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CHAPTER 14. Managing Clusters


This chapter describes aspects of managing clusters (including clustered tables and indexes), and includes the following topics:

Before attempting tasks described in this chapter, familiarize yourself with the concepts[*], "Guidelines for Managing Schema Objects."

Guidelines for Managing Clusters

A cluster provides an optional method of storing table data. A cluster is comprised of a group of tables that share the same data blocks, which are grouped together because they share common columns and are often used together. For example, the EMP and DEPT table share the DEPTNO column. When you cluster the EMP and DEPT tables (see Figure 14 - 1), Oracle physically stores all rows for each department from both the EMP and DEPT tables in the same data blocks. You should not use clusters for tables that are frequently accessed individually.

Because clusters store related rows of different tables together in the same data blocks, properly used clusters offer two primary benefits:

After creating a cluster, you can create tables in the cluster. However, before any rows can be inserted into the clustered tables, a cluster index must be created. Using clusters does not affect the creation of additional indexes on the clustered tables; they can be created and dropped as usual.

Figure 14 - 1. Clustered Table Data

The following sections describe guidelines to consider when managing clusters, and includes the following topics:

See Also: For more information about clusters, see the Oracle7 Server Concepts manual.

Cluster Appropriate Tables

Use clusters to store one or more tables that are primarily queried (not predominantly inserted into or updated) and for which the queries often join data of multiple tables in the cluster or retrieve related data from a single table.

Choose Appropriate Columns for the Cluster Key

Choose cluster key columns carefully. If multiple columns are used in queries that join the tables, make the cluster key a composite key. In general, the characteristics that indicate a good cluster index are the same as those for any index.

A good cluster key has enough unique values so that the group of rows corresponding to each key value fills approximately one data block. Having too few rows per cluster key value can waste space and result in negligible performance gains. Cluster keys that are so specific that only a few rows share a common value can cause wasted space in blocks, unless a small SIZE was specified at cluster creation time (see below).

Too many rows per cluster key value can cause extra searching to find rows for that key. Cluster keys on values that are too general (for example, MALE and FEMALE) result in excessive searching and can result in worse performance than with no clustering.

A cluster index cannot be unique or include a column defined as LONG.

See Also: For information about characteristics of a good index, see "Guidelines for Managing Indexes" [*].

Specify Data Block Space Use

By specifying the PCTFREE and PCTUSED parameters during the creation of a cluster, you can affect the space utilization and amount of space reserved for updates to the current rows in the data blocks of a cluster's data segment. Note that PCTFREE and PCTUSED parameters set for tables created in a cluster are ignored; clustered tables automatically use the settings set for the cluster.

See Also: For more information about setting PCTFREE and PCTUSED, see "Managing the Space Usage of Data Blocks" [*] .

Specify the Space Required by an Average Cluster Key and Its Associated Rows

The CREATE CLUSTER command has an optional argument, SIZE, which is the estimated number of bytes required by an average cluster key and its associated rows. Oracle uses the SIZE parameter when performing the following tasks:

SIZE does not limit the space that can be used by a given cluster key. For example, if SIZE is set such that two cluster keys can fit in one data block, any amount of the available data block space can still be used by either of the cluster keys.

By default, Oracle stores only one cluster key and its associated rows in each data block of the cluster's data segment. Although block size can vary from one operating system to the next, the rule of one key per block is maintained as clustered tables are imported to other databases on other machines.

If all the rows for a given cluster key value cannot fit in one block, the blocks are chained together to speed access to all the values with the given key. The cluster index points to the beginning of the chain of blocks, each of which contains the cluster key value and associated rows. If the cluster SIZE is such that more than one key fits in a block, blocks can belong to more than one chain.

Specify the Location of Each Cluster and Cluster Index

If you have the proper privileges and tablespace quota, you can create a new cluster and the associated cluster index in any tablespace that is currently online. Always specify the TABLESPACE option in a CREATE CLUSTER/INDEX statement to identify the tablespace to store the new cluster or index.

The cluster and its cluster index can be created in different tablespaces. In fact, creating a cluster and its index in different tablespaces that are stored on different storage devices allows table data and index data to be retrieved simultaneously with minimal disk contention.

