Oracle根据数据块ITL查找UNDO前镜像

<pre name="code" class="sql">v$transaction: 列出了没有提交的活动事务

2. XID的结构

eg
SQL> ---session1

SQL> drop table test_undo1 purge;

Table dropped.

SQL> create table test_undo1 as select * from dba_objects;

Table created.
SQL> set linesize 200
SQL> select rowid,
       dbms_rowid.rowid_object(rowid) object_id,
       dbms_rowid.rowid_relative_fno(rowid) file_id,
       dbms_rowid.rowid_block_number(rowid) block_id,
       dbms_rowid.rowid_row_number(rowid) num,
       rowidtochar(rowid)
  from test_undo1
 where object_id = 1051;  2    3    4    5    6    7    8  

ROWID		    OBJECT_ID	 FILE_ID   BLOCK_ID	   NUM ROWIDTOCHAR(ROWID)
------------------ ---------- ---------- ---------- ---------- ------------------
AAApgPAALAAACkWAAC     169999	      11      10518	     2 AAApgPAALAAACkWAAC

SQL> select sid from v$mystat where rownum<2;

       SID
----------
      1625

SQL> delete from test_undo1  where object_id=1051;


1 row deleted.

SQL> SQL> 

产生一个事务:
SQL> select xidusn,xidslot,xidsqn,ubablk,ubafil,ubarec,to_char(START_SCN,'999999999999') from v$transaction;

    XIDUSN    XIDSLOT	  XIDSQN     UBABLK	UBAFIL	   UBAREC TO_CHAR(START
---------- ---------- ---------- ---------- ---------- ---------- -------------
	 1	   17	  173289	303	     2	       29   57693468601

其中XINUSN  表示回滚段号Undo Segement number


XIDSLOT 事务槽号

XIDSQN 事务编号

UBABLK  UBA 块号

-----dump 数据块
SQL>  alter system dump datafile 11 block 10518;

System altered.

Block header dump:  0x02c02916
 Object id on Block? Y
 seg/obj: 0x2980f  csc: 0x0d.6ecc779f  itc: 3  flg: -  typ: 1 - DATA
     fsl: 0  fnx: 0x0 ver: 0x01

 Itl           Xid                  Uba         Flag  Lck        Scn/Fsc
0x01   0xffff.000.00000000  0x00000000.0000.00  C---    0  scn 0x000d.6ecc779f
0x02   0x0001.011.0002a4e9  0x0080012f.4243.1d  ----    1  fsc 0x004e.00000000
0x03   0x0000.000.00000000  0x00000000.0000.00  ----    0  fsc 0x0000.00000000

data_block_dump,data header at 0x14055274
===============
tsiz: 0x1f88
hsiz: 0xba
pbl: 0x14055274
bdba: 0x02c02916
     76543210
flag=--------
ntab=1
nrow=84
frre=-1
fsbo=0xba
fseo=0x3eb
avsp=0x331
tosp=0x381
0xe:pti[0]      nrow=84 offs=0
0x12:pri[0]     offs=0x1f35
0x14:pri[1]     offs=0x1eea
0x16:pri[2]     offs=0x1e9a
0x18:pri[3]     offs=0x1e50


其中Lck 表示锁住的行

从上面我的数据块dump,我们可以发现，该事务所对应的ITL是0x02 .(因为lck是1,flag是---,表示未提交)

Itl           Xid                  Uba         Flag  Lck        Scn/Fsc
0x02   0x0001.011.0002a4e9  0x0080012f.4243.1d  ----    1  fsc 0x004e.00000000

下面开始分解Xid:
0x0001:回滚段编号,转换后为1,说明该事务使用的是第1号回滚段

011:事务槽编号(slot),转换后为17,说明对应undo segment header的transaction table记录中的index是17

0002a4e9:173289序号(同一个事务可能具有多个SCN，实际上每一个DML操作都有一个SCN）实际上对应undo segment header中transaction table的wrap#值


SQL> select xidusn,xidslot,xidsqn,ubablk,ubafil,ubarec,to_char(START_SCN,'9999999999999') from v$transaction;

    XIDUSN    XIDSLOT	  XIDSQN     UBABLK	UBAFIL	   UBAREC TO_CHAR(START_
---------- ---------- ---------- ---------- ---------- ---------- --------------
	 1	   17	  173289	303	     2	       29    5769346860



SQL> select name from v$rollname where usn=1;

NAME
------------------------------
_SYSSMU1$

-------undo segment header dump (_SYSSMU8$)


dump 回滚段:
SQL> alter system dump undo header '_SYSSMU1$';

System altered.

