SQL> exec dbms_stats.gather_schema_stats ( 'HR');

PL/SQL procedure successfully completed.

SQL> column segment_name format a25
SQL> select s.segment_name , s.blocks,t.blocks
  2  from dba_segments s , dba_tables t
  3  where s.segment_name = t.table_name
  4  and s.owner = 'HR';

SEGMENT_NAME                  BLOCKS     BLOCKS
------------------------- ---------- ----------
REGIONS                            8          5
LOCATIONS                          8          5
DEPARTMENTS                        8          5
JOBS                               8          5
EMPLOYEES                          8          5
JOB_HISTORY                        8          5
BIG_TABLE                       2816       2764
TAB1                               8          5
TAB2                               8          5
T                                  8          5

10 rows selected.

SQL> exec dbms_stats.gather_table_stats('HR','BIG_TABLE');

PL/SQL procedure successfully completed.

SQL> SELECT BLOCKS, EMPTY_BLOCKS ,NUM_ROWS FROM DBA_TABLES
  2  WHERE TABLE_NAME = 'BIG_TABLE' AND OWNER='HR';

    BLOCKS EMPTY_BLOCKS   NUM_ROWS
---------- ------------ ----------
      5284            8     381096


SQL> SELECT BLOCKS FROM DBA_segments
  2  WHERE segment_NAME = 'BIG_TABLE' AND OWNER='HR';

    BLOCKS
----------
      5376


SQL> delete hr.big_table where rownum <= 100000;

100000 rows deleted.

SQL> commit;

Commit complete.

SQL> SELECT BLOCKS FROM DBA_segments
  2  WHERE segment_NAME = 'BIG_TABLE' AND OWNER='HR';

    BLOCKS
----------
      5376

SQL> SELECT BLOCKS, EMPTY_BLOCKS ,NUM_ROWS FROM DBA_TABLES
  2  WHERE TABLE_NAME = 'BIG_TABLE' AND OWNER='HR';

    BLOCKS EMPTY_BLOCKS   NUM_ROWS
---------- ------------ ----------
      5284            8     381096

Note : bcoz delete never reset HVM. you have to used import or export / alter table ... move option for reset HVM.

SQL> host exp hr/hr tables=big_table

Export: Release 10.1.0.2.0 - Production on Sat Nov 25 16:01:49 2006

Copyright (c) 1982, 2004, Oracle.  All rights reserved.


Connected to: Oracle Database 10g Enterprise Edition Release 10.1.0.2.0 - Produc
tion
With the Partitioning, OLAP and Data Mining options
Export done in WE8MSWIN1252 character set and AL16UTF16 NCHAR character set

About to export specified tables via Conventional Path ...
. . exporting table                      BIG_TABLE     281808 rows exported
Export terminated successfully without warnings.

SQL> drop table hr.big_table purge;

Table dropped.


SQL> host imp hr/hr tables=big_table

Import: Release 10.1.0.2.0 - Production on Sat Nov 25 16:02:43 2006

Copyright (c) 1982, 2004, Oracle.  All rights reserved.


Connected to: Oracle Database 10g Enterprise Edition Release 10.1.0.2.0 - Produc
tion
With the Partitioning, OLAP and Data Mining options

Export file created by EXPORT:V10.01.00 via conventional path
import done in WE8MSWIN1252 character set and AL16UTF16 NCHAR character set
. importing HR's objects into HR
. . importing table                    "BIG_TABLE"     281808 rows imported
Import terminated successfully without warnings.

SQL> exec dbms_stats.gather_table_stats('HR','BIG_TABLE');

PL/SQL procedure successfully completed.

SQL> SELECT BLOCKS, EMPTY_BLOCKS ,NUM_ROWS FROM DBA_TABLES
  2  WHERE TABLE_NAME = 'BIG_TABLE' AND OWNER='HR';

    BLOCKS EMPTY_BLOCKS   NUM_ROWS
---------- ------------ ----------
      3898            0     281808


SQL> SELECT BLOCKS  FROM DBA_segments
  2  WHERE segment_NAME = 'BIG_TABLE' AND OWNER='HR';

    BLOCKS
----------
      3968

SQL>