Purpose

Use the TRUNCATE CLUSTER statement to remove all rows from a cluster. By default, Oracle Database also performs the following tasks:

• Deallocates all space used by the removed rows except that specified by the MINEXTENTS storage parameter

• Sets the NEXT storage parameter to the size of the last extent removed from the segment by the truncation process

Removing rows with the TRUNCATE statement can be more efficient than dropping and re-creating a cluster. Dropping and re-creating a cluster invalidates dependent objects of the cluster, requires you to regrant object privileges on the cluster, and requires you to re-create the indexes and cluster on the table and respecify its storage parameters. Truncating has none of these effects.

Removing rows with the TRUNCATE CLUSTER statement can be faster than removing all rows with the DELETE statement, especially if the cluster has numerous indexes and other dependencies.

Prerequisites

To truncate a cluster, the cluster must be in your schema or you must have DROP ANY TABLE system privilege.

Syntax

truncate_cluster::=

Description of the illustration truncate_cluster.eps

Semantics

CLUSTER Clause

Specify the schema and name of the cluster to be truncated. You can truncate only an indexed cluster, not a hash cluster. If you omit schema, then the database assumes the cluster is in your own schema.

When you truncate a cluster, the database also automatically deletes all data in the indexes of the cluster tables.

STORAGE Clauses

The STORAGE clauses let you determine what happens to the space freed by the truncated rows. The DROP STORAGE clause and REUSE STORAGE clause also apply to the space freed by the data deleted from associated indexes.

DROP STORAGE

Specify DROP STORAGE to deallocate all space from the deleted rows from the cluster except the space allocated by the MINEXTENTS parameter of the cluster. This space can subsequently be used by other objects in the tablespace. Oracle Database also sets the NEXT storage parameter to the size of the last extent removed from the segment in the truncation process. This is the default.

REUSE STORAGE

Specify REUSE STORAGE to retain the space from the deleted rows allocated to the cluster. Storage values are not reset to the values when the table or cluster was created. This space can subsequently be used only by new data in the cluster resulting from insert or update operations. This clause leaves storage parameters at their current settings.

If you have specified more than one free list for the object you are truncating, then the REUSE STORAGE clause also removes any mapping of free lists to instances and resets the high-water mark to the beginning of the first extent.

Examples

Truncating a Cluster: Example

The following statement removes all rows from all tables in the personnel cluster, but leaves the freed space allocated to the tables:

TRUNCATE CLUSTER personnel REUSE STORAGE;

The preceding statement also removes all data from all indexes on the tables in the personnel cluster.