SQL> select segment_name , bytes from
  2  dba_segments
  3  where segment_name='EMP' and owner='SCOTT';

SEGMENT_NAME
---------------------------------------
     BYTES
----------
EMP
     65536

scott@9i > desc dept
 Name                                                  Null?    Type
 ----------------------------------------------------- -------- ------------------------------------
 DEPTNO                                                         NUMBER(2)
 DNAME                                                          VARCHAR2(14)
 LOC                                                            VARCHAR2(13)

scott@9i > alter table dept modify (dname char(2000), loc char(2000));

Table altered.


--
-- Let us check
--
scott@9i > @ddl
Enter value for table_name: DEPT


  CREATE TABLE "SCOTT"."DEPT"
   (    "DEPTNO" NUMBER(2,0),
        "DNAME" CHAR(2000),
        "LOC" CHAR(2000)
   ) PCTFREE 10 PCTUSED 40 INITRANS 1 MAXTRANS 255 NOCOMPRESS LOGGING
  STORAGE(INITIAL 65536 NEXT 1048576 MINEXTENTS 1 MAXEXTENTS 2147483645
  PCTINCREASE 0 FREELISTS 1 FREELIST GROUPS 1 BUFFER_POOL DEFAULT)
  TABLESPACE "USERS"
 ;

--
-- Collect statistics
-- 
scott@9i > analyze table dept compute statistics;

Table analyzed.

-- 
- Current allocations
-- 
scott@9i > get get_ext
  1  select extent_id, bytes, blocks
  2  from user_extents
  3  where segment_name = 'DEPT'
  4* and segment_type = 'TABLE'
scott@9i > /

 EXTENT_ID      BYTES     BLOCKS
---------- ---------- ----------
         0      65536          8


--
-- I am inserting some data, in a loop and collect stats again.
--
scott@9i > get dept_loopinsert
  1   begin
  2   for mag in 1..5 loop
  3       insert into dept (select * from dept);
  4   end loop;
  5   commit;
  6*  end;
scott@9i > /

PL/SQL procedure successfully completed.

scott@9i > analyze table dept compute statistics;

Table analyzed.

--
-- You can see now, many extents are allocated, many blocks used.
-- Total bytes allocated and total blocks allocated are 
-- report at the end.
-- 

scott@9i > get get_ext
  1  select extent_id, bytes, blocks
  2  from user_extents
  3  where segment_name = 'DEPT'
  4* and segment_type = 'TABLE'

scott@9i > compute sum of BYTES BLOCKS on report
scott@9i > break on report
scott@9i > /

 EXTENT_ID      BYTES     BLOCKS
---------- ---------- ----------
         0      65536          8
         1      65536          8
         2      65536          8
         3      65536          8
         4      65536          8
         5      65536          8
         6      65536          8
         7      65536          8
         8      65536          8
         9      65536          8
        10      65536          8
        11      65536          8
        12      65536          8
        13      65536          8
        14      65536          8
        15      65536          8
        16    1048576        128
           ---------- ----------
sum           2097152        256

17 rows selected.

--
-- So the allocated size is 2097152 bytes
-- Let us cross check that.
-- NOTE: heading is size_in_megs. ignore and read it as size_in_bytes.
--

scott@9i > get size
  1  column segment_name format a15
  2* Select segment_name,bytes  size_in_megs  from user_segments where segment_name='DEPT';
scott@9i > @size

SEGMENT_NAME    SIZE_IN_MEGS
--------------- ------------
DEPT                 2097152

--
-- Now using DBMS_SPACE builtin-package as mentioned here
-- http://download-west.oracle.com/docs/cd/B10501_01/appdev.920/a96612/d_space2.htm#1000756
--  i have little pl/sql that does a few things for us.
-- I used UNUSED_SPACE to find the unused space .
-- I also used the same to cross-check above.

scott@9i > get get_space_u_dbms
  1   DECLARE
  2   total_blocks number;
  3   total_bytes number;
  4   unused_blocks number;
  5   unused_bytes number;
  6   last_used_extent_file_id number;
  7   last_used_extent_block_id  number;
  8   last_used_block            number;
  9   begin
 10   DBMS_SPACE.UNUSED_SPACE('SCOTT', 'DEPT', 'TABLE', total_blocks => total_blocks ,
 11                                                     total_bytes => total_bytes,
 12                                                     unused_blocks =>unused_blocks,
 13                                                     unused_bytes => unused_bytes,
 14                                                     last_used_extent_file_id => last_used_extent_file_id,
 15                                                     last_used_extent_block_id =>last_used_extent_block_id ,
 16                                                     last_used_block =>   last_used_block  );
 17   DBMS_OUTPUT.PUT_LINE ( 'total_blocks '|| total_blocks);
 18   DBMS_OUTPUT.PUT_LINE ( 'total_bytest '|| total_bytes);
 19   DBMS_OUTPUT.PUT_LINE ( 'unused_block '|| unused_blocks);
 20   DBMS_OUTPUT.PUT_LINE ( 'unused_bytes '|| unused_bytes);
 21*  end;
scott@9i > /
total_blocks 256
total_bytest 2097152
unused_block 64
unused_bytes 524288

PL/SQL procedure successfully completed.


