About Block Dump

Question

Hi,
Recently I try to learn BBED to analyze data block.
I create a test table, and run query as below:
BBED@linora>select file_id, block_id from dba_extents
2 where segment_name = 'BLOCK_LAB';
FILE_ID BLOCK_ID
9 1825
I knew that the block 1825 is the table BLOCK_LAB's header block. The data block is in the immediate next block ,so in this example, will be 1826.
But when I run below query, it's result not as expect!
BBED@linora>SELECT
2 dbms_rowid.rowid_relative_fno(rowid) REL_FNO,
3 dbms_rowid.rowid_block_number(rowid) BLOCKNO,
4 dbms_rowid.rowid_row_number(rowid) ROWNO
5 from block_lab
6 where id=1
7 /
REL_FNO BLOCKNO ROWNO
9 1832 0
how could this happened?
Edited by: user12957738 on 2013-6-6 上午12:31

Answer 1

Fung.Kong wrote:
Hi,
Recently I try to learn BBED to analyze data block.
I create a test table, and run query as below:
BBED@linora>select file_id, block_id from dba_extents
2 where segment_name = 'BLOCK_LAB';
FILE_ID BLOCK_ID
9 1825
I knew that the block 1825 is the table BLOCK_LAB's header block. The data block is in the immediate next block ,so in this example, will be 1826.The first block of the first extent isn't necessarily the segment header block; and the first block used for an insert isn't necessarily the first block after the segment header block. You have to check whether you're using freelists or bitmaps for segment space management, and if it's freelist management you have to allow for the freelist blocks if you've declared multiple freelist groups for a segment, and if it's bitmap management the first used block might be almost anywhere in the first extent.
Quick demo to show the potential mismatch between dba_extents and dba_segments:
SQL> select segment_name, file_id, block_id from dba_extents where owner = 'TEST_USER' and extent_id = 0;
SEGMENT_NAME                FILE_ID   BLOCK_ID
T1                                5        128
T2                                6        128
2 rows selected.
SQL> select segment_name, header_file, header_block from dba_segments where owner = 'TEST_USER';
SEGMENT_NAME             HEADER_FILE HEADER_BLOCK
T1                                 5          128
T2                                 6          131File 5 is using freelist management - the segment header block is the first block of the first extent
FIle 6 is using ASSM - with a relatively small first extent size - the segment header block follows 2 level 1 bitmap blocks and a level 2 bitmap block.
Regards
Jonathan Lewis