A single-table hash cluster is an optimized version of a hash cluster that supports only one table at a time. A one-to-one mapping exists between hash keys and rows.

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A single-table hash cluster can be beneficial when users require rapid access to a table by primary key. For example, users often look up an employee record in the employees table by employee_id.

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A sorted hash cluster stores the rows corresponding to each value of the hash function in such a way that the database can efficiently return them in sorted order. The database performs the optimized sort internally. For applications that always consume data in sorted order, this technique can mean faster retrieval of data. For example, an application might always sort on the order_date column of the orders table.

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See also

Oracle Database Administrator’s Guide to learn how to create single-table and sorted hash clusters

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During database creation, create a locally managed SYSTEM tablespace. A locally managed tablespace uses a bitmap stored in each data file to manage the extents.

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• Specify the EXTENT MANAGEMENT LOCAL clause in the CREATE DATABASE statement to create a locally managed SYSTEM tablespace.

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If you do not specify the EXTENT MANAGEMENT LOCAL clause, then by default the database creates a dictionary-managed SYSTEM tablespace. Dictionary-managed tablespaces are deprecated.

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If you create your database with a locally managed SYSTEM tablespace, and if you are not using Oracle Managed Files, then ensure that the following conditions are met:

• You specify the DEFAULT TEMPORARY TABLESPACE clause in the CREATE DATABASE statement.
• You include the UNDO TABLESPACE clause in the CREATE DATABASE statement.

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In queries of a hash cluster, the database determines how to hash the key values input by the user.

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For example, users frequently execute queries such as the following, entering different department ID numbers for p_id:

CopySELECT *
FROM   employees
WHERE  department_id = :p_id;
 
SELECT * 
FROM   departments 
WHERE  department_id = :p_id;

SELECT * 
FROM   employees e, departments d 
WHERE  e.department_id = d.department_id
AND    d.department_id = :p_id;

If a user queries employees in department_id=20, then the database might hash this value to bucket 77. If a user queries employees in department_id=10, then the database might hash this value to bucket 15. The database uses the internally generated hash value to locate the block that contains the employee rows for the requested department.

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The following illustration depicts a hash cluster segment as a horizontal row of blocks. As shown in the graphic, a query can retrieve data in a single I/O.

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Figure 2-6 Retrieving Data from a Hash Cluster
Description of “Figure 2-6 Retrieving Data from a Hash Cluster”

A limitation of hash clusters is the unavailability of range scans on nonindexed cluster keys. Assume no separate index exists for the hash cluster created in Hash Cluster Creation. A query for departments with IDs between 20 and 100 cannot use the hashing algorithm because it cannot hash every possible value between 20 and 100. Because no index exists, the database must perform a full scan.

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To protect your database, specify passwords for users SYS and SYSTEM.

• In the CREATE DATABASE statement, include clauses that specify the password for users SYS and SYSTEM.

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The clauses of the CREATE DATABASE statement used for specifying the passwords for users SYS and SYSTEM are:

• USER SYS IDENTIFIED BY password
• USER SYSTEM IDENTIFIED BY password

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If you omit these clauses, then these users are assigned the default passwords change_on_install and manager, respectively. A record is written to the alert log indicating that the default passwords were used. To protect your database, you must change these passwords using the ALTER USER statement immediately after database creation.

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Oracle strongly recommends that you specify these clauses, even though they are optional in this release of Oracle Database. The default passwords are commonly known, and if you neglect to change them later, then you leave database vulnerable to attack by malicious users.

When choosing a password, keep in mind that passwords are case-sensitive. Also, there may be password formatting requirements for your database. See the section entitled “How Oracle Database Checks the Complexity of Passwords” in Oracle Database Security Guide for more information.

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To create a hash cluster, you use the same CREATE CLUSTER statement as for an indexed cluster, with the addition of a hash key. The number of hash values for the cluster depends on the hash key.

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The cluster key, like the key of an indexed cluster, is a single column or composite key shared by the tables in the cluster. A hash key value is an actual or possible value inserted into the cluster key column. For example, if the cluster key is department_id, then hash key values could be 10, 20, 30, and so on.

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Oracle Database uses a hash function that accepts an infinite number of hash key values as input and sorts them into a finite number of buckets. Each bucket has a unique numeric ID known as a hash value. Each hash value maps to the database block address for the block that stores the rows corresponding to the hash key value (department 10, 20, 30, and so on).

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In the following example, the number of departments that are likely to exist is 100, so HASHKEYS is set to 100:

CopyCREATE CLUSTER employees_departments_cluster
   (department_id NUMBER(4))
SIZE 8192 HASHKEYS 100;

After you create employees_departments_cluster, you can create the employees and departments tables in the cluster. You can then load data into the hash cluster just as in the indexed cluster.

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You can use the CREATE DATABASE clauses to simplify the creation and management of your database.

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When you execute a CREATE DATABASE statement, Oracle Database performs at least these operations:

• Creates the data files for the database
• Creates the control files for the database
• Creates the online redo logs for the database and establishes the ARCHIVELOG mode

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• Creates the SYSTEM tablespace
• Creates the SYSAUX tablespace
• Creates the data dictionary
• Sets the character set that stores data in the database
• Sets the database time zone
• Mounts and opens the database for use

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A hash cluster is like an indexed cluster, except the index key is replaced with a hash function. No separate cluster index exists. In a hash cluster, the data is the index.