--
--Let us dellocate the unused space
-- 

scott@9i > alter table dept deallocate unused;

Table altered.

scott@9i > @get_space_u_dbms
total_blocks 192
total_bytest 1572864
unused_block 0
unused_bytes 0

PL/SQL procedure successfully completed.

-- crosscheck?
--

PL/SQL procedure successfully completed.

scott@9i > @size

SEGMENT_NAME    SIZE_IN_MEGS
--------------- ------------
DEPT                 1572864

scott@9i >  compute sum of BYTES BLOCKS on report
scott@9i > break on report
scott@9i > @get_ext

 EXTENT_ID      BYTES     BLOCKS
---------- ---------- ----------
         0      65536          8
         1      65536          8
         2      65536          8
         3      65536          8
         4      65536          8
         5      65536          8
         6      65536          8
         7      65536          8
         8      65536          8
         9      65536          8
        10      65536          8
        11      65536          8
        12      65536          8
        13      65536          8
        14      65536          8
        15      65536          8
        16     524288         64
           ---------- ----------
sum           1572864        192

17 rows selected.

scott@9i > @size
SEGMENT_NAME    SIZE_IN_BYTES
--------------- -------------
DEPT                  2097152

scott@9i > @get_ext

 EXTENT_ID      BYTES     BLOCKS
---------- ---------- ----------
         0      65536          8
         1      65536          8
         2      65536          8
         3      65536          8
         4      65536          8
         5      65536          8
         6      65536          8
         7      65536          8
         8      65536          8
         9      65536          8
        10      65536          8
        11      65536          8
        12      65536          8
        13      65536          8
        14      65536          8
        15      65536          8
        16    1048576        128
           ---------- ----------
sum           2097152        256

17 rows selected.


scott@9i > get avg_space
  1* select avg_space, blocks, empty_blocks from user_tables where table_name ='DEPT';

--
-- Here EMPTY_BLOCKS are the ones that are never used and above HWM.
--      BLOCKS are ones that are used(1 block for header information). It may or may not be completely used.
--      AVG_SPACE is the average space per block.
-- DEALLOCATE UNSED will release the space above HWM.
-- If we do table re-org, we can see the actual space (avg_space) will vary.
-- after any such operation, collect statistics.
-- 
scott@9i > @avg_space

 AVG_SPACE     BLOCKS EMPTY_BLOCKS
---------- ---------- ------------
      4937        180           76

-- Total blocks is sum of BLOCKS+EMPTY_BLOCKS from above and from get_ext.sql
-- Total Bytes is sum BYTES from get_ext.sql
-- 

-- First we do a re-org. Export /import or a simple move to the same tablespace as it was will do the job.
-- You can AVG_SPACE per block is lesser after the re-org. Also you save a bunch in BLOCKS.
-- And Your EMPTY blocks that were never used is Incremented.
--
scott@9i > alter table dept move tablespace users;

Table altered.

scott@9i > get ana_dept
  1  analyze table dept delete statistics;
  2* analyze table dept compute statistics;
scott@9i > @ana_dept

Table analyzed.


Table analyzed.
scott@9i > @avg_space

 AVG_SPACE     BLOCKS EMPTY_BLOCKS
---------- ---------- ------------
      4032        140          116

scott@9i > @get_space_u_dbms
total_blocks 256
total_bytest 2097152
unused_block 116
unused_bytes 950272

PL/SQL procedure successfully completed.

--
-- When you issue deallocate unused, anything above HWM is released.
--

scott@9i > alter table dept deallocate unused
  2  ;

Table altered.

scott@9i > @ana_dept

Table analyzed.


Table analyzed.

scott@9i > @avg_space

 AVG_SPACE     BLOCKS EMPTY_BLOCKS
---------- ---------- ------------
      4032        140            4

scott@9i > @get_space_u_dbms
total_blocks 144
total_bytest 1179648
unused_block 4
unused_bytes 32768

PL/SQL procedure successfully completed.

scott@9i > @get_ext

 EXTENT_ID      BYTES     BLOCKS
---------- ---------- ----------
         0      65536          8
         1      65536          8
         2      65536          8
         3      65536          8
         4      65536          8
         5      65536          8
         6      65536          8
         7      65536          8
         8      65536          8
         9      65536          8
        10      65536          8
        11      65536          8
        12      65536          8
        13      65536          8
        14      65536          8
        15      65536          8
        16     131072         16
           ---------- ----------
sum           1179648        144

17 rows selected.

