What are the clusters? What are their advantages?

A cluster contains data from one or more tables. All these tables have one or more columns in common. All the rows from all the tables that share the same cluster key.

Example: cluster named workers with the cluster key column department, a cluster size of 512 bytes, and storage parameter values

CREATE CLUSTER workers
(department NUMBER(4))
SIZE 512
STORAGE (initial 100K next 50K);

Advantages:

• Clusters can speed up join queries.
• Storage is saved since storing the field or fields comprising the Cluster Key are stored once instead of multiple times.
• Clustering does not does not affect the relational model (table schemas).