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 ALTER ANY CLUSTER system privilege.

Syntax

alter_cluster::=

Text description of alter_cluster

storage_clause: See storage_clause

allocate_extent_clause::=

Text description of allocate_extent_clause

deallocate_unused_clause::=

Text description of deallocate_unused_clause

parallel_clause::=

Text description of parallel_clause

Keywords and Parameters

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

See Also: CREATE CLUSTER for a description of these parameters storage_clause

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: 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

allocate_extent_clause

Specify the ALLOCATE EXTENT clause to explicitly allocate a new extent for the cluster.

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

SIZE

Use the SIZE parameter to specify the size of the extent in bytes. Use K or M to specify the extent size in kilobytes or megabytes.

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.

DATAFILE

Use the DATAFILE parameter to specify one of the datafiles in the cluster's tablespace to contain the new extent. If you omit this parameter, Oracle chooses the datafile.

INSTANCE

Use the INSTANCE parameter to make the new extent available to the specified instance. An instance is identified by the value of its initialization parameter INSTANCE_NUMBER. If you omit INSTANCE, the extent is available to all instances.

Note: Use this parameter only if you are using Oracle with Real Application Clusters.

deallocate_unused_clause

Specify the DEALLOCATE UNUSED clause to explicitly deallocate unused space at the end of the cluster and makes the freed space available for other segments. Only unused space above the high water mark can be freed.

KEEP

Use the KEEP parameter to specify the number of bytes above the high water mark that the cluster will have after deallocation. If the number of remaining extents is less than MINEXTENTS, then MINEXTENTS is set to the current number of extents. If the initial extent becomes smaller than INITIAL, then INITIAL is set to the value of the current initial extent. If you omit KEEP, all unused space is freed.

See Also: ALTER TABLE for a more complete 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: 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.

See Also: "Notes on the parallel_clause" for CREATE TABLE

Examples

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

Modifying a Cluster Example

The following 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;