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With an indexed table or indexed cluster, Oracle Database locates table rows using key values stored in a separate index. To find or store a row in an indexed table or table cluster, the database must perform at least two I/Os:

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• One or more I/Os to find or store the key value in the index
• Another I/O to read or write the row in the table or table cluster

To find or store a row in a hash cluster, Oracle Database applies the hash function to the cluster key value of the row. The resulting hash value corresponds to a data block in the cluster, which the database reads or writes on behalf of the issued statement.

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Hashing is an optional way of storing table data to improve the performance of data retrieval. Hash clusters may be beneficial when the following conditions are met:

• A table is queried much more often than modified.
• The hash key column is queried frequently with equality conditions, for example, WHERE department_id=20. For such queries, the cluster key value is hashed. The hash key value points directly to the disk area that stores the rows.
• You can reasonably guess the number of hash keys and the size of the data stored with each key value.

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When you execute a CREATE DATABASE statement, Oracle Database performs several operations. The actual operations performed depend on the clauses that you specify in the CREATE DATABASE statement and the initialization parameters that you have set.

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• About CREATE DATABASE Statement Clauses
  You can use the CREATE DATABASE clauses to simplify the creation and management of your database.
• Protecting Your Database: Specifying Passwords for Users SYS and SYSTEM
  To protect your database, specify passwords for users SYS and SYSTEM.

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• Creating a Locally Managed SYSTEM Tablespace
  During database creation, create a locally managed SYSTEM tablespace. A locally managed tablespace uses a bitmap stored in each data file to manage the extents.
• Specify Data File Attributes for the SYSAUX Tablespace
  The SYSAUX tablespace is created by default, but you can specify its data file attributes during database creation.
• Using Automatic Undo Management: Creating an Undo Tablespace
  Automatic undo management uses an undo tablespace.

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• Creating a Default Permanent Tablespace
  Oracle strongly recommends that you create a default permanent tablespace. Oracle Database assigns to this tablespace any non-SYSTEM users for whom you do not explicitly specify a different permanent tablespace.
• Creating a Default Temporary Tablespace
  When you create a default temporary tablespace, Oracle Database assigns it as the temporary tablespace for users who are not explicitly assigned a temporary tablespace.
• Specifying Oracle Managed Files at Database Creation
  You can minimize the number of clauses and parameters that you specify in your CREATE DATABASE statement by using the Oracle Managed Files feature.
• Supporting Bigfile Tablespaces During Database Creation
  Oracle Database simplifies management of tablespaces and enables support for extremely large databases by letting you create bigfile tablespaces.
• Specifying the Database Time Zone and Time Zone File
  Oracle Database datetime and interval data types and time zone support make it possible to store consistent information about the time of events and transactions.
• Specifying FORCE LOGGING Mode
  Some data definition language statements (such as CREATE TABLE) allow the NOLOGGING clause, which causes some database operations not to generate redo records in the database redo log. The NOLOGGING setting can speed up operations that can be easily recovered outside of the database recovery mechanisms, but it can negatively affect media recovery and standby databases.

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An index cluster is a table cluster that uses an index to locate data. The cluster index is a B-tree index on the cluster key. A cluster index must be created before any rows can be inserted into clustered tables.

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Example 2-6 Creating a Table Cluster and Associated Index

Assume that you create the cluster employees_departments_cluster with the cluster key department_id, as shown in the following example:

CopyCREATE CLUSTER employees_departments_cluster
   (department_id NUMBER(4))
SIZE 512;

CREATE INDEX idx_emp_dept_cluster 
   ON CLUSTER employees_departments_cluster;

Because the HASHKEYS clause is not specified, employees_departments_cluster is an indexed cluster. The preceding example creates an index named idx_emp_dept_cluster on the cluster key department_id.

Example 2-7 Creating Tables in an Indexed Cluster

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You create the employees and departments tables in the cluster, specifying the department_id column as the cluster key, as follows (the ellipses mark the place where the column specification goes):

CopyCREATE TABLE employees ( ... )
   CLUSTER employees_departments_cluster (department_id);
 
CREATE TABLE departments ( ... )
   CLUSTER employees_departments_cluster (department_id);

Assume that you add rows to the employees and departments tables. The database physically stores all rows for each department from the employees and departments tables in the same data blocks. The database stores the rows in a heap and locates them with the index.

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Figure 2-5 shows the employees_departments_cluster table cluster, which contains employees and departments. The database stores rows for employees in department 20 together, department 110 together, and so on. If the tables are not clustered, then the database does not ensure that the related rows are stored together.

Figure 2-5 Clustered Table Data
Description of “Figure 2-5 Clustered Table Data”

The B-tree cluster index associates the cluster key value with the database block address (DBA) of the block containing the data. For example, the index entry for key 20 shows the address of the block that contains data for employees in department 20:

Copy20,AADAAAA9d

The cluster index is separately managed, just like an index on a nonclustered table, and can exist in a separate tablespace from the table cluster.

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Take a full backup of the database to ensure that you have a complete set of files from which to recover if a media failure occurs.

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• Back up the database.

Step 15: (Optional) Enable Automatic Instance Startup

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You might want to configure the Oracle instance to start automatically when its host computer restarts.

• Configure the Oracle instance to start automatically when its host computer restarts.

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See your operating system documentation for instructions. For example, on Windows, use the following command to configure the database service to start the instance upon computer restart: ORADIM -EDIT -SID sid -STARTMODE AUTO -SRVCSTART SYSTEM [-SPFILE]

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