SQL Statements: ALTER CLUSTER to ALTER SEQUENCE, 4 of 21

ALTER CLUSTER

Purpose

Use the ALTER CLUSTER statement to redefine storage and parallelism characteristics of a cluster.

Note:

You cannot use this statement to change the number or the name of columns in the cluster key, and you cannot change the tablespace in which the cluster is stored.

See Also:

CREATE CLUSTER for information on creating a cluster
DROP CLUSTER and DROP TABLE for information on removing tables from a cluster

Prerequisites

The cluster must be in your own schema or you must have the ALTER ANY CLUSTER system privilege.

Syntax

alter_cluster::=

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Text description of alter_cluster

physical_attributes_clause::=

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Text description of physical_attributes_clause

(storage_clause)

allocate_extent_clause::=

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Text description of allocate_extent_clause

deallocate_unused_clause::=

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Text description of deallocate_unused_clause

parallel_clause::=

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Text description of parallel_clause

Semantics

schema

Specify the schema containing the cluster. If you omit schema, Oracle assumes the cluster is in your own schema.

cluster

Specify the name of the cluster to be altered.

physical_attributes_clause

Use this clause to change the values of the PCTUSED, PCTFREE, INITRANS, and MAXTRANS parameters of the cluster.

Use the STORAGE clause to change the storage characteristics of the cluster.

Restriction on Physical Attributes

You cannot change the values of the storage parameters INITIAL and MINEXTENTS for a cluster.

See Also:

physical_attributes_clause for a full description of this clause
storage_clause for a full description of the storage parameters

SIZE integer

Use the SIZE clause to specify the number of cluster keys that will be stored in data blocks allocated to the cluster.

Restriction on SIZE

You can change the SIZE parameter only for an indexed cluster, not for a hash cluster.

See Also:

CREATE CLUSTER for a description of the SIZE parameter and "Modifying a Cluster: Example"

allocate_extent_clause

Specify the allocate_extent_clause to explicitly allocate a new extent for the cluster.

When you explicitly allocate an extent with this clause, Oracle does not evaluate the cluster's storage parameters and determine a new size for the next extent to be allocated (as it does when you create a table). Therefore, specify SIZE if you do not want Oracle to use a default value.

Restriction on Allocating Extents

You can allocate a new extent only for an indexed cluster, not for a hash cluster.

See Also:

allocate_extent_clause for a full description of this clause and "Deallocating Unused Space: Example"

deallocate_unused_clause

Use the deallocate_unused_clause to explicitly deallocate unused space at the end of the cluster and make the freed space available for other segments.

See Also:

deallocate_unused_clause for a full description of this clause

CACHE | NOCACHE

CACHE

Specify CACHE if you want the blocks retrieved for this cluster to be placed at the most recently used end of the least recently used (LRU) list in the buffer cache when a full table scan is performed. This clause is useful for small lookup tables.

NOCACHE

Specify NOCACHE if you want the blocks retrieved for this cluster to be placed at the least recently used end of the LRU list in the buffer cache when a full table scan is performed. This is the default behavior.

parallel_clause

Specify the parallel_clause to change the default degree of parallelism for queries and DML on the cluster.

Restriction on Parallelized Clusters

If the tables in cluster contain any columns of LOB or user-defined object type, this statement as well as subsequent INSERT, UPDATE, or DELETE operations on cluster are executed serially without notification.

Note:

The syntax of the parallel_clause supersedes syntax appearing in earlier releases of Oracle. Superseded syntax is still supported for backward compatibility, but may result in slightly different behavior than that documented.

NOPARALLEL

Specify NOPARALLEL for serial execution. This is the default.

PARALLEL

Specify PARALLEL if you want Oracle to select a degree of parallelism equal to the number of CPUs available on all participating instances times the value of the PARALLEL_THREADS_PER_CPU initialization parameter.

PARALLEL integer

Specification of integer indicates the degree of parallelism, which is the number of parallel threads used in the parallel operation. Each parallel thread may use one or two parallel execution servers. Normally Oracle calculates the optimum degree of parallelism, so it is not necessary for you to specify integer.

Examples

The following examples modify the clusters that were created in the "Examples" section of CREATE CLUSTER on .

Modifying a Cluster: Example

The next statement alters the personnel cluster:

ALTER CLUSTER personnel
   SIZE 1024 CACHE;

Oracle allocates 1024 bytes for each cluster key value and turns on the cache attribute. Assuming a data block size of 2 kilobytes, future data blocks within this cluster contain 2 cluster keys in each data block, or 2 kilobytes divided by 1024 bytes.

Deallocating Unused Space: Example

The following statement deallocates unused space from the language cluster, keeping 30 kilobytes of unused space for future use:

ALTER CLUSTER language 
   DEALLOCATE UNUSED KEEP 30 K;