CREATE CLUSTER

Purpose

Use the CREATE CLUSTER statement to create a cluster. A cluster is a schema object that contains data from one or more tables, all of which have one or more columns in common. Oracle stores together all the rows (from all the tables) that share the same cluster key.

For information on existing clusters, query the USER_CLUSTERS, ALL_CLUSTERS, and DBA_CLUSTERS data dictionary views.

Prerequisites

To create a cluster in your own schema, you must have CREATE CLUSTER system privilege. To create a cluster in another user's schema, you must have CREATE ANY CLUSTER system privilege. Also, the owner of the schema to contain the cluster must have either space quota on the tablespace containing the cluster or the UNLIMITED TABLESPACE system privilege.

Oracle does not automatically create an index for a cluster when the cluster is initially created. Data manipulation language (DML) statements cannot be issued against clustered tables until you create a cluster index.

Syntax

create_cluster::=

Text description of create_cluster

(physical_attributes_clause::=)

physical_attributes_clause::=

Text description of physical_attributes_clause

(storage_clause)

parallel_clause::=

Text description of parallel_clause

Semantics

schema

Specify the schema to contain the cluster. If you omit schema, Oracle creates the cluster in your current schema.

cluster

Specify is the name of the cluster to be created.

After you create a cluster, you add tables to it. A cluster can contain a maximum of 32 tables. After you create a cluster and add tables to it, the cluster is transparent. You can access clustered tables with SQL statements just as you can access nonclustered tables.

column

Specify one or more names of columns in the cluster key. You can specify up to 16 cluster key columns. These columns must correspond in both datatype and size to columns in each of the clustered tables, although they need not correspond in name.

You cannot specify integrity constraints as part of the definition of a cluster key column. Instead, you can associate integrity constraints with the tables that belong to the cluster.

datatype

Specify the datatype of each cluster key column.

Restrictions on Cluster Datatypes

• You cannot specify a cluster key column of datatype LONG, LONG RAW, REF, nested table, varray, BLOB, CLOB, BFILE, or user-defined object type.
• You cannot use the HASH IS clause if any column datatype is not INTEGER or NUMBER with scale 0.
• You can specify a column of type ROWID, but Oracle does not guarantee that the values in such columns are valid rowids.

  See Also: "Datatypes" for information on datatypes

physical_attributes_clause

The physical_attributes_clause lets you specify the storage characteristics of the cluster. Each table in the cluster uses these storage characteristics as well. If you do not specify values for these parameters, Oracle uses the following defaults:

• PCTFREE: 10
• PCTUSED: 40
• INITRANS: 2 or the default value of the cluster's tablespace, whichever is greater
• MAXTRANS: the default value for the cluster's tablespace

  See Also: physical_attributes_clause for a complete description of the parameters of the physical_attributes_clause storage_clause for a complete description of the storage_clause, including default values

SIZE

Specify the amount of space in bytes reserved to store all rows with the same cluster key value or the same hash value. Use K or M to specify this space in kilobytes or megabytes. This space determines the maximum number of cluster or hash values stored in a data block. If SIZE is not a divisor of the data block size, Oracle uses the next largest divisor. If SIZE is larger than the data block size, Oracle uses the operating system block size, reserving at least one data block for each cluster or hash value.

Oracle also considers the length of the cluster key when determining how much space to reserve for the rows having a cluster key value. Larger cluster keys require larger sizes. To see the actual size, query the KEY_SIZE column of the USER_CLUSTERS data dictionary view. (This does not apply to hash clusters, because hash values are not actually stored in the cluster.)

If you omit this parameter, Oracle reserves one data block for each cluster key value or hash value.

TABLESPACE

Specify the tablespace in which the cluster is created.

INDEX Clause

Specify INDEX to create an indexed cluster. In an indexed cluster, Oracle stores together rows having the same cluster key value. Each distinct cluster key value is stored only once in each data block, regardless of the number of tables and rows in which it occurs.

After you create an indexed cluster, you must create an index on the cluster key before you can issue any data manipulation language (DML) statements against a table in the cluster. This index is called the cluster index.

HASHKEYS Clause

Specify the HASHKEYS clause to create a hash cluster and specify the number of hash values for a hash cluster. In a hash cluster, Oracle stores together rows that have the same hash key value. The hash value for a row is the value returned by the cluster's hash function.

Oracle rounds up the HASHKEYS value to the nearest prime number to obtain the actual number of hash values. The minimum value for this parameter is 2. If you omit both the INDEX clause and the HASHKEYS parameter, Oracle creates an indexed cluster by default.

When you create a hash cluster, Oracle immediately allocates space for the cluster based on the values of the SIZE and HASHKEYS parameters.

