Bitmap blocks. Was: How does Oracle set the High Water Mark?

From: Feighery Raymond <Raymond.Feighery_at_churchill.com>
Date: Wed, 10 Mar 2004 14:34:10 -0000
Message-ID: <817D2444710B934B9F7B8A1DAAF432D601F2ADD6@brcexm03>

OK. This seems to be a feature of Bitmap Blocks (good article on Metalink Note:149516.1) and Automatic Space Segment Management.

When I run dbms_space.space_usage I get:

Unformatted Blocks = 0       
FS1 Blocks            = 0    
FS2 Blocks            = 0    
FS3 Blocks            = 0    
FS4 Blocks            = 13   
Full Blocks           = 0

where
FS1 means 0-25% free space within a block

FS2 means 25-50%  free space within a block   
FS3 means 50-75%  free space within a block   
FS4 means 75-100% free space within a block

So my 1 number row is somehow causing 13 blocks to be used (although partially).

Does this mean that everytime I want to read my 1 row, I have to scan 13 blocks (ouch!)?

Ray

-----Original Message-----

From: Feighery Raymond [mailto:Raymond.Feighery_at_churchill.com] Sent: Wednesday, March 10, 2004 1:56 PM
To: 'oracle-l_at_freelists.org'
Subject: How does Oracle set the High Water Mark?

Oracle 9.2.0.4 Solaris 8

How does Oracle set the HWM? In the example below, I create a table and insert one row. I would expect only one block to be used, instead Oracle seems to have pushed the HWM up to 13 blocks. I can imagine that this might be done to "pre-mark" rows as used during bulk inserts, but how does Oracle decide where to set the HWM?

SQL> select * from dba_tablespaces where tablespace_name = 'EXAMPLE'; TABLESPACE_NAME BLOCK_SIZE INITIAL_EXTENT NEXT_EXTENT MIN_EXTENTS
------------------------------ ---------- -------------- -----------

MAX_EXTENTS PCT_INCREASE MIN_EXTLEN STATUS CONTENTS LOGGING FOR EXTENT_MAN
----------- ------------ ---------- --------- --------- --------- ---

ALLOCATIO PLU SEGMEN DEF_TAB_
--------- --- ------ --------

EXAMPLE 4096 65536 1
2147483645 65536 ONLINE PERMANENT LOGGING NO LOCAL SYSTEM NO AUTO DISABLED SQL> create table t (col1 number(1));
Table created.
SQL> analyze table t compute statistics; Table analyzed.
SQL> insert into t values(1);
1 row created.
SQL> commit;
Commit complete.
SQL> select blocks, empty_blocks from user_tables where table_name = 'T'; BLOCKS EMPTY_BLOCKS
---------- ------------

0 16
SQL> analyze table t compute statistics; Table analyzed.
SQL> select blocks, empty_blocks from user_tables where table_name = 'T'; BLOCKS EMPTY_BLOCKS
---------- ------------

13 3

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should also delete the email and any attachment from your systems and should
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Churchill Insurance Group plc or its affiliates or subsidiaries. Thank you. 

Churchill Insurance Group plc.  Company Registration Number - 2280426.
England. 

Registered Office: Churchill Court, Westmoreland Road, Bromley, Kent BR1
1DP. 




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Received on Wed Mar 10 2004 - 08:33:13 CST