You can force the writing of redo records even when NOLOGGING is specified in DDL statements.

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To put the database into FORCE LOGGING mode:

• Include the FORCE LOGGING clause in the CREATE DATABASE statement.

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If you do not specify this clause, then the database is not placed into FORCE LOGGING mode.

Use the ALTER DATABASE statement to place the database into FORCE LOGGING mode after database creation. This statement can take a considerable time for completion, because it waits for all unlogged direct writes to complete.

You can cancel FORCE LOGGING mode using the following SQL statement:

ALTER DATABASE NO FORCE LOGGING;

Independent of specifying FORCE LOGGING for the database, you can selectively specify FORCE LOGGING or NO FORCE LOGGING at the tablespace level. However, if FORCE LOGGING mode is in effect for the database, it takes precedence over the tablespace setting. If it is not in effect for the database, then the individual tablespace settings are enforced. Oracle recommends that either the entire database is placed into FORCE LOGGING mode, or individual tablespaces be placed into FORCE LOGGING mode, but not both.

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The FORCE LOGGING mode is a persistent attribute of the database. That is, if the database is shut down and restarted, it remains in the same logging mode. However, if you re-create the control file, the database is not restarted in the FORCE LOGGING mode unless you specify the FORCE LOGGING clause in the CREATE CONTROL FILE statement.

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Temporary tables are useful in applications where a result set must be buffered.

For example, a scheduling application enables college students to create optional semester course schedules. A row in a global temporary table represents each schedule. During the session, the schedule data is private. When the student chooses a schedule, the application moves the row for the chosen schedule to a permanent table. At the end of the session, the database automatically drops the schedule data that was in the global temporary table.

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Private temporary tables are useful for dynamic reporting applications. For example, a customer resource management (CRM) application might connect as the same user indefinitely, with multiple sessions active at the same time. Each session creates a private temporary table named ORA$PTT_crm for each new transaction. The application can use the same table name for every session, but change the definition. The data and definition are visible only to the session. The table definition persists until the transaction ends or the table is manually dropped.

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Segment Allocation in Temporary Tables

Like permanent tables, global temporary tables are persistent objects that are statically defined in the data dictionary. For private temporary tables, metadata exists only in memory, but can reside in the temporary tablespace on disk.

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For global and private temporary tables, the database allocates temporary segments when a session first inserts data. Until data is loaded in a session, the table appears empty. For transaction-specific temporary tables, the database deallocates temporary segments at the end of the transaction. For session-specific temporary tables, the database deallocates temporary segments at the end of the session.

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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.

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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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Oracle Database lets you force the writing of redo records even when NOLOGGING has been specified in DDL statements. The database never generates redo records for temporary tablespaces and temporary segments, so forced logging has no affect for objects.

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• Using the FORCE LOGGING Clause
  You can force the writing of redo records even when NOLOGGING is specified in DDL statements.
• Performance Considerations of FORCE LOGGING Mode
  FORCE LOGGING mode results in some performance degradation.

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A temporary table holds data that exists only for the duration of a transaction or session.

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Data in a temporary table is private to the session. Each session can only see and modify its own data.

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You can create either a global temporary table or a private temporary table. The following table shows the essential differences between them.

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Table 2-5 Temporary Table Characteristics

Characteristic	Global	Private
Naming rules	Same as for permanent tables	Must be prefixed with ORA$PTT_
Visibility of table definition	All sessions	Only the session that created the table
Storage of table definition	Disk	Memory only
Types	Transaction-specific (ON COMMIT DELETE ROWS) or session-specific (ON COMMIT PRESERVE ROWS)	Transaction-specific (ON COMMIT DROP DEFINITION) or session-specific (ON COMMIT PRESERVE DEFINITION)

A third type of temporary table, known as a cursor-duration temporary table, is created by the database automatically for certain types of queries.

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All databases that share information must use the same time zone data file.

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The database server always uses the large time zone file by default.

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To use the small time zone file on the client and know that all your data will refer only to regions in the small file:

• Set the ORA_TZFILE environment variable on the client to the full path name of the timezone version.dat file on the client, where version matches the time zone file version that is being used by the database server.

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If you are already using the default larger time zone file on the client, then it is not practical to change to the smaller time zone file, because the database may contain data with time zones that are not part of the smaller file.

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Two time zone files are included in a subdirectory of the Oracle home directory. The time zone files contain the valid time zone names.

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The following information is also included for each time zone:

• Offset from Coordinated Universal Time (UTC)
• Transition times for Daylight Saving Time
• Abbreviations for standard time and Daylight Saving Time

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The default time zone file is ORACLE_HOME/oracore/zoneinfo/timezlrg_11.dat. A smaller time zone file with fewer time zones can be found in ORACLE_HOME/oracore/zoneinfo/timezone_11.dat.

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To view the time zone names in the file being used by your database, use the following query:

SELECT * FROM V$TIMEZONE_NAMES;

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Interleaved ordering uses a technique that is similar to a Z-order.

Interleaved ordering enables the database to prune I/O based on any subset of predicates in the clustering columns. Interleaved ordering is useful for dimensional hierarchies in a data warehouse.

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As with attribute-clustered tables with linear ordering, Oracle Database supports interleaved ordering on single or multiple tables that are connected through a primary-foreign key relationship. Columns in tables other than the attribute-clustered table must be linked by foreign key and joined to the attribute-clustered table.

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Large data warehouses frequently organize data in a star schema. A dimension table uses a parent-child hierarchy and is connected to a fact table by a foreign key. Clustering a fact table by interleaved order enables the database to use a special function to skip values in dimension columns during table scans.

