How to Decrease fragmentation level of a table.

Question

Hello Everyone,
There is a table present in my production database.
Its version is Oracle 10G(10.2.0.1.0)
i)Its size is 26 GB with fragmentation level.
ii)It has more then 48 crores of rows.
iii)Its size is 20 GB without fragmentation level.(So 6 GB of unused space is wasted).
Following are my doubts regarding the above points.
i) How to decrease the fragmentation level of the table.
ii) if there is any way to decrease the fragmentation level,how much time it will take to perform the particular activity so as to decrease the fragmentation level.

Answer 1

Well Mine is Oracle 10G. Hence the production database is Locally Managed.Well it has more then 48 millions of rows in my table
Definition of Fragmentation :-
When rows are not stored contiguously, or if rows are split onto more than one block, performance decreases because these rows require additional block accesses.
When a lot of DML operations are applied on a table, the table will become fragmented because DML does not release free space from the table below the HWM.
Definition of High Water Mark(HWM):-
HWM is an indicator of USED BLOCKS in the database. Blocks below the high water mark (used blocks) have at least once contained data. This data might have been deleted. Since Oracle knows that blocks beyond the high water mark don't have data, it only reads blocks up to the high water mark when doing a full table scan.
Please go through the below example if you have any doubts.
EXAMPLE:-
How to find table fragmentation?
SQL> select count(*) from big1;
1000000 rows selected.
SQL> delete from big1 where rownum <= 300000;
300000 rows deleted.
SQL> commit;
Commit complete.
SQL> update big1 set object_id = 0 where rownum <=350000;
342226 rows updated.
SQL> commit;
Commit complete.
SQL> exec dbms_stats.gather_table_stats('SCOTT','BIG1');
PL/SQL procedure successfully completed.
Now lets determine the size of table
TABLE SIZE(with fragmentation)
SQL> select table_name,round((blocks*8),2)||'kb' "size"
2 from user_tables
3 where table_name = 'BIG1';
TABLE_NAME size
BIG1 72952kb
ACTUAL DATA in table:
SQL> select table_name,round((num_rows*avg_row_len/1024),2)||'kb' "size"
2 from user_tables
3 where table_name = 'BIG1';
TABLE_NAME size
BIG1 30604.2kb
Note = 72952 - 30604 = 42348 Kb is wasted space in table
Edited by: 855956 on May 3, 2011 12:11 PM