Clusters:

A Cluster is an optional technique of storing table data. Cluster is a set of tables which share the similar data blocks since they share common columns and are frequently used together.

For instance, the EMP and DEPT table share the DEPTNO column. When you use cluster command on the EMP and DEPT tables. An Oracle physically stores all rows for every department from both the EMP and DEPT tables in the similar data blocks.

Since clusters store associated rows of variant tables together in the simialr data blocks, properly used clusters offer two primary advantages:

Disk I/O is decrease and access time improves for joins of clustered tables.

In a cluster, a cluster key value is the value of the cluster key columns for a particular row. Every cluster key value is stored only once each in the cluster and the cluster index and no matter how many rows of different tables hold the value.

Thus, less storage might be needed to store associated table and index data in a cluster than is necessary in non-clustered table format.

For instance, notice how each cluster key (each DEPTNO) is stored just once for various rows which contain the similar value in both the DEPT and EMP tables.