Estimate Cluster Size and Set Storage Parameters

The benefits of estimating a cluster's size before creating one follow:

Whether or not you estimate table size before creation, you can explicitly set storage parameters when creating each non-clustered table. Any storage parameter that you do not explicitly set when creating or subsequently altering a table automatically uses the corresponding default storage parameter set for the tablespace in which the table resides. Clustered tables also automatically use the storage parameters of the cluster.

See Also: For information about estimating the size of schema objects, including clusters, see Appendix A.

Creating Clusters

This section describes how to create clusters, and includes the following topics:

To create a cluster in your schema, you must have the CREATE CLUSTER system privilege and a quota for the tablespace intended to contain the cluster or the UNLIMITED TABLESPACE system privilege.

To create a cluster in another user's schema, you must have the CREATE ANY CLUSTER system privilege and the owner must have a quota for the tablespace intended to contain the cluster or the UNLIMITED TABLESPACE system privilege.

You can create a cluster using the SQL command CREATE CLUSTER. The following statement creates a cluster named EMP_DEPT, which stores the EMP and DEPT tables, clustered by the DEPTNO column:

CREATE CLUSTER emp_dept (deptno NUMBER(3))
   PCTUSED 80
   PCTFREE 5
   SIZE 600
   TABLESPACE users
   STORAGE (INITIAL 200k
      NEXT 300K
      MINEXTENTS 2
      MAXEXTENTS 20
      PCTINCREASE 33);

Creating Clustered Tables

To create a table in a cluster, you must have either the CREATE TABLE or CREATE ANY TABLE system privilege. You do not need a tablespace quota or the UNLIMITED TABLESPACE system privilege to create a table in a cluster.

You can create a table in a cluster using the SQL command CREATE TABLE with the CLUSTER option. The EMP and DEPT tables can be created in the EMP_DEPT cluster using the following statements:

CREATE TABLE dept (
   deptno NUMBER(3) PRIMARY KEY, . . . )
   CLUSTER emp_dept (deptno);
CREATE TABLE emp (
   empno NUMBER(5) PRIMARY KEY,
   ename VARCHAR2(15) NOT NULL,
   . . .
   deptno NUMBER(3) REFERENCES dept)
   CLUSTER emp_dept (deptno); 

Note: You can specify the schema for a clustered table in the CREATE TABLE statement; a clustered table can be in a different schema than the schema containing the cluster.

Creating Cluster Indexes

To create a cluster index, one of the following conditions must be true:

In either case, you must also have either a quota for the tablespace intended to contain the cluster index, or the UNLIMITED TABLESPACE system privilege.

A cluster index must be created before any rows can be inserted into any clustered table. The following statement creates a cluster index for the EMP_DEPT cluster:

CREATE INDEX emp_dept_index
   ON CLUSTER emp_dept
   INITRANS 2
   MAXTRANS 5
   TABLESPACE users
   STORAGE (INITIAL 50K
      NEXT 50K
      MINEXTENTS 2
      MAXEXTENTS 10
      PCTINCREASE 33)
   PCTFREE 5;

The cluster key establishes the relationship of the tables in the cluster. Several storage settings are explicitly specified for the cluster and cluster index.

See Also: See Chapter 20 for more information about system privileges, and Chapter 19 for information about tablespace quotas.

Altering Clusters

You can alter an existing cluster to change the following settings:

To alter a cluster, your schema must contain the cluster or you must have the ALTER ANY CLUSTER system privilege.

When you alter data block space usage parameters (PCTFREE and PCTUSED) or the cluster size parameter (SIZE) of a cluster, the new settings apply to all data blocks used by the cluster, including blocks already allocated and blocks subsequently allocated for the cluster. Blocks already allocated for the table are reorganized when necessary (not immediately).

When you alter the transaction entry settings (INITRANS, MAXTRANS) of a cluster, a new setting for INITRANS applies only to data blocks subsequently allocated for the cluster, while a new setting for MAXTRANS applies to all blocks (already and subsequently allocated blocks) of a cluster.

The storage parameters INITIAL and MINEXTENTS cannot be altered. All new settings for the other storage parameters affect only extents subsequently allocated for the cluster.