--
-- Base data
-- they match

scott@9i > analyze table emp compute statistics;

Table analyzed.

scott@9i > select segment_name,blocks from user_segments where segment_name='EMP';

SEGMENT_     BLOCKS
-------- ----------
EMP             768

scott@9i > select table_name,num_rows,blocks,empty_blocks, blocks+empty_blocks from user_tables where table_name='EMP';

TABL   NUM_ROWS     BLOCKS EMPTY_BLOCKS BLOCKS+EMPTY_BLOCKS
---- ---------- ---------- ------------ -------------------
EMP      114688        684           84                 768

--
-- Now let us deallocate unused and do the same.
-- Still, they match

scott@9i > alter table emp deallocate unused;

Table altered.

scott@9i >  analyze table emp compute statistics;

Table analyzed.

scott@9i >  select segment_name,blocks from user_segments where segment_name='EMP';

SEGMENT_     BLOCKS
-------- ----------
EMP             704

scott@9i >  select table_name,num_rows,blocks,empty_blocks, blocks+empty_blocks from user_tables where table_name='EMP';

TABL   NUM_ROWS     BLOCKS EMPTY_BLOCKS BLOCKS+EMPTY_BLOCKS
---- ---------- ---------- ------------ -------------------
EMP      114688        684           20                 704

--
-- Let us reset the HWM by moving to same tablespace.
-- They still match

scott@9i > alter table emp move tablespace users;

Table altered.

scott@9i >  analyze table emp compute statistics;

Table analyzed.

scott@9i >  select segment_name,blocks from user_segments where segment_name='EMP';

SEGMENT_     BLOCKS
-------- ----------
EMP             768

scott@9i > select table_name,num_rows,blocks,empty_blocks, blocks+empty_blocks from user_tables where table_name='EMP';

TABL   NUM_ROWS     BLOCKS EMPTY_BLOCKS BLOCKS+EMPTY_BLOCKS
---- ---------- ---------- ------------ -------------------
EMP      114688        696           72                 768

user_segments.blocks=user_tables.block + user_tables.empty_blocks + 1;
Please see this session;

scott@9i > create table mytable (c1 number);

Table created.

scott@9i >  insert into mytable values (23333333333333333333333333333333333333333);

1 row created.

scott@9i > begin
  2  for mag in 1..12 loop
  3  insert into mytable (select * from mytable);
  4  end loop;
  5  commit;
  6  end;
  7  /

PL/SQL procedure successfully completed.

scott@9i > analyze table mytable compute statistics;

Table analyzed.

scott@9i > column segment_name format a10
scott@9i >  select segment_name,blocks from user_segments where segment_name='MYTABLE';

SEGMENT_NA     BLOCKS
---------- ----------
MYTABLE            18

scott@9i >  select table_name,num_rows,blocks,empty_blocks, blocks+empty_blocks from user_tables where table_name='MYTABLE';

TABLE_NAME                       NUM_ROWS     BLOCKS EMPTY_BLOCKS BLOCKS+EMPTY_BLOCKS
------------------------------ ---------- ---------- ------------ -------------------
MYTABLE                              4096         17            0                  17

scott@9i > alter table mytable move tablespace users;

Table altered.

scott@9i > analyze table mytable compute statistics;

Table analyzed.

scott@9i >  select segment_name,blocks from user_segments where segment_name='MYTABLE';

SEGMENT_NA     BLOCKS
---------- ----------
MYTABLE            18

scott@9i >  select table_name,num_rows,blocks,empty_blocks, blocks+empty_blocks from user_tables where table_name='MYTABLE';

TABLE_NAME                       NUM_ROWS     BLOCKS EMPTY_BLOCKS BLOCKS+EMPTY_BLOCKS
------------------------------ ---------- ---------- ------------ -------------------
MYTABLE                              4096         16            1                  17

Table SAVINGS does not have partiotions or sub-partitions.

1. dbms_stats.GATHER_TABLE_STATS(ownname=>'TEST',
			  tabname=>'SAVINGS',
			  granularity => 'DEFAULT'
			  ); 

Ref: Metalink note 236935.1, the granularity Default = Table + Partition .

SQL> select blocks from user_segments where segment_name = 'SAVINGS' and segment_type='TABLE';

   BLOCKS
---------
    408775

SQL> select blocks, empty_blocks, blocks + empty_blocks from user_tables where table_name = 'SAVINGS' ;

   BLOCKS EMPTY_BLOCKS BLOCKS+EMPTY_BLOCKS
--------- ------------ -------------------
   408694            0              408694


2. alter table SAVINGS compute statistics;

Ref: ANALYZE command collects statistics only at the lowest level and derives higher level statistics by aggregation.

Here Lowest level is TABLE.

SQL> select blocks from user_segments where segment_name = 'SAVINGS' and segment_type='TABLE';

   BLOCKS
---------
    408775

SQL> select blocks, empty_blocks, blocks + empty_blocks from user_tables where table_name = 'SAVINGS' ;

   BLOCKS EMPTY_BLOCKS BLOCKS+EMPTY_BLOCKS
--------- ------------ -------------------
   408694            0              408694


If, user_segments.blocks=user_tables.block + user_tables.empty_blocks + 1;

I do not understand why there is a difference even after Analyzing.

Brian