Dumping a block in Oracle

It is possible to peek into the datafiles and see what data they contain. In order to demonstrate that, I create a table (dump_table), fill some data in it and dump it into a file for further inspecting.
The SQL statement to dump one or more blocks is


alter system dump datafile <number datafile> block min <first number> block max <last block>

Here's the table that's going to be filled:


create table dump_table (


  a varchar2(4000),


  b varchar2(4000),


  c varchar2(4000),


  d varchar2(4000));


insert into dump_table values(


  'aaaaaaaaaa','bbbbbbbbbb','cccccccccc','dddddddddd');




insert into dump_table values(


  'AAAAAAAAAA','BBBBBBBBBB','CCCCCCCCCC','DDDDDDDDDD');




commit;

Dumping the Segment's Header Block

Each table (actually each segment) has a segment header.
First, the file and the block number must be determined before the block can be dumpe

 


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




HEADER_FILE    HEADER_BLOCK


--------------------     -----------------------


                        3                       108639


alter system dump datafile 3 block 108639;

Location of dumpfile

This command dumps the block in a more or less readable form. Where is it dumped? The directory in which it is dumped is found like this:


select value from v$parameter where name = 'user_dump_dest';



The name of the dumped file is then determined like so:


select spid


from v$session s, v$process p


where p.addr = s.paddr


and s.audsid = sys_context('userenv','sessionid')

 

This returns a number. Append this number to ORA and append a suffix .trc
See find the trace file for a complete script.
Here's the interesting part of the dump file:


Start dump data blocks tsn: 3 file#: 3 minblk 49 maxblk 49


buffer tsn: 3 rdba: 0x00c00031 (3/49)


scn: 0x0000.0005c24e seq: 0x01 flg: 0x04 tail: 0xc24e1001


frmt: 0x02 chkval: 0x49ed type: 0x10=DATA SEGMENT HEADER - UNLIMITED


  Extent Control Header


  -----------------------------------------------------------------


  Extent Header:: spare1: 0      spare2: 0      #extents: 1      #blocks: 15    


                  last map  0x00000000  #maps: 0      offset: 2080  


      Highwater::  0x00c00033  ext#: 0      blk#: 1      ext size: 15    


  #blocks in seg. hdr's freelists: 1     


  #blocks below: 1     


  mapblk  0x00000000  offset: 0     


                   Unlocked


     Map Header:: next  0x0000000
ca12
0  #extents: 1    obj#: 5927   flag: 0x40000000


  Extent Map


  -----------------------------------------------------------------


   0x00c00032  length: 15    


  


  nfl = 1, nfb = 1 typ = 1 nxf = 0 ccnt = 1


  SEG LST:: flg: USED   lhd: 0x00c00032 ltl: 0x00c00032 


End dump data blocks tsn: 3 file#: 3 minblk 49 maxblk 49

 

The number after obj# (that is 5927) is found in sys.obj$:


select name from obj$ where obj# = 5927;



returns DUMP_TABLE.

 

Dumping a block from a rowid

The following procedure can be used to dump a database block that contains a row whose rowid is known.
The user who compiles this procedure must have the rights to select v$parameter, v$process, v$session, otherwise he'll get an ora-00942 table or view does not exist.
 


create or replace procedure dump_block_from_rowid(p_rowid rowid)


is




  dump_file varchar2(4000);


   


begin




  execute immediate '


    alter system dump datafile ' ||


      dbms_rowid.rowid_relative_fno(p_rowid) || '


    block ' ||


      dbms_rowid.rowid_block_number(p_rowid);




  select 


    u_dump.value || '/' || instance.value || '_ora_' || sys.v_$process.spid || '.trc'


  into 


    dump_file


  from 


               sys.v_$parameter u_dump 


    cross join sys.v_$parameter instance


    cross join sys.v_$process 


          join sys.v_$session 


            on sys.v_$process.addr = sys.v_$session.paddr


  where 


   u_dump.name   = 'user_dump_dest' and 


   instance.name = 'instance_name' and


   sys.v_$session.audsid=sys_context('userenv','sessionid');




  dbms_output.put_line('  dumped block to: ');


  dbms_output.put_line('  ' ||  dump_file);


end;


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