Clusters are groups of one or more tables physically stored together because they share common columns and are often used together. This improves disk access time.The related columns of the tables in a cluster are called the cluster key. The cluster key is indexed so that rows of the cluster can be retrieved with a minimum amount of I/O.No matter how many tables within the cluster contain the cluster key value, it is stored only once each in the cluster and the cluster index. Therefore, less storage is required.Whether or not a table is part of a cluster is transparent to users and to applications. Data stored in a clustered table is accessed by SQL in the same way as data stored in a nonclustered table.