The number of leaf_blocks does not decrease after delete

Question

hi All,
this the example from Tom's book <<expert oracle database architecture>>, may anyone explain why the leaf_block oes not decrease to 523 after delete.
db version is 10.2.0.3, SEGMENT SPACE MANAGEMENT MANUAL in the tablespace system
sys@CS10G> drop table big_table purge;
Table dropped.
sys@CS10G> @big_table
Table created.
Table altered.
Enter value for 1: 500000
old   3:     l_rows number := &1;
new   3:     l_rows number := 500000;
Enter value for 1: 500000
old   9:         where rownum <= &1;
new   9:         where rownum <= 500000;
PL/SQL procedure successfully completed.
Table altered.
PL/SQL procedure successfully completed.
  COUNT(*)
    500000
sys@CS10G> declare
  2  l_freelist_blocks number;
  3  begin
  4  dbms_space.free_blocks
  5  (
  6  segment_owner => user,
  7  segment_name => 'BIG_TABLE_PK',
  8  segment_type => 'INDEX',
  9  freelist_group_id => 0,
10  free_blks => l_freelist_blocks
11  );
12  dbms_output.put_line(' block on freelist = ' || l_freelist_blocks);
13  end;
14  /
block on freelist = 0
PL/SQL procedure successfully completed.
sys@CS10G> select leaf_blocks from user_indexes where index_name = 'BIG_TABLE_PK';
LEAF_BLOCKS
       1043
sys@CS10G> delete  from big_table where id <= 250000;
250000 rows deleted.
sys@CS10G> commit;
Commit complete.
sys@CS10G> declare
  2  l_freelist_blocks number;
  3  begin
  4  dbms_space.free_blocks
  5  (
  6  segment_owner => user,
  7  segment_name => 'BIG_TABLE_PK',
  8  segment_type => 'INDEX',
  9  freelist_group_id => 0,
10  free_blks => l_freelist_blocks
11  );
12  dbms_output.put_line(' block on freelist = ' || l_freelist_blocks);
13  end;
14  /
block on freelist = 520
PL/SQL procedure successfully completed.
sys@CS10G>
sys@CS10G> select leaf_blocks from user_indexes where index_name = 'BIG_TABLE_PK';
LEAF_BLOCKS
       1043Edited by: Sidney Chen on Aug 15, 2009 3:10 PM

Answer 1

Sidney Chen wrote:
hi All,
this the example from Tom's book <<expert oracle database architecture>>, may anyone explain why the leaf_block oes not decrease to 523 after delete.It's just the way it works - when a leaf block is emptied it is linked into the freelist, but also stays linked into the correct place in the index structure. It will be detached from the structure by a coalesce, or when Oracle needs to use a free block to handle new data.
If you want a couple of reasonable guesses why:
<ul>
a) it probably makes read-consistency of indexes easier to deal with
b) unlinking a block from the structure is expensive (you have to modify the blocks to the left, the right, and above) so Oracle takes the gamble that you're likely to re-insert data in the same block in the near future.
</ul>
Regards
Jonathan Lewis
http://jonathanlewis.wordpress.com
http://www.jlcomp.demon.co.uk
"Science is more than a body of knowledge; it is a way of thinking"
Carl Sagan