********************************************************************************
Undo Segment:  _SYSSMU1$ (1)
********************************************************************************
  Extent Control Header
  -----------------------------------------------------------------
  Extent Header:: spare1: 0      spare2: 0      #extents: 4      #blocks: 271
                  last map  0x00000000  #maps: 0      offset: 4080
      Highwater::  0x00800131  ext#: 2      blk#: 40     ext size: 128
  #blocks in seg. hdr's freelists: 0
  #blocks below: 0
  mapblk  0x00000000  offset: 2
                   Unlocked
     Map Header:: next  0x00000000  #extents: 4    obj#: 0      flag: 0x40000000
  Extent Map
  -----------------------------------------------------------------
   0x0080000a  length: 7
   0x00801611  length: 8
   0x00800109  length: 128
   0x00800789  length: 128

 Retention Table
  -----------------------------------------------------------
 Extent Number:0  Commit Time: 1408576610
 Extent Number:1  Commit Time: 1408576610
 Extent Number:2  Commit Time: 0
 Extent Number:3  Commit Time: 1408576610

  TRN CTL:: seq: 0x4243 chd: 0x002a ctl: 0x0028 inc: 0x00000000 nfb: 0x0002
            mgc: 0x8201 xts: 0x0068 flg: 0x0001 opt: 2147483646 (0x7ffffffe)
            uba: 0x0080012f.4243.1d scn: 0x000d.6ecc61a1
Version: 0x01
  FREE BLOCK POOL::
    uba: 0x00000000.4243.1c ext: 0x2  spc: 0x1166
    uba: 0x00800130.4243.02 ext: 0x2  spc: 0x1f06
    uba: 0x00800131.4243.16 ext: 0x2  spc: 0xfb6
    uba: 0x00000000.423d.01 ext: 0x2  spc: 0x1f88
    uba: 0x00000000.2919.01 ext: 0x2  spc: 0x1f88
  TRN TBL::

  index  state cflags  wrap#    uel         scn            dba            parent-xid    nub     stmt_num    cmt
  ------------------------------------------------------------------------------------------------
   0x0a    9    0x00  0x2a4f0  0x0008  0x000d.6ecc69cc  0x0080011f  0x0000.000.00000000  0x00000001   0x00000000  1408591580
   0x0b    9    0x00  0x2a4e6  0x002f  0x000d.6ecc72a2  0x00000000  0x0000.000.00000000  0x00000000   0x00000000  1408597235
   0x0c    9    0x00  0x2a4dd  0x000e  0x000d.6ecc6d05  0x0080011f  0x0000.000.00000000  0x00000001   0x00000000  1408593634
   0x0d    9    0x00  0x2a4ea  0x0022  0x000d.6ecc6384  0x0080011d  0x0000.000.00000000  0x00000001   0x00000000  1408587630
   0x0e    9    0x00  0x2a4ee  0x0017  0x000d.6ecc6e3b  0x0080011e  0x0000.000.00000000  0x00000001   0x00000000  1408594533
   0x0f    9    0x00  0x2a4e6  0x000a  0x000d.6ecc694a  0x0080011f  0x0000.000.00000000  0x00000001   0x00000000  1408591232
   0x10    9    0x00  0x2a4e1  0x0029  0x000d.6ecc728f  0x00000000  0x0000.000.00000000  0x00000000   0x00000000  1408597235
   0x11   10    0x80  0x2a4e9  0x0002  0x000d.6ecc77b9  0x0080012f  0x0000.000.00000000  0x00000001   0x00000000  0
   0x12    9    0x00  0x2a4f2  0x0028  0x000d.6ecc773f  0x0080012f  0x0000.000.00000000  0x00000001   0x00000000  1408600205
   0x13    9    0x00  0x2a4eb  0x002c  0x000d.6ecc68e0  0x0080011f  0x0000.000.00000000  0x00000001   0x00000000  1408591232
   0x14    9    0x00  0x2a4eb  0x001c  0x000d.6ecc6360  0x0080011d  0x0000.000.00000000  0x00000001   0x00000000  1408587630
   0x15    9    0x00  0x2a4ea  0x000d  0x000d.6ecc6370  0x0080011d  0x0000.000.00000000  0x00000001   0x00000000  1408587630
   0x16    9    0x00  0x2a4e9  0x0002  0x000d.6ecc7406  0x0080012f  0x0000.000.00000000  0x00000001   0x00000000  1408597835
   0x17    9    0x00  0x2a44f  0x0019  0x000d.6ecc6e4b  0x0080011e  0x0000.000.00000000  0x00000001   0x00000000  1408594533
   0x18    9    0x00  0x2a4f0  0x001d  0x000d.6ecc6548  0x0080011f  0x0000.000.00000000  0x00000001   0x00000000  1408588831
 

