|Oracle9i SQL Reference
Release 2 (9.2)
Part Number A96540-02
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
DBA_CLUSTERS data dictionary views.
To create a cluster in your own schema, you must have
CLUSTER system privilege. To create a cluster in another user's schema, you must have
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
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.
Specify the schema to contain the cluster. If you omit
schema, Oracle creates the cluster in your current schema.
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.
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.
Specify the datatype of each cluster key column.
REF, nested table, varray,
BFILE, or user-defined object type.
ISclause if any column datatype is not
NUMBERwith scale 0.
ROWID, but Oracle does not guarantee that the values in such columns are valid rowids.
"Datatypes" for information on datatypes
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:
INITRANS: 2 or the default value of the cluster's tablespace, whichever is greater
MAXTRANS: the default value for the cluster's tablespace
Specify the amount of space in bytes reserved to store all rows with the same cluster key value or the same hash value. Use
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.
Specify the tablespace in which the cluster is created.
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.
You cannot create a cluster index for a hash cluster, and you need not create an index on a hash cluster key. If you specify neither
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
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.
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.
Specify an expression to be used as the hash function for the hash cluster. The expression:
USERor the datetime functions
If you omit the
IS clause, Oracle uses an internal hash function for the hash cluster.
For information on existing hash functions, query the
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.
Oracle9i Database Reference for information on the data dictionary views
parallel_clause lets you parallelize the creation of the cluster.
The syntax of the
NOPARALLEL for serial execution. This is the default.
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.
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
If the tables in
cluster contain any columns of LOB or user-defined object type, this statement as well as subsequent
DELETE operations on
cluster are executed serially without notification.
"Notes on the parallel_clause" for
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 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 if you do not want to use the row level dependency tracking feature. This is the default.
Oracle9i Advanced Replication for information about the use of row-level dependency tracking in replication environments
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 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.
The following statement creates a cluster named
personnel with the cluster key column
department, a cluster size of 512 bytes, and storage parameter values:
The following statement creates the cluster index on the cluster key of
After creating the cluster index, you can add tables to the index and perform DML operations on those tables.
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;
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
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
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);
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: