Home > Oracle, Query Tuning > Table Fragmentation on Oracle

Table Fragmentation on Oracle

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.

Note that table fragmentation is different from file fragmentation. 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.

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.

DDL statement always resets the HWM.

  • 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.

  • 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

The difference between two values is 60% and Pctfree 10% (default) – so, the table has 50% extra space which is wasted because there is no data.

  • How to reset HWM / remove fragemenation?

For that we need to reorganize the fragmented table.

We have four options to reorganize fragmented tables:

1. alter table … move + rebuild indexes
2. export / truncate / import
3. create table as select ( CTAS)
4. dbms_redefinition

  • Option: 1 “alter table … move + rebuild indexes”
SQL> alter table BIG1 move;

Table altered.

SQL> select status,index_name from user_indexes
2 where table_name = 'BIG1';

STATUS     INDEX_NAME
--------  ------------
UNUSABLE   BIGIDX

SQL> alter index bigidx rebuild;

Index altered.

SQL> select status,index_name from user_indexes
2 where table_name = 'BIG1';

STATUS    INDEX_NAME
-------- ------------
VALID      BIGIDX

SQL> exec dbms_stats.gather_table_stats('SCOTT','BIG1');

PL/SQL procedure successfully completed.

SQL> select table_name,round((blocks*8),2)||'kb' "size"
2 from user_tables
3 where table_name = 'BIG1';

TABLE_NAME   size
----------- -----------
BIG1         38224kb

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         30727.37kb


  • Option: 2 “Create table as select”
SQL> create table big2 as select * from big1;

Table created.

SQL> drop table big1 purge;

Table dropped.

SQL> rename big2 to big1;

Table renamed.

SQL> exec dbms_stats.gather_table_stats('SCOTT','BIG1');

PL/SQL procedure successfully completed.

SQL> select table_name,round((blocks*8),2)||'kb' "size"
2 from user_tables
3 where table_name = 'BIG1';

TABLE_NAME    size
------------ --------- 
BIG1          85536kb

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         68986.97kb

SQL> select status from user_indexes
2 where table_name = 'BIG1';

no rows selected

SQL> --Note we need to create all indexes.

  • Option: 3 “export / truncate / import”
SQL> select table_name, round((blocks*8),2)||'kb' "size"
2 from user_tables
3 where table_name = 'BIG1';

TABLE_NAME   size
----------- ---------
BIG1         85536kb

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          42535.54kb

SQL> select status from user_indexes where table_name = 'BIG1';

STATUS
--------
VALID

SQL> exit
Disconnected from Oracle Database 10g Enterprise Edition Release 10.1.0.5.0 - Pr
oduction
With the Partitioning, OLAP and Data Mining options

C:\>exp scott/tiger@Orcl file=c:\big1.dmp tables=big1

Export: Release 10.1.0.5.0 - Production on Sat Jul 28 16:30:44 2007

Copyright (c) 1982, 2005, Oracle. All rights reserved.

Connected to: Oracle Database 10g Enterprise Edition Release 10.1.0.5.0 - 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 BIG1 468904 rows exported
Export terminated successfully without warnings.

C:\>sqlplus scott/tiger@orcl

SQL*Plus: Release 10.1.0.5.0 - Production on Sat Jul 28 16:31:12 2007

Copyright (c) 1982, 2005, Oracle. All rights reserved.

Connected to:
Oracle Database 10g Enterprise Edition Release 10.1.0.5.0 - Production
With the Partitioning, OLAP and Data Mining options

SQL> truncate table big1;

Table truncated.

SQL> exit
Disconnected from Oracle Database 10g Enterprise Edition Release 10.1.0.5.0 - Pr
oduction
With the Partitioning, OLAP and Data Mining options

C:\>imp scott/tiger@Orcl file=c:\big1.dmp ignore=y

Import: Release 10.1.0.5.0 - Production on Sat Jul 28 16:31:54 2007

Copyright (c) 1982, 2005, Oracle. All rights reserved.

Connected to: Oracle Database 10g Enterprise Edition Release 10.1.0.5.0 - 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 SCOTT's objects into SCOTT
. . importing table "BIG1" 468904 rows imported
Import terminated successfully without warnings.

C:\>sqlplus scott/tiger@orcl

SQL*Plus: Release 10.1.0.5.0 - Production on Sat Jul 28 16:32:21 2007

Copyright (c) 1982, 2005, Oracle. All rights reserved.

Connected to:
Oracle Database 10g Enterprise Edition Release 10.1.0.5.0 - Production
With the Partitioning, OLAP and Data Mining options

SQL> select table_name, round((blocks*8),2)||'kb' "size"
2 from user_tables
3 where table_name = 'BIG1';

TABLE_NAME    size
----------- -----------
BIG1          85536kb

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          42535.54kb

SQL> exec dbms_stats.gather_table_stats('SCOTT','BIG1');

PL/SQL procedure successfully completed.