To alter a cluster, use the SQL command ALTER CLUSTER. The following statement alters the EMP_DEPT cluster:

ALTER CLUSTER emp_dept
   PCTFREE 30
   PCTUSED 60;

Altering Cluster Tables and Cluster Indexes

You can alter clustered tables using the SQL command ALTER TABLE. However, any data block space parameters, transaction entry parameters, or storage parameters you set in an ALTER TABLE statement for a clustered table generate an error message (ORA-01771, "illegal option for a clustered table"). Oracle uses the parameters of the cluster for all clustered tables. Therefore, you can use the ALTER TABLE command only to add or modify columns, or add, drop, enable, or disable integrity constraints or triggers for a clustered table.

Note: When estimating the size of cluster indexes, remember that the index is on each cluster key, not the actual rows; therefore, each key will only appear once in the index.

Manually Allocating Storage for a Cluster

Oracle dynamically allocates additional extents for the data segment of a cluster as required. In some circumstances, however, you might want to allocate an additional extent for a cluster explicitly. For example, when using the Oracle Parallel Server, you can allocate an extent of a cluster explicitly for a specific instance.

You allocate a new extent for a cluster using the SQL command ALTER CLUSTER with the ALLOCATE EXTENT option.

See Also: For information about altering tables, see "Altering Tables" [*].

You alter cluster indexes exactly as you do other indexes. For more information, see "Altering an Index" [*].

For more information about the CLUSTER parameter in the ALTER CLUSTER command, see the Oracle7 Parallel Server Concepts & Administration guide.

Dropping Clusters

This section describes aspects of dropping clusters, and includes the following topics:

A cluster can be dropped if the tables within the cluster are no longer necessary. When a cluster is dropped, so are the tables within the cluster and the corresponding cluster index; all extents belonging to both the cluster's data segment and the index segment of the cluster index are returned to the containing tablespace and become available for other segments within the tablespace.

Dropping Clustered Tables

To drop a cluster, your schema must contain the cluster or you must have the DROP ANY CLUSTER system privilege. You do not have to have additional privileges to drop a cluster that contains tables, even if the clustered tables are not owned by the owner of the cluster.

Clustered tables can be dropped individually without affecting the table's cluster, other clustered tables, or the cluster index. A clustered table is dropped just as a non-clustered table is dropped--with the SQL command DROP TABLE.

Note: When you drop a single table from a cluster, Oracle deletes each row of the table individually. To maximize efficiency when you intend to drop an entire cluster, drop the cluster including all tables by using the DROP CLUSTER command with the INCLUDING TABLES option. Drop an individual table from a cluster (using the DROP TABLE command) only if you want the rest of the cluster to remain.

See Also: For information about dropping a table, see "Dropping Tables" [*].

Dropping Cluster Indexes

A cluster index can be dropped without affecting the cluster or its clustered tables. However, clustered tables cannot be used if there is no cluster index; you must re-create the cluster index to allow access to the cluster. Cluster indexes are sometimes dropped as part of the procedure to rebuild a fragmented cluster index.

To drop a cluster that contains no tables, and its cluster index, use the SQL command DROP CLUSTER. For example, the following statement drops the empty cluster named EMP_DEPT:

DROP CLUSTER emp_dept;

If the cluster contains one or more clustered tables and you intend to drop the tables as well, add the INCLUDING TABLES option of the DROP CLUSTER command, as follows:

DROP CLUSTER emp_dept INCLUDING TABLES;

If the INCLUDING TABLES option is not included and the cluster contains tables, an error is returned.

If one or more tables in a cluster contain primary or unique keys that are referenced by FOREIGN KEY constraints of tables outside the cluster, the cluster cannot be dropped unless the dependent FOREIGN KEY constraints are also dropped. This can be easily done using the CASCADE CONSTRAINTS option of the DROP CLUSTER command, as shown in the following example:

DROP CLUSTER emp_dept INCLUDING TABLES CASCADE CONSTRAINTS;

Oracle returns an error if you do not use the CASCADE CONSTRAINTS option and constraints exist.

See Also: For information about dropping an index, see "Dropping Indexes" [*].




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