state 9标示inactive 

10 标示active 
  index  state cflags  wrap#    uel         scn            dba            parent-xid    nub     stmt_num    cmt
  ------------------------------------------------------------------------------------------------
  0x11   10    0x80  0x2a4e9  0x0002  0x000d.6ecc77b9  0x0080012f  0x0000.000.00000000  0x00000001   0x00000000  0

这一行中的 dba地址 标示的是undo block 

 0x11  ：17 :事务槽编号(slot),转换后为17,说明对应undo segment header中的transaction table记录中的index是17


########################################################################################################################################
SQL> select xidusn,xidslot,xidsqn,ubablk,ubafil,ubarec,to_char(START_SCN,'999999999999') from v$transaction;

    XIDUSN    XIDSLOT	  XIDSQN     UBABLK	UBAFIL	   UBAREC TO_CHAR(START
---------- ---------- ---------- ---------- ---------- ---------- -------------
	 1	   17	  173289	303	     2	       29   57693468601

其中XINUSN  表示回滚段号Undo Segement number


XIDSLOT 事务槽号

XIDSQN 事务编号

UBABLK  UBA 块号


dump 数据块:
从上面我的数据块dump,我们可以发现，该事务所对应的ITL是0x02 .(因为lck是1,flag是---,表示未提交)

Itl           Xid                  Uba         Flag  Lck        Scn/Fsc
0x02   0x0001.011.0002a4e9  0x0080012f.4243.1d  ----    1  fsc 0x004e.00000000

下面开始分解Xid:
0x0001:回滚段编号,转换后为1,说明该事务使用的是第1号回滚段

011:事务槽编号(slot),转换后为17,说明对应undo segment header的transaction table记录中的index是17

0002a4e9:173289序号(同一个事务可能具有多个SCN，实际上每一个DML操作都有一个SCN）实际上对应undo segment header中transaction table的wrap#值


数据块Uba 0x0080012f.4243.1d  对应 undo header '_SYSSMU1$' dba 0x0080012f

SQL>  select dbms_utility.data_block_address_file(to_number(substr(uba, 3, 8),
                                                          'xxxxxxxxxxxx')) as undo_file#,
           dbms_utility.data_block_address_block(to_number(substr(uba, 3, 8),
                                                           'xxxxxxxxxxxx')) as undo_block,
           to_number(substr(uba, 12, 4), 'xxxxxxxxxxxx') undo_sequence,
           to_number(substr(uba, 17, 2), 'xxxxxxxxxxxx') undo_record
      from (select '0x00c0016e.0085.06' uba from dual);  2    3    4    5    6    7  


UNDO_FILE# UNDO_BLOCK UNDO_SEQUENCE UNDO_RECORD
---------- ---------- ------------- -----------
         3        366           133           6




uba = Address of the last undo block used + Sequence + Last Entry in UNDO record map

###########################################################################################################################################



我们可以看到transaction table中状态处于active的slot是index  0x11 . 跟我们前面的内容刚好对上.