SINGLE TABLE

SINGLE TABLE indicates that the cluster is a type of hash cluster containing only one table. This clause can provide faster access to rows than would result if the table were not part of a cluster.

Restriction on Single Table Clusters

Only one table can be present in the cluster at a time. However, you can drop the table and create a different table in the same cluster.

HASH IS expr

Specify an expression to be used as the hash function for the hash cluster. The expression:

• Must evaluate to a positive value
• Must contain at least one column with referenced columns of any datatype as long as the entire expression evaluates to a number of scale 0. For example: NUM_COLUMN * length(VARCHAR2_COLUMN)
• Cannot reference user-defined PL/SQL functions
• Cannot reference the pseudocolumns LEVEL or ROWNUM
• Cannot reference the user-related functions USERENV, UID, or USER or the datetime functions CURRENT_DATE, CURRENT_TIMESTAMP, DBTIMEZONE, EXTRACT (datetime), FROM_TZ, LOCALTIMESTAMP, NUMTODSINTERVAL, NUMTOYMINTERVAL, SESSIONTIMEZONE, SYSDATE, SYSTIMESTAMP, TO_DSINTERVAL, TO_TIMESTAMP, TO_DATE, TO_TIMESTAMP_TZ, TO_YMINTERVAL, and TZ_OFFSET.
• Cannot evaluate to a constant
• Cannot be a scalar subquery expression
• Cannot contain columns qualified with a schema or object name (other than the cluster name)

If you omit the HASH IS clause, Oracle uses an internal hash function for the hash cluster.

For information on existing hash functions, query the USER_, ALL_, and DBA_CLUSTER_HASH_EXPRESSIONS data dictionary tables.

The cluster key of a hash column can have one or more columns of any datatype. Hash clusters with composite cluster keys or cluster keys made up of noninteger columns must use the internal hash function.

parallel_clause

The parallel_clause lets you parallelize the creation of the cluster.

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.

Restriction on Parallelizing Cluster Creation

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.

NOROWDEPENDENCIES | ROWDEPENDENCIES

This clause lets you specify whether cluster will use row-level dependency tracking. With this feature, each row in the tables that make up the cluster has a system change number (SCN) that represents a time greater than or equal to the commit time of the last transaction that modified the row. You cannot change this setting after cluster is created.

ROWDEPENDENCIES

Specify ROWDEPENDENCIES if you want to enable row-level dependency tracking. This setting is useful primarily to allow for parallel propagation in replication environments. It increases the size of each row by 6 bytes.

NOROWDEPENDENCIES

Specify NOROWDEPENDENCIES if you do not want to use the row level dependency tracking feature. This is the default.

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.

Examples

Creating a Cluster: Example

The following statement creates a cluster named personnel with the cluster key column department, a cluster size of 512 bytes, and storage parameter values:

CREATE CLUSTER personnel
   (department NUMBER(4))
SIZE 512 
STORAGE (initial 100K next 50K);

Cluster Keys: Example

The following statement creates the cluster index on the cluster key of personnel:

CREATE INDEX idx_personnel ON CLUSTER personnel;
 

After creating the cluster index, you can add tables to the index and perform DML operations on those tables.

Adding Tables to a Cluster: Example

The following statements create some departmental tables from the sample hr.employees table and add them to the personnel cluster created in the earlier example:

CREATE TABLE dept_10
   CLUSTER personnel (department_id)
   AS SELECT * FROM employees WHERE department_id = 10;

CREATE TABLE dept_20
   CLUSTER personnel (department_id)
   AS SELECT * FROM employees WHERE department_id = 20;

Hash Clusters: Examples

The following statement creates a hash cluster named language with the cluster key column cust_language, a maximum of 10 hash key values, each of which is allocated 512 bytes, and storage parameter values:

CREATE CLUSTER language (cust_language VARCHAR2(3))
   SIZE 512 HASHKEYS 10
   STORAGE (INITIAL 100k next 50k);

Because the preceding statement omits the HASH IS clause, Oracle uses the internal hash function for the cluster.

The following statement creates a hash cluster named address with the cluster key made up of the columns postal_code and country_id, and uses a SQL expression containing these columns for the hash function:

CREATE CLUSTER address
   (postal_code NUMBER, country_id CHAR(2))
   HASHKEYS 20
   HASH IS MOD(postal_code + country_id, 101);

Single-Table Hash Clusters: Example

The following statement creates a single-table hash cluster named cust_orders with the cluster key customer_id and a maximum of 100 hash key values, each of which is allocated 512 bytes:

CREATE CLUSTER cust_orders (customer_id NUMBER(6))
   SIZE 512 SINGLE TABLE HASHKEYS 100;