The most important step in creating a cluster is deciding which table or tables to cluster, and how. Although a clustered table can deliver a terrific performance improvement in the right circumstances, a poorly chosen cluster can decrease performance. Once you have properly selected the table or tables you wish to cluster, you must go through a three-step process to create this database structure.

The following Tooltip illustrates how to create a cluster using SQL:

Once you create a cluster, you then create the table or tables that the cluster will contain.
The syntax for creating tables that are a part of a cluster is exactly the same syntax that is used for non-clustered tables, with one exception. The final clause in the CREATE TABLE statement is:

CLUSTER cluster_name (column_name)

The cluster_name is the same name that was given to the cluster in the CREATE CLUSTER command. The column_name is a list of columns in the table being created that match up with the columns in the already created cluster.

The final step is to create a cluster index. You will learn to do this in Lesson 5 of this module.

The following simplified code is an example of creating a cluster and the tables it will contain:

CREATE CLUSTER orders (order_id NUMBER) SIZE 512 K;
CREATE TABLE order_header( order_number NUMBER,
 customer_number NUMBER)
 CLUSTER orders (order_number);
CREATE TABLE order_detail(
 order_number NUMBER,
 detail_line VARCHAR2(100))
 CLUSTER orders (order_number);

The next lesson explains how to size a cluster properly.