What are clusters in Oracle

Can any one explain me about the concept of clusters in oracle, it would be better if some one illustrate it by an example. I tried a lot in Google to get the some idea about clusters but with no luck.

All the links which i got after googling had given me some theoritical information that clusters are used to provide faster access to data by storing the relevant data in a physical area so that when two tables accessing data they dont have to move over two data files they can find it in same place(if the tables are using a cluster).

Needed a pratical example to check out the execution speed etc., I know its not so simple to explain, This EE has got many talented people. I had just started to use this site hardly for a month and seen a various experts comments and articles which are really helpful.

I hope my question will be answered in a best possible way..
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Wasim Akram ShaikAsked:
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DavidSenior Oracle Database AdministratorCommented:
In an Oracle real application cluster environment, two or more computers (each with an instance) concurrently access a single database. This allows an application or user to connect to either computer and have access to a single coordinated set of data.  So, the benefit is to split the processing load between independent hosts.  And, in the event of a node failure, the remaining RAC nodes can absorb the load without requiring a database outage.
Wasim Akram ShaikAuthor Commented:
ok.. but what about create cluster command?
what does that mean?

i understand that clusters are used in RAC environment. Is it not possible to create a cluster in a single node database using create cluster command???
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DavidSenior Oracle Database AdministratorCommented:
To the best of my knowledge, no, it is not possible.  CREATE CLUSTER, otoh, refers to the ability to pair up (cluster) two or more tables in one database, so that the tables are effectively stored together based upon columns in common.  EMP & SAL are typical examples.

Wasim Akram ShaikAuthor Commented:
in that case, i dont have a RAC installed on database hosted on my local machine.

however i was able to execute the below commands successfully from the link you mentioned..

C:\Documents and Settings\Administrator>sqlplus scott/local

SQL*Plus: Release - Production on Sat Dec 31 11:17:18 2011

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

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

SQL> CREATE CLUSTER language (cust_language VARCHAR2(3))
  2     SIZE 512 HASHKEYS 10
  3     STORAGE (INITIAL 100k next 50k);

Cluster created.

SQL>    drop cluster language;

Cluster dropped.


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if i dont have RAC then how cluster got created? i am a little bit confused.. i have gone through the oracle documentation, but still the concept is not clear...!!!
it is a totally different concept. A cluster is a schema object that contains data from one or more tables, all of which have one or more columns in common. Oracle Database stores together all the rows from all the tables that share the same cluster key. You can use it for fast accessing the tables that are usually fetched/joined together.
On the other hand Oracle RAC is setting up two or more machines running in parallel to provide fail over structure.
slightwv (䄆 Netminder) Commented:

As ytarkan pointed out, a cluster can mean different things depending on the context in which it is used.

The CREATE CLUSTER command has already been talked about:  It allows you to store frequently accessed data together so it can be retrieved with fewer block reads from disk.

I assume you have been through the docs:


The second link talks about when to use it and when not to use it.
Wasim Akram ShaikAuthor Commented:
I think, we have not got practical examples for this.. thanks for all your help..

will try to learn this..
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