分析drop col对于数据存储块做了什么

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标题:分析drop col对于数据存储块做了什么

作者:惜分飞©版权所有[未经本人同意,不得以任何形式转载,否则有进一步追究法律责任的权利.]

oracle 的alter table drop col具体内部是对于数据存储块操作的,如果drop col之后dul之类的工具是否可以恢复,这里我通过具体测试,结合bbed,dump block等方法来说明该问题
1.创建测试表,并写入硬盘

SQL> create table xff.t_xifenfei as select object_id,owner,object_name from dba_objects;

Table created.

SQL> desc xff.t_xifenfei
 Name                                      Null?    Type
 ----------------------------------------- -------- ----------------------------
 OBJECT_ID                                          NUMBER
 OWNER                                              VARCHAR2(30)
 OBJECT_NAME                                        VARCHAR2(128)


SQL> alter system checkpoint;

System altered.

SQL> alter system checkpoint;

System altered.

2.找出来测试表一个block分析drop col对于存储的影响

SQL> select   rowid,
  2  dbms_rowid.rowid_relative_fno(rowid)rel_fno,
  3  dbms_rowid.rowid_block_number(rowid)blockno,
dbms_rowid.rowid_row_number(rowid) rowno,object_id
  4    5  from xff.t_xifenfei where rownum<5;

ROWID                 REL_FNO    BLOCKNO      ROWNO  OBJECT_ID
------------------ ---------- ---------- ---------- ----------
AAAZ9wAAEAAAJojAAA          4      39459          0         20
AAAZ9wAAEAAAJojAAB          4      39459          1         46
AAAZ9wAAEAAAJojAAC          4      39459          2         28
AAAZ9wAAEAAAJojAAD          4      39459          3         15

3. dump block,并且记录该block 1,2,和最后一条记录

SQL> oradebug setmypid
Statement processed.
SQL> alter system dump datafile 4 block 39459;

System altered.

SQL> oradebug TRACEFILE_NAME
/home/u01/diag/rdbms/orcl/orcl/trace/orcl_ora_14069.trc


block_row_dump:
tab 0, row 0, @0x1f70
tl: 16 fb: --H-FL-- lb: 0x0  cc: 3
col  0: [ 2]  c1 15
col  1: [ 3]  53 59 53
col  2: [ 5]  49 43 4f 4c 24
tab 0, row 1, @0x1f5e
tl: 18 fb: --H-FL-- lb: 0x0  cc: 3
col  0: [ 2]  c1 2f
col  1: [ 3]  53 59 53
col  2: [ 7]  49 5f 55 53 45 52 31
…………
tab 0, row 288, @0x589
tl: 22 fb: --H-FL-- lb: 0x0  cc: 3
col  0: [ 3]  c2 03 5b
col  1: [ 3]  53 59 53
col  2: [10]  49 5f 4a 4f 42 5f 4e 45 58 54

4. 使用bbed查看该block 1,2,和最后一条记录

[oracle@localhost ~]$ bbed password=blockedit blocksize=8192 filename=/usr/local/oradata/qsng/users01.dbf 

BBED: Release 2.0.0.0.0 - Limited Production on Sun Apr 3 22:25:28 2016

Copyright (c) 1982, 2011, Oracle and/or its affiliates.  All rights reserved.

************* !!! For Oracle Internal Use only !!! ***************

BBED> set block 39459
        BLOCK#          39459

BBED> map
 File: /usr/local/oradata/qsng/users01.dbf (0)
 Block: 39459                                 Dba:0x00000000
------------------------------------------------------------
 KTB Data Block (Table/Cluster)

 struct kcbh, 20 bytes                      @0       

 struct ktbbh, 96 bytes                     @20      

 struct kdbh, 14 bytes                      @124     

 struct kdbt[1], 4 bytes                    @138     

 sb2 kdbr[289]                              @142     

 ub1 freespace[821]                         @720     

 ub1 rowdata[6647]                          @1541    

 ub4 tailchk                                @8188    


BBED> p *kdbr[0]
rowdata[6631]
-------------
ub1 rowdata[6631]                           @8172     0x2c

