ALTER SESSION

Purpose

Use the ALTER SESSION statement to set or modify any of the conditions or parameters that affect your connection to the database. The statement stays in effect until you disconnect from the database.

Prerequisites

To enable and disable the SQL trace facility, you must have ALTER SESSION system privilege.

To enable or disable resumable space allocation, you must have the RESUMABLE system privilege.

You do not need any privileges to perform the other operations of this statement unless otherwise indicated.

Syntax

alter_session::=

Description of the illustration alter_session.gif

alter_session_set_clause ::=

Description of the illustration alter_session_set_clause.gif

Semantics

ADVISE Clause

The ADVISE clause sends advice to a remote database to force a distributed transaction. The advice appears in the ADVICE column of the DBA_2PC_PENDING view on the remote database (the values are 'C' for COMMIT, 'R' for ROLLBACK, and '  ' for NOTHING). If the transaction becomes in doubt, then the administrator of that database can use this advice to decide whether to commit or roll back the transaction.

You can send different advice to different remote databases by issuing multiple ALTER SESSION statements with the ADVISE clause in a single transaction. Each such statement sends advice to the databases referenced in the following statements in the transaction until another such statement is issued.

CLOSE DATABASE LINK Clause

Specify CLOSE DATABASE LINK to close the database link dblink. When you issue a statement that uses a database link, Oracle Database creates a session for you on the remote database using that link. The connection remains open until you end your local session or until the number of database links for your session exceeds the value of the initialization parameter OPEN_LINKS. If you want to reduce the network overhead associated with keeping the link open, then use this clause to close the link explicitly if you do not plan to use it again in your session.

ENABLE | DISABLE COMMIT IN PROCEDURE

Procedures and stored functions written in PL/SQL can issue COMMIT and ROLLBACK statements. If your application would be disrupted by a COMMIT or ROLLBACK statement not issued directly by the application itself, then specify DISABLE COMMIT IN PROCEDURE clause to prevent procedures and stored functions called during your session from issuing these statements.

You can subsequently allow procedures and stored functions to issue COMMIT and ROLLBACK statements in your session by issuing the ENABLE COMMIT IN PROCEDURE.

Some applications automatically prohibit COMMIT and ROLLBACK statements in procedures and stored functions. Refer to your application documentation for more information.

ENABLE | DISABLE GUARD

The security_clause of ALTER DATABASE lets you prevent anyone other than the SYS user from making any changes to data or database objects on the primary or standby database. This clause lets you override that setting for the current session.

PARALLEL DML | DDL | QUERY

The PARALLEL parameter determines whether all subsequent DML, DDL, or query statements in the session will be considered for parallel execution. This clause enables you to override the degree of parallelism of tables during the current session without changing the tables themselves. Uncommitted transactions must either be committed or rolled back prior to executing this clause for DML.

ENABLE Clause

Specify ENABLE to execute subsequent statements in the session in parallel. This is the default for DDL and query statements.

• DML: DML statements are executed in parallel mode if a parallel hint or a parallel clause is specified.

• DDL: DDL statements are executed in parallel mode if a parallel clause is specified.

• QUERY: Queries are executed in parallel mode if a parallel hint or a parallel clause is specified

Restriction on the ENABLE clause You cannot specify the optional PARALLEL integer with ENABLE.

DISABLE Clause

Specify DISABLE to execute subsequent statements in the session serially. This is the default for DML statements.

• DML: DML statements are executed serially.

• DDL: DDL statements are executed serially.

• QUERY: Queries are executed serially.

Restriction on the DISABLE clause You cannot specify the optional PARALLEL integer with DISABLE.

FORCE Clause

FORCE forces parallel execution of subsequent statements in the session. If no parallel clause or hint is specified, then a default degree of parallelism is used. This clause overrides any parallel_clause specified in subsequent statements in the session but is overridden by a parallel hint.

• DML: Provided no parallel DML restrictions are violated, subsequent DML statements in the session are executed with the default degree of parallelism, unless a degree is specified in this clause.

• DDL: Subsequent DDL statements in the session are executed with the default degree of parallelism, unless a degree is specified in this clause. Resulting database objects will have associated with them the prevailing degree of parallelism.

  Specifying FORCE DDL automatically causes all tables created in this session to be created with a default level of parallelism. The effect is the same as if you had specified the parallel_clause (with the default degree) in the CREATE TABLE statement.

• QUERY: Subsequent queries are executed with the default degree of parallelism, unless a degree is specified in this clause.

PARALLEL integer Specify an integer to explicitly specify a degree of parallelism:

• For FORCE DDL, the degree overrides any parallel clause in subsequent DDL statements.

• For FORCE DML and QUERY, the degree overrides the degree currently stored for the table in the data dictionary.

• A degree specified in a statement through a hint will override the degree being forced.

The following types of DML operations are not parallelized regardless of this clause:

• Operations on cluster tables

• Operations with embedded functions that either write or read database or package states

• Operations on tables with triggers that could fire

• Operations on tables or schema objects containing object types, or LONG or LOB datatypes.