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Example 2-10 Interleaved Ordering Example

Suppose your data warehouse contains a sales fact table and its two dimension tables: customers and products. Most queries have predicates on the customers table hierarchy (cust_state_province, cust_city) and the products hierarchy (prod_category, prod_subcategory). You can use interleaved ordering for the sales table as shown in the partial statement in the following example:

CopyCREATE TABLE sales
(
   prod_id NUMBER NOT NULL
,  cust_id NUMBER NOT NULL
,  amount_sold NUMBER(10,2) ...
)
CLUSTERING sales
   JOIN products ON (sales.prod_id = products.prod_id)
   JOIN customers ON (sales.cust_id = customers.cust_id)
   BY INTERLEAVED ORDER
   (
     (  products.prod_category
     ,  products.prod_subcategory
     ),
     (  customers.cust_state_province
     ,  customers.cust_city
     )
   )
WITH MATERIALIZED ZONEMAP;

Note

The columns specified in the BY INTERLEAVED ORDER clause need not be in actual dimension tables, but they must be connected through a primary-foreign key relationship.

Suppose an application queries the sales, products, and customers tables in a join. The query specifies the customers.prod_category and customers_cust_state_province columns in the predicate as follows:

CopySELECT cust_city, prod_sub_category, SUM(amount_sold)
FROM   sales, products, customers
WHERE  sales.prod_id = products.prod_id 
AND    sales.cust_id = customers.cust_id
AND    customers.prod_category = 'Boys' 
AND    customers.cust_state_province = 'England - Norfolk' 
GROUP BY cust_city, prod_sub_category;

In the preceding query, the prod_category and cust_state_province columns are part of the clustering definition shown in the CREATE TABLE example. During the scan of the sales table, the database can consult the zone map and access only the rowids in this zone.

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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.

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• Setting the Database Time Zone
  You can set the database time zone with the SET TIME_ZONE clause of the CREATE DATABASE statement.
• About the Database Time Zone Files
  Two time zone files are included in a subdirectory of the Oracle home directory. The time zone files contain the valid time zone names.
• Specifying the Database Time Zone File
  All databases that share information must use the same time zone data file.

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Setting the Database Time Zone

You can set the database time zone with the SET TIME_ZONE clause of the CREATE DATABASE statement.

• Set the database time zone when the database is created by using the SET TIME_ZONE clause of the CREATE DATABASE statement.

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If you do not set the database time zone, then it defaults to the time zone of the host operating system.

You can change the database time zone for a session by using the SET TIME_ZONE clause of the ALTER SESSION statement.

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The SYSTEM and SYSAUX tablespaces are always created with the default tablespace type. However, you optionally can explicitly override the default tablespace type for the UNDO and DEFAULT TEMPORARY tablespace during the CREATE DATABASE operation.

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• Specify an UNDO TABLESPACE clause or a DEFAULT TEMPORARY TABLESPACE clause that overrides the default tablespace type.

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For example, you can create a bigfile UNDO tablespace in a database with the default tablespace type of smallfile as follows:

CREATE DATABASE mynewdb
…
BIGFILE UNDO TABLESPACE undotbs1
DATAFILE ‘/u01/oracle/oradata/mynewdb/undotbs01.dbf’
SIZE 200M REUSE AUTOEXTEND ON MAXSIZE UNLIMITED;

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You can create a smallfile DEFAULT TEMPORARY tablespace in a database with the default tablespace type of bigfile as follows:

CREATE DATABASE mynewdb
SET DEFAULT BIGFILE TABLESPACE
…
SMALLFILE DEFAULT TEMPORARY TABLESPACE tempts1
TEMPFILE ‘/u01/oracle/oradata/mynewdb/temp01.dbf’
SIZE 20M REUSE
…

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A linear ordering scheme for a table divides rows into ranges based on user-specified attributes in a specific order. Oracle Database supports linear ordering on single or multiple tables that are connected through a primary-foreign key relationship.

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For example, the sales table divides the cust_id and prod_id columns into ranges, and then clusters these ranges together on disk. When you specify the BY LINEAR ORDER directive for a table, significant I/O reduction can occur when a predicate specifies either the prefix column or all columns in the directive.

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Assume that queries of sales often specify either a customer ID or a combination of a customer ID and product ID. You can create an attribute-clustered table so that such queries benefit from I/O reduction:

CopyCREATE TABLE sales
(
   prod_id     NOT NULL NUMBER
,  cust_id     NOT NULL NUMBER
,  amount_sold NUMBER(10,2) ...
)
CLUSTERING 
  BY LINEAR ORDER (cust_id, prod_id)
  YES ON LOAD YES ON DATA MOVEMENT
  WITH MATERIALIZED ZONEMAP;

Queries that qualify both columns cust_id and prod_id, or the prefix cust_id experience I/O reduction. Queries that qualify prod_id only do not experience significant I/O reduction because prod_id is the suffix of the BY LINEAR ORDER clause. The following examples show how the database can reduce I/O during table scans.

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Example 2-8 Specifying Only cust_id

An application issues the following query:

CopySELECT * FROM sales WHERE cust_id = 100;

Because the sales table is a BY LINEAR ORDER cluster, the database must only read the zones that include the cust_id value of 100.

Example 2-9 Specifying prod_id and cust_id

An application issues the following query:

CopySELECT * FROM sales WHERE cust_id = 100 AND prod_id = 2300;

Because the sales table is a BY LINEAR ORDER cluster, the database must only read the zones that include the cust_id value of 100 and prod_id value of 2300.

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