Undo Internal

SQL> ----session 1
SQL> create table t as select object_id,owner from dba_objects where rownum < 1000;

表已创建。

SQL> alter table t add constraint t_id_pk primary key(object_id);

表已更改。

SQL>  select distinct dbms_rowid.rowid_block_number(rowid) block_id from t where object_id=555;

  BLOCK_ID
----------
       491
SQL> select distinct dbms_rowid.rowid_relative_fno(rowid) file_id  from t;

   FILE_ID
----------
         5


SQL> select file_name,file_id from dba_data_files;

FILE_NAME                         FILE_ID
------------------------------ ----------
/oracle/june/users01.dbf                4
/oracle/june/undotbs01.dbf              3
/oracle/june/sysaux01.dbf               2
/oracle/june/system01.dbf               1
/oracle/june/example01.dbf              5

SQL> update t set owner='killdb.com' where object_id=555;

已更新 1 行。

SQL> select xidusn,xidslot,xidsqn,ubablk,ubafil,ubarec,START_SCN from v$transaction;

    XIDUSN    XIDSLOT     XIDSQN     UBABLK     UBAFIL     UBAREC  START_SCN
---------- ---------- ---------- ---------- ---------- ---------- ----------
         6         15        916        366          3          6     968248
 

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

       SID
----------
        29

SQL> select spid from v$process where addr=(select paddr from v$session  where sid=29);

SPID
------------------------
1614
SQL> alter system flush buffer_cache;

系统已更改。




SQL> select * from t where object_id=555;

 OBJECT_ID OWNER
---------- ------------------------------
       555 SYS




但是 他还要读 原来的 

----Session 3
dtrace1.d脚本内容:
#!/usr/sbin/dtrace -s 
#pragma D option quiet

struct arg {
  int32_t arg0;
  int32_t arg1;
  int32_t arg2;
  int32_t arg3;
};

struct arg *arg;

pid$1::kkspsc0:entry{
  i=0;  
}

pid$1::kcbgtcr:entry{
  i++;
  arg=(struct arg *)copyin(arg0,sizeof(arg));
  printf("%5d %s:%s:%s:%s 0x%08x[%d,%d]
",i,probeprov,probemod,probefunc,probename, arg->arg1,arg->arg1>>22,arg->arg1&0x3fffff);
}


-bash-3.2# ./dtrace1.d 1614
    1 pid1614:oracle:kcbgtcr:entry 0x014001f3[5,499]  --索引块
    2 pid1614:oracle:kcbgtcr:entry 0x014001f5[5,501]  --索引块
    3 pid1614:oracle:kcbgtcr:entry 0x014001eb[5,491]  --表块
    4 pid1614:oracle:kcbgtcr:entry 0x00c000d0[3,208]  --_SYSSMU6_112126372$ 段头
    5 pid1614:oracle:kcbgtcr:entry 0x00c0016e[3,366]  --_SYSSMU6_112126372$ 

SQL> select owner, segment_name, segment_type    
  from dba_extents    
 where file_id = 5   
   and 499 between block_id    
   and block_id + blocks - 1  2    3    4    5  ;



OWNER                          SEGMENT_NAME                                                                      SEGMENT_TYPE
------------------------------ --------------------------------------------------------------------------------- ------------------
TEST                           T_ID_PK                                                                           INDEX


pid1614:oracle:kcbgtcr:entry 0x014001f3[5,499]  ---读的是索引块


SQL> select owner, segment_name, segment_type    
  from dba_extents    
 where file_id = 5   
   and 501 between block_id    
   and block_id + blocks - 1  2    3    4    5  ;

OWNER                          SEGMENT_NAME                                                                      SEGMENT_TYPE
------------------------------ --------------------------------------------------------------------------------- ------------------
TEST                           T_ID_PK                                                                           INDEX


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

OWNER                          SEGMENT_NAME                                                                       EXTENT_ID    FILE_ID   BLOCK_ID     BLOCKS
------------------------------ --------------------------------------------------------------------------------- ---------- ---------- ---------- ----------
TEST                           T_ID_PK                                                                                    0          5        496          8

EXTENT_ID	NUMBER	 	Extent number in the segment 在segment中extent号

BLOCK_ID	NUMBER	 	Starting block number of the extent

BLOCKS	NUMBER	 	Size of the extent in Oracle blocks



SQL> select owner,segment_name,header_file,header_block  from dba_segments where segment_name='T_ID_PK';

OWNER                          SEGMENT_NAME         HEADER_FILE HEADER_BLOCK
------------------------------ -------------------- ----------- ------------
TEST                           T_ID_PK                        5          498

HEADER_FILE	NUMBER	 	ID of the file containing the segment header

HEADER_BLOCK	NUMBER	 	ID of the block containing the segment header


起始块是496 块头是498


pid1614:oracle:kcbgtcr:entry 0x014001eb[5,491] ----读取的表

SQL> select owner, segment_name, segment_type    
  from dba_extents    
 where file_id = 5   
   and 491 between block_id    
   and block_id + blocks - 1  2    3    4    5  ;

