Read before image

Question

Hi.
Say , my transaction started at T1..
Someone did some modification after T1, now ITL entries of the block is open.
Then a commit issued and itl entries are cleaned.
During my transaction how does oracle understand if the row was modified after T1 and read the
before image from undo?
ITL entries are already cleaned, so it has to check something else..

Answer 1

Pascal Nouma wrote:
I got again confused aman...
I know how scn incremented in controlfile and datafile headers.Well, you need to read alot still. I don't think that you have grasped the concept of data files and control file scn yet.
However I still have some doubt, how it is stored inside block ?Where else it would be ? If you don't believe me, let's do a block dump and see,
SQL*Plus: Release 10.2.0.1.0 - Production on Sun Oct 11 08:00:37 2009
Copyright (c) 1982, 2005, Oracle.  All rights reserved.
Connected to an idle instance.
SQL> startup
ORACLE instance started.
Total System Global Area  167772160 bytes
Fixed Size                  1247900 bytes
Variable Size              67110244 bytes
Database Buffers           96468992 bytes
Redo Buffers                2945024 bytes
Database mounted.
Database opened.
SQL> conn aman/aman
Connected.
SQL> create table test_scn( a number);
Table created.
SQL> select header_file, header_block from  dba_segments where segment_name='TEST_SCN';
HEADER_FILE HEADER_BLOCK
          4           43
SQL>
SQL> select header_file, header_block from  dba_segments where segment_name='TEST_SCN';
HEADER_FILE HEADER_BLOCK
          4           43
SQL> alter session set tracefile_identifier='testscn';
Session altered.
SQL> alter system dump datafile 4 block 43;
System altered.
SQL>
*** TRACE DUMP CONTINUED FROM FILE  ***
Dump file e:\oracle\product\10.2.0\admin\orcl\udump\orcl_ora_2360_testscn.trc
Sun Oct 11 08:02:38 2009
ORACLE V10.2.0.1.0 - Production vsnsta=0
vsnsql=14 vsnxtr=3
Oracle Database 10g Enterprise Edition Release 10.2.0.1.0 - Production
With the Partitioning, OLAP and Data Mining options
Windows XP Version V5.1 Service Pack 2
CPU                 : 1 - type 586
Process Affinity    : 0x00000000
Memory (Avail/Total): Ph:2048M/3315M, Ph+PgF:3836M/5200M, VA:1728M/2047M
Instance name: orcl
Redo thread mounted by this instance: 1
Oracle process number: 19
Windows thread id: 2360, image: ORACLE.EXE (SHAD)
*** 2009-10-11 08:02:38.359
*** ACTION NAME:() 2009-10-11 08:02:38.359
*** MODULE NAME:(SQL*Plus) 2009-10-11 08:02:38.359
*** SERVICE NAME:(SYS$USERS) 2009-10-11 08:02:38.359
*** SESSION ID:(156.10) 2009-10-11 08:02:38.359
Start dump data blocks tsn: 4 file#: 4 minblk 43 maxblk 43
buffer tsn: 4 rdba: 0x0100002b (4/43)
*scn: 0x0000.00188d5a* seq: 0x03 flg: 0x00 tail: 0x8d5a2303
frmt: 0x02 chkval: 0x0000 type: 0x23=PAGETABLE SEGMENT HEADER
Hex dump of block: st=0, typ_found=1Its the segment header block which I have dumped and have shown you!
HTH
Aman....