Oracle7 Server Administrator's Guide

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Dropping Clusters

This section describes aspects of dropping clusters, and includes the following topics:

A cluster can be dropped if the tables within the cluster are no longer necessary. When a cluster is dropped, so are the tables within the cluster and the corresponding cluster index; all extents belonging to both the cluster's data segment and the index segment of the cluster index are returned to the containing tablespace and become available for other segments within the tablespace.

Dropping Clustered Tables

To drop a cluster, your schema must contain the cluster or you must have the DROP ANY CLUSTER system privilege. You do not have to have additional privileges to drop a cluster that contains tables, even if the clustered tables are not owned by the owner of the cluster.

Clustered tables can be dropped individually without affecting the table's cluster, other clustered tables, or the cluster index. A clustered table is dropped just as a non-clustered table is dropped--with the SQL command DROP TABLE.

Note: When you drop a single table from a cluster, Oracle deletes each row of the table individually. To maximize efficiency when you intend to drop an entire cluster, drop the cluster including all tables by using the DROP CLUSTER command with the INCLUDING TABLES option. Drop an individual table from a cluster (using the DROP TABLE command) only if you want the rest of the cluster to remain.

See Also: For information about dropping a table, see "Dropping Tables" [*].

Dropping Cluster Indexes

A cluster index can be dropped without affecting the cluster or its clustered tables. However, clustered tables cannot be used if there is no cluster index; you must re-create the cluster index to allow access to the cluster. Cluster indexes are sometimes dropped as part of the procedure to rebuild a fragmented cluster index.

To drop a cluster that contains no tables, and its cluster index, use the SQL command DROP CLUSTER. For example, the following statement drops the empty cluster named EMP_DEPT:

DROP CLUSTER emp_dept;

If the cluster contains one or more clustered tables and you intend to drop the tables as well, add the INCLUDING TABLES option of the DROP CLUSTER command, as follows:

DROP CLUSTER emp_dept INCLUDING TABLES;

If the INCLUDING TABLES option is not included and the cluster contains tables, an error is returned.

If one or more tables in a cluster contain primary or unique keys that are referenced by FOREIGN KEY constraints of tables outside the cluster, the cluster cannot be dropped unless the dependent FOREIGN KEY constraints are also dropped. This can be easily done using the CASCADE CONSTRAINTS option of the DROP CLUSTER command, as shown in the following example:

DROP CLUSTER emp_dept INCLUDING TABLES CASCADE CONSTRAINTS;

Oracle returns an error if you do not use the CASCADE CONSTRAINTS option and constraints exist.

See Also: For information about dropping an index, see "Dropping Indexes" [*].

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