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x$bh.dbablk values

From: Daniel W. Fink <optimaldba_at_yahoo.com>
Date: Fri, 23 May 2003 15:11:47 -0800
Message-ID: <F001.005A202B.20030523151147@fatcity.com>


I am working on determining which objects have 'hot blocks'. In two different sessions, I issue "select count(*) from random_data where rowid_rownum in (1,2,3);" repeatedly to see what happens with the touch count (x$bh.tch).
In another session, I look for the blocks related to this object by issuing the statement:
select x.owner, x.segment_name, x.segment_type, b.dbarfil, b.dbablk, b.class, b.state, sum(b.tc
from dba_extents x, x$bh b

where b.dbarfil = x.file_id
  and b.dbablk between x.block_id and (x.block_id + blocks - 1)
  and x.owner = 'BCA'

group by x.owner, x.segment_name, x.segment_type, b.dbarfil, b.dbablk, b.class, b.state;

and the output is

OWNER      SEGMENT_NAME              SEGMENT_TYPE       DBARFIL     
DBABLK      CLASS      STATE SUM(B.TCH)
---------- ------------------------- --------------- ---------- 
---------- ---------- ---------- ---
BCA        HWM_DATA                  TABLE                   12      
12809          4          3          2
BCA        FRAG_DATA                 TABLE                   12      
19209          4          3          2
BCA        PLAN_TABLE                TABLE                   12      
25609          4          3          2
BCA        RANDOM_DATA               TABLE                   12       
6409          4          1          5
BCA        RANDOM_DATA               TABLE                   12      
12729          1          1          0
BCA        RANDOM_DATA               TABLE                   12      
12730          1          1          0
BCA        RANDOM_DATA               TABLE                   12      
12731          1          1          0
BCA        RANDOM_DATA               TABLE                   12      
12732          1          1          0
BCA        RANDOM_DATA               TABLE                   12      
12733          1          1          0
BCA        RANDOM_DATA               TABLE                   12      
12745          1          1          0
BCA        RANDOM_DATA               TABLE                   12      
12746          1          1          0
BCA        RANDOM_DATA               TABLE                   12      
12747          1          1          0
BCA        RANDOM_DATA               TABLE                   12      
12748          1          1          0
BCA        RANDOM_DATA               TABLE                   12      
12749          1          1          0
BCA        RANDOM_DATA               TABLE                   12      
12750          1          1          0
BCA        RANDOM_DATA               TABLE                   12      
12751          1          1          0
BCA        RANDOM_DATA               TABLE                   12      
12752          1          1          0
BCA        RANDOM_DATA               TABLE                   12      
12753          1          1          0
BCA        RANDOM_DATA               TABLE                   12      
12754          1          1          0
BCA        RANDOM_DATA               TABLE                   12      
12755          1          1          0
BCA        IX_SD_ROWNUM              INDEX                   13       
1929          4          3          2
BCA        PK_FD_REC_NO              INDEX                   13       
8993          4          3          2
BCA        PK_HD_REC_NO              INDEX                   13       
8969          4          3          2
BCA        PK_RD_REC_NO              INDEX                   13       
3977          4          3          2
BCA        PK_SD_REC_NO              INDEX                   13          
9          4          3          2
BCA        IX_RD_SMALL_RN            INDEX                   13       
3985          4          3          2
BCA        SEQUENTIAL_DATA           TABLE                   12          
9          4          3          2

The oddity is that I have restarted the instance and have only issued queries against the random_data table. Since I am the only user on the system, I know that no other sessions are accessing the objects. The interesting bit in all this is that the blocks other than random_data listed in x$bh are the segment headers.

select segment_name, file_id, block_id, blocks, block_id+blocks-1 from dba_extents
where owner = 'BCA'

   and extent_id = 0
order by file_id, block_id;

SEGMENT_NAME                 FILE_ID   BLOCK_ID     BLOCKS BLOCK_ID+BLOCKS-1
------------------------- ---------- ---------- ---------- -----------------
SEQUENTIAL_DATA                   12          9          8                16
RANDOM_DATA                       12       6409          8              6416
HWM_DATA                          12      12809          8             12816
FRAG_DATA                         12      19209          8             19216
PLAN_TABLE                        12      25609          8             25616
PK_SD_REC_NO                      13          9          8                16
IX_SD_ROWNUM                      13       1929          8              1936
PK_RD_REC_NO                      13       3977          8              3984
IX_RD_SMALL_RN                    13       3985          8              3992
PK_HD_REC_NO                      13       8969          8              8976
PK_FD_REC_NO                      13       8993          8              9000


The questions are
Why are the segment headers being returned by my first query? Is there something wrong with the sql statement? Am I not reading the dbablk value properly?
Is there a process that is reading these segment headers?

--

Daniel W. Fink
http://www.optimaldba.com

--

Please see the official ORACLE-L FAQ: http://www.orafaq.net
--

Author: Daniel W. Fink
  INET: optimaldba_at_yahoo.com

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