL> 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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