An indexed cluster is a table cluster that uses an index to locate data. The cluster
index is a B-tree index on the cluster key. A cluster index must be created before any
rows can be inserted into clustered tables.
Assume that you create the cluster employees_departments_cluster with the cluster
key department_id, as shown in Example 2–8. Because the HASHKEYS clause is not
specified, this cluster is an indexed cluster. Afterward, you create an index named
idx_emp_dept_cluster on this cluster key.