大家注意看slot 0x11的wrap#值,是不是刚好对上前面的xid中的第3部分内容 ？

cflags 表示什么？   表示事务的状态,0x80表示未commit

uel    表示什么？   表示事务当前所处于的extent区域(0x0002 ,转换为10进制后为2)

scn    表示什么？   该事务开始的scn,等于v$transaction.start_scn。


SQL> select * from v$rollstat where usn=1;

       USN	LATCH	 EXTENTS     RSSIZE	WRITES	    XACTS	GETS
---------- ---------- ---------- ---------- ---------- ---------- ----------
     WAITS    OPTSIZE	 HWMSIZE    SHRINKS	 WRAPS	  EXTENDS  AVESHRINK
---------- ---------- ---------- ---------- ---------- ---------- ----------
 AVEACTIVE STATUS	       CUREXT	  CURBLK
---------- --------------- ---------- ----------
	 1	    1	       4    2220032	140460		1	 354
	 0		 2220032	  0	     0		0	   0
	 0 ONLINE		    2	      40


可以可以发现，uel值正好等于v$rollstat.curext值(Current extent)= 2，0x0002表示第2个extent.

SQL> select owner,segment_name,extent_id,file_id,block_id,blocks from dba_extents where segment_name='_SYSSMU1$';

OWNER	   SEGMENT_NAME  EXTENT_ID    FILE_ID	BLOCK_ID     BLOCKS
---------- ------------ ---------- ---------- ---------- ----------
SYS	   _SYSSMU1$		 0	    2	       9	  8
SYS	   _SYSSMU1$		 1	    2	    5649	  8
SYS	   _SYSSMU1$		 2	    2	     265	128
SYS	   _SYSSMU1$		 3	    2	    1929	128

BLOCK_ID ：extent的起始块

blocks extent的块数

和如下对应
   0x0080000a  length: 7
   0x00801611  length: 8
   0x00800109  length: 128
   0x00800789  length: 128



查看创建UNOD的sql：

  CREATE UNDO TABLESPACE "UNDOTBS1" DATAFILE 
  '/u03/oradata/jhoa/undotbs01.dbf' SIZE 26214400
  AUTOEXTEND ON NEXT 5242880 MAXSIZE 32767M
  BLOCKSIZE 8192
  EXTENT MANAGEMENT LOCAL AUTOALLOCATE
   ALTER DATABASE DATAFILE 
  '/u03/oradata/jhoa/undotbs01.dbf' RESIZE 2537553920
 




SQL> select dbms_utility.data_block_address_file(TO_NUMBER('800109', 'XXXXXXXX')) file_id,
       dbms_utility.data_block_address_block(TO_NUMBER('800109',
                                                       'XXXXXXXX')) block_id
  from dual;
  2    3    4  
   FILE_ID   BLOCK_ID
---------- ----------
	 2	  265


SQL> select 265 + 128 from dual;

   265+128
----------
       393

undo block 303正好落在265 --- 393


这里的地址就是UBA：


  TRN CTL:: seq: 0x4243 chd: 0x002a ctl: 0x0028 inc: 0x00000000 nfb: 0x0002
            mgc: 0x8201 xts: 0x0068 flg: 0x0001 opt: 2147483646 (0x7ffffffe)
            uba: 0x0080012f.4243.1d scn: 0x000d.6ecc61a1


10 标示active 
  index  state cflags  wrap#    uel         scn            dba            parent-xid    nub     stmt_num    cmt
  ------------------------------------------------------------------------------------------------
  0x11   10    0x80  0x2a4e9  0x0002  0x000d.6ecc77b9  0x0080012f  0x0000.000.00000000  0x00000001   0x00000000  0


SQL> select dbms_utility.data_block_address_file(TO_NUMBER('080012f', 'XXXXXXXX')) file_id,
       dbms_utility.data_block_address_block(TO_NUMBER('080012f',
                                                       'XXXXXXXX')) block_id
  from dual;
  2    3    4  
   FILE_ID   BLOCK_ID
---------- ----------
	 2	  303

转换uba的地址后得到的块号是303