[20130721]ORACLE 12C Invisible Columns.txt

[20130721]ORACLE 12C Invisible Columns.txt

在ORACLE 12C中有了Invisible Columns的概念,就是在表中真实的存在该列,但是通过设置Invisible可以实现不显示该列。
自己测试看看。

SQL> select banner from v$version ;

BANNER

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

Oracle Database 12c Enterprise Edition Release 12.1.0.1.0 - 64bit Production

PL/SQL Release 12.1.0.1.0 - Production

CORE    12.1.0.1.0      Production

TNS for 64-bit Windows: Version 12.1.0.1.0 - Production

NLSRTL Version 12.1.0.1.0 - Production

SQL> create table t (a number,b number ,c number);

Table created.

SQL> select object_id,data_object_id from dba_objects where wner=user and object_name='T';

 OBJECT_ID DATA_OBJECT_ID

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

     92476          92476

SQL> select obj#,col#,segcol#,SEGCOLLENGTH,OFFSET, NAME,TYPE#,LENGTH , INTCOL# from sys.col$ where obj#= 92476;

      OBJ#       COL#    SEGCOL# SEGCOLLENGTH     OFFSET NAME                      TYPE#     LENGTH    INTCOL#

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

     92476          1          1           22          0 A                             2         22       1

     92476          2          2           22          0 B                             2         22       2

     92476          3          3           22          0 C                             2         22       3

--因为我创建的是普通表，注意看COL#,SEGCOL#,INTCOL#相同。

--intcol#为创建表时的列顺序，col#初始情况下跟intcol#相同。而segcol#表示列在数据段上存储时的顺序。

--如果是IOT，cluster table可能不同。

SQL> ALTER TABLE t MODIFY (a invisible);

Table altered.

SQL> select obj#,col#,segcol#,SEGCOLLENGTH,OFFSET, NAME,TYPE#,LENGTH , INTCOL# from sys.col$ where obj#= 92476;

      OBJ#       COL#    SEGCOL# SEGCOLLENGTH     OFFSET NAME                      TYPE#     LENGTH    INTCOL#

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

     92476          0          1           22          0 A                             2         22       1

     92476          1          2           22          0 B                             2         22       2

     92476          2          3           22          0 C                             2         22       3

SQL> ALTER TABLE t MODIFY (a visible);

Table altered.

SQL> select obj#,col#,segcol#,SEGCOLLENGTH,OFFSET, NAME,TYPE#,LENGTH , INTCOL# from sys.col$ where obj#= 92476;

      OBJ#       COL#    SEGCOL# SEGCOLLENGTH     OFFSET NAME                      TYPE#     LENGTH    INTCOL#

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

     92476          3          1           22          0 A                             2         22       1

     92476          1          2           22          0 B                             2         22       2

     92476          2          3           22          0 C                             2         22       3

--可以发现COL#的顺序发现了变化，利用这个特性可以实现修改显示select *时列的展示顺序(Column Ordering)。

SQL> set linesize 40

SQL> desc t;

 Name              Null?    Type

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

 B                          NUMBER

 C                          NUMBER

 A                          NUMBER

--但是要注意如果插入数据：

insert into t values(1,2,3);

commit ;

SQL> select * from t;

         B          C          A

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

         1          2          3

-- 实际上按照desc t显示的顺序，如果你看数据块，可以发现保持A,B,C的顺序。

SQL> @lookup_rowid AAAWk8AAJAAAACtAAA

    OBJECT       FILE      BLOCK        ROW DBA

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

     92476          9        173          0 9,173

SQL> alter system dump datafile 9 block 173 ;

System altered.

Block header dump:  0x024000ad

 Object id on Block? Y

 seg/obj: 0x1693c  csc: 0x00.28e176  itc: 2  flg: E  typ: 1 - DATA

     brn: 0  bdba: 0x24000a8 ver: 0x01 opc: 0

     inc: 0  exflg: 0

 Itl           Xid                  Uba         Flag  Lck        Scn/Fsc

0x01   0x0008.020.00000869  0x01402866.0111.21  --U-    1  fsc 0x0000.0028e177

0x02   0x0000.000.00000000  0x00000000.0000.00  ----    0  fsc 0x0000.00000000

bdba: 0x024000ad

data_block_dump,data header at 0x179d6064

===============

tsiz: 0x1f98

hsiz: 0x14

pbl: 0x179d6064

     76543210

flag=--------

ntab=1

nrow=1

frre=-1

fsbo=0x14

fseo=0x1f8c

avsp=0x1f78

tosp=0x1f78

0xe:pti[0]    nrow=1    ffs=0

0x12:pri[0]    ffs=0x1f8c

block_row_dump:

tab 0, row 0, @0x1f8c

tl: 12 fb: --H-FL-- lb: 0x1  cc: 3

col  0: [ 2]  c1 04

col  1: [ 2]  c1 02

col  2: [ 2]  c1 03

end_of_block_dump

End dump data blocks tsn: 3 file#: 9 minblk 173 maxblk 173

SQL> select dump(A,16) c20 ,dump(b,16) c20,dump(c,16) c20 from t ;

C20                  C20                  C20

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

Typ=2 Len=2: c1,4    Typ=2 Len=2: c1,2    Typ=2 Len=2: c1,3