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 (see Figure-1), 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:
• Disk I/O is reduced and access time improves for joins of clustered tables.
• The cluster key is the column, or group of columns, that the clustered
tables have in common. You specify the columns of the cluster key when
creating the cluster. You subsequently specify the same columns when
creating every table added to the cluster. Each cluster key value is stored
only once each in the cluster and the cluster index, no matter how many rows
of different tables contain the value.
Therefore, less storage might be required to store related table and index data in a cluster than is necessary in non-clustered table format. For example, in Figure 1, 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.
Figure 1 Clustered Table Data