BBED> x /rncc
rowdata[6631]                               @8172    
-------------
flag@8172: 0x2c (KDRHFL, KDRHFF, KDRHFH)
lock@8173: 0x00
cols@8174:    3

col    0[2] @8175: 20 
col    1[3] @8178: SYS
col    2[5] @8182: ICOL$


BBED> d
 File: /usr/local/oradata/qsng/users01.dbf (0)
 Block: 39459            Offsets: 8172 to 8191           Dba:0x00000000
------------------------------------------------------------------------
 2c000302 c1150353 59530549 434f4c24 02067576 

 <32 bytes per line>

BBED> p *kdbr[1]
rowdata[6613]
-------------
ub1 rowdata[6613]                           @8154     0x2c

BBED> x /rncc
rowdata[6613]                               @8154    
-------------
flag@8154: 0x2c (KDRHFL, KDRHFF, KDRHFH)
lock@8155: 0x00
cols@8156:    3

col    0[2] @8157: 46 
col    1[3] @8160: SYS
col    2[7] @8164: I_USER1


BBED> d
 File: /usr/local/oradata/qsng/users01.dbf (0)
 Block: 39459            Offsets: 8154 to 8191           Dba:0x00000000
------------------------------------------------------------------------
 2c000302 c12f0353 59530749 5f555345 52312c00 0302c115 03535953 0549434f 
 4c240206 7576 

 <32 bytes per line>

BBED> p *kdbr[288]
rowdata[0]
----------
ub1 rowdata[0]                              @1541     0x2c

BBED> x /rncc
rowdata[0]                                  @1541    
----------
flag@1541: 0x2c (KDRHFL, KDRHFF, KDRHFH)
lock@1542: 0x00
cols@1543:    3

col    0[3] @1544: 290 
col    1[3] @1548: SYS
col   2[10] @1552: I_JOB_NEXT


BBED> set count 32
        COUNT           32

BBED> d
 File: /usr/local/oradata/qsng/users01.dbf (0)
 Block: 39459            Offsets: 1541 to 1572           Dba:0x00000000
------------------------------------------------------------------------
 2c000303 c2035b03 5359530a 495f4a4f 425f4e45 58542c00 0303c203 5a035359 

 <32 bytes per line>

5. 删除中间列,并且写入硬盘

SQL> ALTER TABLE XFF.T_XIFENFEI DROP COLUMN owner;

Table altered.

SQL> alter system checkpoint;

System altered.

SQL> /

System altered.

6. 查询确定相同行所在block没有发生改变

SQL> select   rowid,
  2  dbms_rowid.rowid_relative_fno(rowid)rel_fno,
  3  dbms_rowid.rowid_block_number(rowid)blockno,
dbms_rowid.rowid_row_number(rowid) rowno,object_id
  4    5  from xff.t_xifenfei where rownum<5;

ROWID                 REL_FNO    BLOCKNO      ROWNO  OBJECT_ID
------------------ ---------- ---------- ---------- ----------
AAAZ9wAAEAAAJojAAA          4      39459          0         20
AAAZ9wAAEAAAJojAAB          4      39459          1         46
AAAZ9wAAEAAAJojAAC          4      39459          2         28
AAAZ9wAAEAAAJojAAD          4      39459          3         15

7. drop col之后dump block继续分析

SQL> alter system dump datafile 4 block 39459;

System altered.

SQL>  select value from v$diag_info where name='Default Trace File';

VALUE
--------------------------------------------------------------------------------
/home/u01/diag/rdbms/orcl/orcl/trace/orcl_ora_14784.trc

SQL> 

tab 0, row 0, @0x1f70
tl: 12 fb: --H-FL-- lb: 0x2  cc: 2
col  0: [ 2]  c1 15
col  1: [ 5]  49 43 4f 4c 24
tab 0, row 1, @0x1f5e
tl: 14 fb: --H-FL-- lb: 0x2  cc: 2
col  0: [ 2]  c1 2f
col  1: [ 7]  49 5f 55 53 45 52 31
…………
tab 0, row 288, @0x589
tl: 18 fb: --H-FL-- lb: 0x2  cc: 2
col  0: [ 3]  c2 03 5b
col  1: [10]  49 5f 4a 4f 42 5f 4e 45 58 54

8. 使用bbed查看drop col后的数据存储情况

$ bbed password=blockedit blocksize=8192 filename=/usr/local/oradata/qsng/users01.dbf 

BBED: Release 2.0.0.0.0 - Limited Production on Sun Apr 3 22:31:37 2016

Copyright (c) 1982, 2011, Oracle and/or its affiliates.  All rights reserved.