OWNER                          SEGMENT_NAME                                                                      SEGMENT_TYPE
------------------------------ --------------------------------------------------------------------------------- ------------------
TEST                           T                                                                                 TABLE



pid1614:oracle:kcbgtcr:entry 0x00c000d0[3,208]



SQL> select owner, segment_name, segment_type    
  from dba_extents    
 where file_id = 3   
   and 208 between block_id    
   and block_id + blocks - 1  2    3    4    5  ;

OWNER                          SEGMENT_NAME                                                                      SEGMENT_TYPE
------------------------------ --------------------------------------------------------------------------------- ------------------
SYS                            _SYSSMU6_112126372$                                                               TYPE2 UNDO

SQL>  select owner,segment_name,extent_id,file_id,block_id,blocks from dba_extents where segment_name='_SYSSMU6_112126372$';

OWNER                          SEGMENT_NAME                                                                       EXTENT_ID    FILE_ID   BLOCK_ID     BLOCKS
------------------------------ --------------------------------------------------------------------------------- ---------- ---------- ---------- ----------
SYS                            _SYSSMU6_112126372$                                                                        0          3        208          8
SYS                            _SYSSMU6_112126372$                                                                        1          3        216          8
SYS                            _SYSSMU6_112126372$                                                                        2          3        360          8
SYS                            _SYSSMU6_112126372$                                                                        3          3      11136        128
SYS                            _SYSSMU6_112126372$                                                                        4          3       2304        128

读的是_SYSSMU6_112126372$ 段头

SQL> select owner,segment_name,header_file,header_block  from dba_segments where segment_name='_SYSSMU6_112126372$';

OWNER                          SEGMENT_NAME         HEADER_FILE HEADER_BLOCK
------------------------------ -------------------- ----------- ------------
SYS                            _SYSSMU6_112126372$            3          208



    5 pid1614:oracle:kcbgtcr:entry 0x00c0016e[3,366]

SQL> select owner, segment_name, segment_type    
  from dba_extents    
 where file_id = 3   
   and 366 between block_id    
   and block_id + blocks - 1  2    3    4    5  ;

OWNER                          SEGMENT_NAME                                                                      SEGMENT_TYPE
------------------------------ --------------------------------------------------------------------------------- ------------------
SYS                            _SYSSMU6_112126372$                                                               TYPE2 UNDO




SQL> select xidusn,xidslot,xidsqn,ubablk,ubafil,ubarec,START_SCN from v$transaction;

    XIDUSN    XIDSLOT     XIDSQN     UBABLK     UBAFIL     UBAREC  START_SCN
---------- ---------- ---------- ---------- ---------- ---------- ----------
         6         15        916        366          3          6     968248
 
完全一致:
UBAFIL	NUMBER	Undo block address (UBA) filenum

UBABLK	NUMBER	UBA block number

我们可以发现，在构造一致性读的时候会首先读取原表的数据块，然后读取undo segment header,定位到undo block,从而完成一致性读的构造


同样，我们也可以利用event 10200 来观察一致性读的情况，不过我这里测试发现不是太明确.

SQL> conn / as sysdba
已连接。
SQL> oradebug setmypid
已处理的语句
SQL>  alter system flush buffer_cache;

系统已更改。

SQL> alter system flush shared_pool;

系统已更改。

SQL>  alter session set events '10200 trace name context forever,level 10';

会话已更改。

SQL>  select * from test.t where object_id=555; 

 OBJECT_ID OWNER
---------- ------------------------------
       555 SYS

SQL>  alter session set events '10200 trace name context off';

会话已更改。

SQL> oradebug close_Trace
已处理的语句
SQL> oradebug tracefile_name
/oracle/app/diag/rdbms/june/june/trace/june_ora_2027.trc
SQL> 



-bash-3.2# ./dtrace1.d 1614
    1 pid1614:oracle:kcbgtcr:entry 0x014001f3[5,499]  --索引块
    2 pid1614:oracle:kcbgtcr:entry 0x014001f5[5,501]  --索引块
    3 pid1614:oracle:kcbgtcr:entry 0x014001eb[5,491]  --表块
    4 pid1614:oracle:kcbgtcr:entry 0x00c000d0[3,208]  --_SYSSMU6_112126372$ 段头
    5 pid1614:oracle:kcbgtcr:entry 0x00c0016e[3,366]  --_SYSSMU6_112126372$ 


ktrgtc2(): started for block <0x0006 : 0x014001f3> objd: 0x00015a1e --对用T_ID_PK

ktrgtc2(): started for block <0x0006 : 0x014001f5> objd: 0x00015a1e ---T_ID_PK

ktrgtc2(): started for block <0x0006 : 0x014001eb> objd: 0x00015a1d ---T表


kcbchg updating CR fields for 0xbefdc960, 20972011; 0:eec1b
kcbchg new CR fields for 0xbefdc960, 20972011; 0:eec1b
ktrgcm(): completed for block  <0x0006 : 0x014001eb> objd: 0x00015a1d
ktrgtc2(): completed for block <0x0006 : 0x014001eb> objd: 0x00015a1d

