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,dump block等方法来说明该问题
1.创建测试表，并写入硬盘
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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.
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2.找出来测试表一个block分析drop col对于存储的影响
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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
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3. dump block，并且记录该block 1,2,和最后一条记录
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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
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4. 使用bbed查看该block 1,2,和最后一条记录
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[oracle ~]$ 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>
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5. 删除中间列，并且写入硬盘
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SQL> ALTER TABLE XFF.T_XIFENFEI DROP COLUMN owner; Table altered. SQL> alter system checkpoint; System altered. SQL> / System altered.
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6. 查询确定相同行所在block没有发生改变
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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
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7. drop col之后dump block继续分析
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SQL> alter system dump datafile 4 block 39459; System altered. SQL> select value from v$diag_info where name='Default Trace File'; VALUE -------------------------------------------------------------------------------- /home/u01/diag/rdbms/orcl/orcl/trace/orcl_ora_14784.trc SQL> tab 0, row 0, @0x1f70 tl: 12 fb: --H-FL-- lb: 0x2 cc: 2 col 0: [ 2] c1 15 col 1: [ 5] 49 43 4f 4c 24 tab 0, row 1, @0x1f5e tl: 14 fb: --H-FL-- lb: 0x2 cc: 2 col 0: [ 2] c1 2f col 1: [ 7] 49 5f 55 53 45 52 31 ………… tab 0, row 288, @0x589 tl: 18 fb: --H-FL-- lb: 0x2 cc: 2 col 0: [ 3] c2 03 5b col 1: [10] 49 5f 4a 4f 42 5f 4e 45 58 54
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8. 使用bbed查看drop col后的数据存储情况
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$ 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>
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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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