************* !!! For Oracle Internal Use only !!! ***************

BBED> set block 39459
        BLOCK#          39459

BBED> map 
 File: /usr/local/oradata/qsng/users01.dbf (0)
 Block: 39459                                 Dba:0x00000000
------------------------------------------------------------
 KTB Data Block (Table/Cluster)

 struct kcbh, 20 bytes                      @0       

 struct ktbbh, 96 bytes                     @20      

 struct kdbh, 14 bytes                      @124     

 struct kdbt[1], 4 bytes                    @138     

 sb2 kdbr[289]                              @142     

 ub1 freespace[821]                         @720     

 ub1 rowdata[6647]                          @1541    

 ub4 tailchk                                @8188    


BBED> p *kdbr[0]
rowdata[6631]
-------------
ub1 rowdata[6631]                           @8172     0x2c

BBED> x /rncc
rowdata[6631]                               @8172    
-------------
flag@8172: 0x2c (KDRHFL, KDRHFF, KDRHFH)
lock@8173: 0x02
cols@8174:    2

col    0[2] @8175: 20 
col    1[5] @8178: ICOL$


BBED> d
 File: /usr/local/oradata/qsng/users01.dbf (0)
 Block: 39459            Offsets: 8172 to 8191           Dba:0x00000000
------------------------------------------------------------------------
 2c020202 c1150549 434f4c24 434f4c24 0106de78 

 <32 bytes per line>

BBED> p *kdbr[1]
rowdata[6613]
-------------
ub1 rowdata[6613]                           @8154     0x2c

BBED> x /rncc
rowdata[6613]                               @8154    
-------------
flag@8154: 0x2c (KDRHFL, KDRHFF, KDRHFH)
lock@8155: 0x02
cols@8156:    2

col    0[2] @8157: 46 
col    1[7] @8160: I_USER1


BBED> d
 File: /usr/local/oradata/qsng/users01.dbf (0)
 Block: 39459            Offsets: 8154 to 8191           Dba:0x00000000
------------------------------------------------------------------------
 2c020202 c12f0749 5f555345 52315345 52312c02 0202c115 0549434f 4c24434f 
 4c240106 de78 

 <32 bytes per line>

BBED> p *kdbr[288]
rowdata[0]
----------
ub1 rowdata[0]                              @1541     0x2c

BBED> set count 32
        COUNT           32

BBED> x /rncc
rowdata[0]                                  @1541    
----------
flag@1541: 0x2c (KDRHFL, KDRHFF, KDRHFH)
lock@1542: 0x02
cols@1543:    2

col    0[3] @1544: 290 
col   1[10] @1548: I_JOB_NEXT


BBED> d
 File: /usr/local/oradata/qsng/users01.dbf (0)
 Block: 39459            Offsets: 1541 to 1572           Dba:0x00000000
------------------------------------------------------------------------
 2c020203 c2035b0a 495f4a4f 425f4e45 58544e45 58542c02 0203c203 5a09495f 

 <32 bytes per line>

通过上述测试可以得出如下结论:
1. drop col是真的把对应列存储在block中的内容除掉,而且把后面的列的内容前移了,并且以前多于的内容(因为一行内容前移,后面就出现空闲记录不设置为空,而就是最初内容,下次如果行长度发生改变的时候使用,就和类似update把列修改短了一样)
2. drop col只是导致一行的长度变短,但是每行的偏移量未发生改变,也就是说,每行所在的偏移量没有改变,drop col之后,每行后面多了一些空闲空间
3. 根据上面分析的原理,drop col 是真的从block内部把这一列的数据使用后面列的数据覆盖了,因此从原理上而言,dul无法恢复drop col的数据(最后一列有可能可以恢复,因为他不会被覆盖),对于drop col,只能是通过备份不完全恢复,全库闪回,dg延迟应用等方法解决

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