How can I verify if SHRNK or MOVE will reclaim space?

Question

Hi all,
I need to optimize a 9.2 and a 10.2 database.
I suspect fragmentation to be one of the causes of slowness (some queries do a lot of full table scan), but I want to rebuild only the objects which really contain unused .
How can I verify in advance if an ALTER TABLE SHRINK or ALTER TABLE MOVE will really reclaim free space? I'd like to verify if some segments are good candidates for shrinking rather when do an "ALTER TABLE MOVE" on the whole database...
Is DBMS_SPACE.SPACE_USAGE the only thing I can use?
thanks in advance for every answer.
andrea

Answer 1

Normally I use this package from Tom Kyte to check the space usage of the object.
create or replace procedure show_space
( p_segname in varchar2,
  p_owner   in varchar2 default user,
  p_type    in varchar2 default 'TABLE',
  p_partition in varchar2 default NULL )
-- this procedure uses authid current user so it can query DBA_*
-- views using privileges from a ROLE and so it can be installed
-- once per database, instead of once per user that wanted to use it
authid current_user
as
    l_free_blks                 number;
    l_total_blocks              number;
    l_total_bytes               number;
    l_unused_blocks             number;
    l_unused_bytes              number;
    l_LastUsedExtFileId         number;
    l_LastUsedExtBlockId        number;
    l_LAST_USED_BLOCK           number;
    l_segment_space_mgmt        varchar2(255);
    l_unformatted_blocks number;
    l_unformatted_bytes number;
    l_fs1_blocks number; l_fs1_bytes number;
    l_fs2_blocks number; l_fs2_bytes number;
    l_fs3_blocks number; l_fs3_bytes number;
    l_fs4_blocks number; l_fs4_bytes number;
    l_full_blocks number; l_full_bytes number;
    -- inline procedure to print out numbers nicely formatted
    -- with a simple label
    procedure p( p_label in varchar2, p_num in number )
    is
    begin
        dbms_output.put_line( rpad(p_label,40,'.') ||
                              to_char(p_num,'999,999,999,999') );
    end;
begin
   -- this query is executed dynamically in order to allow this procedure
   -- to be created by a user who has access to DBA_SEGMENTS/TABLESPACES
   -- via a role as is customary.
   -- NOTE: at runtime, the invoker MUST have access to these two
   -- views!
   -- this query determines if the object is a ASSM object or not
   begin
      execute immediate
          'select ts.segment_space_management
             from dba_segments seg, dba_tablespaces ts
            where seg.segment_name      = :p_segname
              and (:p_partition is null or
                  seg.partition_name = :p_partition)
              and seg.owner = :p_owner
              and seg.tablespace_name = ts.tablespace_name'
             into l_segment_space_mgmt
            using p_segname, p_partition, p_partition, p_owner;
   exception
       when too_many_rows then
          dbms_output.put_line
          ( 'This must be a partitioned table, use p_partition => ');
          return;
   end;
   -- if the object is in an ASSM tablespace, we must use this API
   -- call to get space information, else we use the FREE_BLOCKS
   -- API for the user managed segments
   if l_segment_space_mgmt = 'AUTO'
   then
     dbms_space.space_usage
     ( p_owner, p_segname, p_type, l_unformatted_blocks,
       l_unformatted_bytes, l_fs1_blocks, l_fs1_bytes,
       l_fs2_blocks, l_fs2_bytes, l_fs3_blocks, l_fs3_bytes,
       l_fs4_blocks, l_fs4_bytes, l_full_blocks, l_full_bytes, p_partition);
     p( 'Unformatted Blocks ', l_unformatted_blocks );
     p( 'FS1 Blocks (0-25)  ', l_fs1_blocks );
     p( 'FS2 Blocks (25-50) ', l_fs2_blocks );
     p( 'FS3 Blocks (50-75) ', l_fs3_blocks );
     p( 'FS4 Blocks (75-100)', l_fs4_blocks );
     p( 'Full Blocks        ', l_full_blocks );
  else
     dbms_space.free_blocks(
       segment_owner     => p_owner,
       segment_name      => p_segname,
       segment_type      => p_type,
       freelist_group_id => 0,
       free_blks         => l_free_blks);
     p( 'Free Blocks', l_free_blks );
  end if;
  -- and then the unused space API call to get the rest of the
  -- information
  dbms_space.unused_space
  ( segment_owner     => p_owner,
    segment_name      => p_segname,
    segment_type      => p_type,
    partition_name    => p_partition,
    total_blocks      => l_total_blocks,
    total_bytes       => l_total_bytes,
    unused_blocks     => l_unused_blocks,
    unused_bytes      => l_unused_bytes,
    LAST_USED_EXTENT_FILE_ID => l_LastUsedExtFileId,
    LAST_USED_EXTENT_BLOCK_ID => l_LastUsedExtBlockId,
    LAST_USED_BLOCK => l_LAST_USED_BLOCK );
    p( 'Total Blocks', l_total_blocks );
    p( 'Total Bytes', l_total_bytes );
    p( 'Total MBytes', trunc(l_total_bytes/1024/1024) );
    p( 'Unused Blocks', l_unused_blocks );
    p( 'Unused Bytes', l_unused_bytes );
    p( 'Last Used Ext FileId', l_LastUsedExtFileId );
    p( 'Last Used Ext BlockId', l_LastUsedExtBlockId );
    p( 'Last Used Block', l_LAST_USED_BLOCK );
end;
/Read about it completely here.
Aman....