Distributed Databases And Fragmentation

Fragmentation independence refers to the ability of end users to store logically related information at different physical locations. There are two types of fragmentation independence: vertical partitioning and horizontal partitioning. Horizontal partitioning permits different rows of the same table to be stored at different remote sites. This is commonly done by organizations that maintain several branch offices, each with an identical set of table structures.
Vertical partitioning refers to the ability of a distributed system to fragment information such that the data columns from the same logical tables are maintained across the network. Oracle accomplishes this with Oracle views that hide specific columns and rows in a table.

A Definition Of Distributed Databases
There is an ongoing debate over the standard definition of distributed database systems. Vendors have implemented distributed database technology in different manners.
To many database vendors, a distributed database is a geographically distributed system composed entirely of one brand of database products. On the other hand, front end applications vendors define a distributed database as a system distributed architecturally, using a blend of database products and access methods
Finally, to hardware vendors, a distributed database is a system composed of different databases running on the same hardware platforms.

Table 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.

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”
[code]
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.

[/code]

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.

Reducing database fragmentation

Excessively fragmented tables or indexes can adversely affect performance. Use the following SQL to identify those database objects that have over 10 extents allocated:

select * from dba_segments where extents > 10;

In general, if a table or index has more than 10 extents then rebuild it to fit into one extent.

A table can only be rebuilt by exporting and then importing it. The database will be unavailable for use by applications during this time. The steps to accomplish this are:

1. Export the table with COMPRESS=Y
2. Drop the table
3. Import the table.

An index can be rebuilt without preventing others from still using it. Firstly change the storage parameters to make the ‘next’ storage parameter larger (perhaps double it). The initial storage value cannot be changed. Then rebuild the index.

COMP332-DDB-notes.pdf