SQL> select table_name, round((blocks*8),2)||'kb' "size"
2 from user_tables
3 where table_name = 'BIG1';

TABLE_NAME   size
----------  ---------
BIG1         51840kb

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         42542.27kb

SQL> select status from user_indexes where table_name = 'BIG1';

STATUS
--------
VALID

SQL> exec dbms_redefinition.can_redef_table('SCOTT','BIG1',-
> dbms_redefinition.cons_use_pk);

PL/SQL procedure successfully completed.


  • Option: 4 “dbms_redefinition”
SQL> create table TABLE1 (
2 no number,
3 name varchar2(20) default 'NONE',
4 ddate date default SYSDATE);

Table created.

SQL> alter table table1 add constraint pk_no primary key(no);

Table altered.

SQL> begin
2 for x in 1..100000 loop
3 insert into table1 ( no , name, ddate)
4 values ( x , default, default);
5 end loop;
6 end;
7 /

PL/SQL procedure successfully completed.

SQL> create or replace trigger tri_table1
2 after insert on table1
3 begin
4 null;
5 end;
6 /

Trigger created.

SQL> select count(*) from table1;

COUNT(*)
----------
100000

SQL> delete table1 where rownum <= 50000;

50000 rows deleted.

SQL> commit;

Commit complete.

SQL> exec dbms_stats.gather_table_stats('SCOTT','TABLE1');

PL/SQL procedure successfully completed.

SQL> select table_name, round((blocks*8),2)||'kb' "size"
2 from user_tables
3 where table_name = 'TABLE1';

TABLE_NAME   size
---------- ---------
TABLE1       2960kb

SQL> select table_name, round((num_rows*avg_row_len/1024),2)||'kb' "size"
2 from user_tables
3 where table_name = 'TABLE1';

TABLE_NAME    size
----------- ------------
TABLE1        822.69kb

SQL> --Minimum Privs required "DBA" role or "SELECT" on dbms_redefinition pkg
SQL> --First check table is condidate for redefinition.
SQL>
SQL> exec sys.dbms_redefinition.can_redef_table('SCOTT',-
> 'TABLE1',-
> sys.dbms_redefinition.cons_use_pk);

PL/SQL procedure successfully completed.

SQL> --After verifying that the table can be redefined online, you manually crea
te an empty interim table (in the same schema as the table to be redefined)
SQL>
SQL> create table TABLE2 as select * from table1 WHERE 1 = 2;

Table created.

SQL> exec sys.dbms_redefinition.start_redef_table ( 'SCOTT',-
> 'TABLE1',-
> 'TABLE2');

PL/SQL procedure successfully completed.

SQL> --This procedure keeps the interim table synchronized with the original tab
le.
SQL>
SQL> exec sys.dbms_redefinition.sync_interim_table ('SCOTT',-
> 'TABLE1',-
> 'TABLE2');

PL/SQL procedure successfully completed.

SQL> --Create PRIMARY KEY on interim table(TABLE2)
SQL> alter table TABLE2
2 add constraint pk_no1 primary key (no);

Table altered.

SQL> create trigger tri_table2
2 after insert on table2
3 begin
4 null;
5 end;
6 /

Trigger created.

SQL> --Disable foreign key on original table if exists before finish this proces
s.
SQL>
SQL> exec sys.dbms_redefinition.finish_redef_table ( 'SCOTT',-
> 'TABLE1',-
> 'TABLE2');

PL/SQL procedure successfully completed.

SQL> exec dbms_stats.gather_table_stats('SCOTT','TABLE1');

PL/SQL procedure successfully completed.

SQL> select table_name, round((blocks*8),2)||'kb' "size"
2 from user_tables
3 where table_name = 'TABLE1';

TABLE_NAME    size
----------- ---------
TABLE1        1376kb

SQL> select table_name, round((num_rows*avg_row_len/1024),2)||'kb' "size"
2 from user_tables
3 where table_name = 'TABLE1';

TABLE_NAME    size
----------- ----------
TABLE1        841.4kb

SQL> select status,constraint_name
2 from user_constraints
3 where table_name = 'TABLE1';

STATUS     CONSTRAINT_NAME
-------- ------------------
ENABLED    PK_NO1

SQL> select status ,trigger_name
2 from user_triggers
3 where table_name = 'TABLE1';

STATUS    TRIGGER_NAME
-------- --------------
ENABLED   TRI_TABLE2

SQL> drop table TABLE2 PURGE;

Table dropped.

 

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