System tablespace space not regained when objects are dropped

Question

Mine is a Oracle 10g 10.2 on windows.
I am importing a export file into a user ,It takes some amount of space in SYSTEM and another tablespace .When I drop the user space in system tablespace is not coming back. ANY IDEA WHY
BEFORE IMPORT
SQL> select sum(bytes)/1024/1024 from dba_segments where owner='SYSTEM';
SUM(BYTES)/1024/1024
22.1875
SQL> select sum(bytes)/1024/1024 from dba_segments where owner='SYS';
SUM(BYTES)/1024/1024
544.1875
SQL> select sum(bytes)/1024/1024 from dba_segments where segment_name='SOURCE$';
SUM(BYTES)/1024/1024
41
I use the following commands to import
SQL>create user <username> identified by <password> default tablespace <tsname> quota unlimited on <tsname>;
SQL>grant create session,imp_full_database to <username>;
imp system file=filename.dmp log=logname.log fromuser=<username> touser=<username> statistics=none
AFTER IMPORT
SQL> select sum(bytes)/1024/1024 from dba_segments where segment_name='SOURCE$';
SUM(BYTES)/1024/1024
53
SQL> select sum(bytes)/1024/1024 from dba_segments where owner='SYSTEM';
SUM(BYTES)/1024/1024
22.1875
SQL> select sum(bytes)/1024/1024 from dba_segments where owner='SYS';
SUM(BYTES)/1024/1024
728.375
AFTER DROPPING THE USER/SCHEMA
SQL> select sum(bytes)/1024/1024 from dba_segments where owner='SYS';
SUM(BYTES)/1024/1024
728.375
SQL> select sum(bytes)/1024/1024 from dba_segments where owner='SYSTEM';
SUM(BYTES)/1024/1024
22.1875
SQL> select sum(bytes)/1024/1024 from dba_segments where segment_name='SOURCE$';
SUM(BYTES)/1024/1024
53
I even tried deleting the objects first and then dropping the user
SQL> delete from source$ where obj# in(select object_id from dba_objects where owner='USERNAME');
211252 rows deleted.
SQL> commit;
Commit complete.
SQL> drop user USERNAME cascade;
User dropped.
The space used by the schema on system tablespace is not coming back.

Answer 1

Hi user509593!
Adding objects to a tablespace requires space in that tablespace. This space is managed in segments and extents. If an extent is fully used (that means 100 % usage) a new extent will be added to a segment. Oracle uses a mechanism called "High Water Mark" to mark the last used extent.
Your problem is that oracle don't set this High Water Mark back if you are dropping objects from a tablespace. Once an extent is marked as it it retains marked as used.
Before Adding Objects:
u = used Extent
x = free Extent
| = High Water Mark
uuuuuuxxxxx
...........|
After Adding Objects:
uuuuuuuxxxx
............|
After dropping objects:
uuuuuuuxxxx
............|
The only chance to get your "unused" space back is to reorganized your tablespace. But before you reorganize something please read the documentation to know all about the costs and traps that comes with reorganization.
Hope this help!
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