A cluster provides an optional method of storing table data. A cluster is made up of a group of tables that share the same data blocks. The tables are grouped together because they share common columns and are often used together. For example, the emp and dept table share the deptno column. When you cluster the emp and dept tables Oracle Database physically stores all rows for each department from both the emp and dept tables in the same data blocks.
Because clusters store related rows of different tables together in the same data blocks, properly used clusters offer two primary benefits:
Therefore, less storage might be required to store related table and index data in a cluster than is necessary in nonclustered table format. For example, notice how each cluster key (each deptno) is stored just once for many rows that contain the same value in both the emp and dept tables.
After creating a cluster, you can create tables in the cluster. However, before any rows can be inserted into the clustered tables, a cluster index must be created. Using clusters does not affect the creation of additional indexes on the clustered tables; they can be created and dropped as usual. You should not use clusters for tables that are frequently accessed individually.
Clustered Table Data
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