这些是什么意思呢？

我们dump [5,491]块
alter system dump datafile 5 block 491;

*** 2014-08-24 06:26:45.039
Dump of buffer cache at level 4 for tsn=6 rdba=20972011


BH (0xb5fb8b58) file#: 5 rdba: 0x014001eb (5/491) class: 1 ba: 0xb58b2000
  set: 3 pool: 3 bsz: 8192 bsi: 0 sflg: 2 pwc: 135,19
  dbwrid: 0 obj: 88605 objn: 88605 tsn: 6 afn: 5 hint: f
  hash: [0xbefdca18,0xe82db060] lru: [0xb5fb83c0,0xbefdd1a0]
  ckptq: [NULL] fileq: [NULL] objq: [NULL] objaq: [NULL]
  st: CR md: NULL tch: 1
  cr: [scn: 0x0.ef4ce],[xid: 0x6.f.394],[uba: 0xc0016e.85.6],[cls: 0x0.ef4ce],[sfl: 0x0],[lc: 0x0.ef4ce]
  flags: only_sequential_access
BH (0xbefdc960) file#: 5 rdba: 0x014001eb (5/491) class: 1 ba: 0xbec60000
  set: 3 pool: 3 bsz: 8192 bsi: 0 sflg: 2 pwc: 135,19
BH (0xbefdc960) file#: 5 rdba: 0x014001eb (5/491) class: 1 ba: 0xbec60000
  set: 3 pool: 3 bsz: 8192 bsi: 0 sflg: 2 pwc: 135,19
  dbwrid: 0 obj: 88605 objn: 88605 tsn: 6 afn: 5 hint: f
  hash: [0xbefdcb50,0xb5fb8c10] lru: [0xbefdc918,0xbefdd2d8]
  ckptq: [NULL] fileq: [NULL] objq: [NULL] objaq: [NULL]
  st: CR md: NULL tch: 1
  cr: [scn: 0x0.eec1b],[xid: 0x6.f.394],[uba: 0xc0016e.85.6],[cls: 0x0.eec1b],[sfl: 0x0],[lc: 0x0.eec1b]
  flags: only_sequential_access
BH (0xbefdca98) file#: 5 rdba: 0x014001eb (5/491) class: 1 ba: 0xbec62000
  set: 3 pool: 3 bsz: 8192 bsi: 0 sflg: 2 pwc: 135,19
  dbwrid: 0 obj: 88605 objn: 88605 tsn: 6 afn: 5 hint: f
  hash: [0xe82db060,0xbefdca18] lru: [0xbefdcdf8,0xbefdc918]
  ckptq: [NULL] fileq: [NULL] objq: [0xb5fb83e8,0xdd792470] objaq: [0xdd792460,0xbefdd1d8]
  st: XCURRENT md: NULL fpin: 'kdswh02: kdsgrp' tch: 0
  flags: block_written_once redo_since_read
  LRBA: [0x0.0.0] LSCN: [0x0.0] HSCN: [0xffff.ffffffff] HSUB: [1]


Block header dump:  0x014001eb
 Object id on Block? Y
 seg/obj: 0x15a1d  csc: 0x00.ef4ce  itc: 3  flg: E  typ: 1 - DATA
     brn: 0  bdba: 0x14001e8 ver: 0x01 opc: 0
     inc: 0  exflg: 0
     inc: 0  exflg: 0

 Itl           Xid                  Uba         Flag  Lck        Scn/Fsc
0x01   0xffff.000.00000000  0x00000000.0000.00  C---    0  scn 0x0000.000ec619
0x02   0x0006.00f.00000394  0x00c0016e.0085.06  ----    1  fsc 0x0000.00000000
0x03   0x0000.000.00000000  0x00000000.0000.00  ----    0  fsc 0x0000.00000000
bdba: 0x014001eb
data_block_dump,data header at 0xfffffd7ffdd34a7c
===============
tsiz: 0x1f80
hsiz: 0x474
pbl: 0xfffffd7ffdd34a7c
     76543210
flag=--------


BH (0xbefdc960) file#: 5 rdba: 0x014001eb (5/491) class: 1 ba: 0xbec60000
  set: 3 pool: 3 bsz: 8192 bsi: 0 sflg: 2 pwc: 135,19
  dbwrid: 0 obj: 88605 objn: 88605 tsn: 6 afn: 5 hint: f
  hash: [0xbefdcb50,0xb5fb8c10] lru: [0xbefdc918,0xbefdd2d8]
  ckptq: [NULL] fileq: [NULL] objq: [NULL] objaq: [NULL]
  st: CR md: NULL tch: 1
  cr: [scn: 0x0.eec1b],[xid: 0x6.f.394],[uba: 0xc0016e.85.6],[cls: 0x0.eec1b],[sfl: 0x0],[lc: 0x0.eec1b]
  flags: only_sequential_access