RESUMABLE Clauses

These clauses let you enable and disable resumable space allocation. This feature allows an operation to be suspended in the event of an out-of-space error condition and to resume automatically from the point of interruption when the error condition is fixed.

ENABLE RESUMABLE

This clause enables resumable space allocation for the session.

TIMEOUT TIMEOUT lets you specify (in seconds) the time during which an operation can remain suspended while waiting for the error condition to be fixed. If the error condition is not fixed within the TIMEOUT period, then Oracle Database aborts the suspended operation.

NAME NAME lets you specify a user-defined text string to help users identify the statements issued during the session while the session is in resumable mode. Oracle Database inserts the text string into the USER_RESUMABLE and DBA_RESUMABLE data dictionary views. If you do not specify NAME, then Oracle Database inserts the default string 'User username(userid), Session sessionid, Instance instanceid'.

DISABLE RESUMABLE

This clause disables resumable space allocation for the session.

alter_session_set_clause

Use the alter_session_set_clause to set parameter values. You can set two types of parameters using this clause:

• Initialization parameters that are dynamic in the scope of the ALTER SESSION statement (listed in "Initialization Parameters and ALTER SESSION")

• Session parameters (listed in "Session Parameters and ALTER SESSION")

You can set values for multiple parameters in the same alter_session_set_clause.

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Initialization Parameters and ALTER SESSION

A list of the initialization parameters that are dynamic in the scope of ALTER SESSION appears in Table 11-1. In PDF and HTML, the parameters are hyperlinks that take you to their full descriptions in Oracle Database Reference. In print, for a description of each parameter, please refer to Oracle Database Reference.

When you set these parameters using ALTER SESSION, the value you set persists only for the duration of the current session.

A number of parameters that can be set using ALTER SESSION are not initialization parameters. That is, you can set them only with ALTER SESSION, not in an initialization parameter file. Those session parameters are described in "Session Parameters and ALTER SESSION".

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Session Parameters and ALTER SESSION

The following parameters are session parameters only, not initialization parameters:

CONSTRAINT[S]

Syntax:

CONSTRAINT[S] = { IMMEDIATE | DEFERRED | DEFAULT }

The CONSTRAINT[S] parameter determines when conditions specified by a deferrable constraint are enforced.

• IMMEDIATE indicates that the conditions specified by the deferrable constraint are checked immediately after each DML statement. This setting is equivalent to issuing the SET CONSTRAINTS ALL IMMEDIATE statement at the beginning of each transaction in your session.

• DEFERRED indicates that the conditions specified by the deferrable constraint are checked when the transaction is committed. This setting is equivalent to issuing the SET CONSTRAINTS ALL DEFERRED statement at the beginning of each transaction in your session.

• DEFAULT restores all constraints at the beginning of each transaction to their initial state of DEFERRED or IMMEDIATE.

CREATE_STORED_OUTLINES

In earlier releases, CREATE_STORED_OUTLINES was a session parameter only. It is now an initialization parameter. Please refer to CREATE_STORED_OUTLINES in the Oracle Database Reference.

CURRENT_SCHEMA

Syntax:

CURRENT_SCHEMA = schema

The CURRENT_SCHEMA parameter changes the current schema of the session to the specified schema. Subsequent unqualified references to schema objects during the session will resolve to objects in the specified schema. The setting persists for the duration of the session or until you issue another ALTER SESSION SET CURRENT_SCHEMA statement.

This setting offers a convenient way to perform operations on objects in a schema other than that of the current user without having to qualify the objects with the schema name. This setting changes the current schema, but it does not change the session user or the current user, nor does it give the session user any additional system or object privileges for the session.

ERROR_ON_OVERLAP_TIME

Syntax:

ERROR_ON_OVERLAP_TIME = {TRUE | FALSE}

The ERROR_ON_OVERLAP_TIME parameter determines how Oracle Database should handle an ambiguous boundary datetime value--that is, a case in which it is not clear whether the datetime is in standard or daylight savings time.

• Specify TRUE to return an error for the ambiguous overlap timestamp.

• Specify FALSE to default the ambiguous overlap timestamp to the standard time. This is the default.

Please refer to "Support for Daylight Saving Times" for more information on boundary datetime values.

FLAGGER

Syntax:

FLAGGER = { ENTRY | INTERMEDIATE | FULL | OFF }

The FLAGGER parameter specifies FIPS flagging, which causes an error message to be generated when a SQL statement issued is an extension of ANSI SQL92. FLAGGER is a session parameter only, not an initialization parameter.

In Oracle Database, there is currently no difference between entry, intermediate, or full level flagging. After flagging is set in a session, a subsequent ALTER SESSION SET FLAGGER statement will work, but generates the message, ORA-00097. This allows FIPS flagging to be altered without disconnecting the session. OFF turns off flagging.

INSTANCE

Syntax:

INSTANCE = integer

Setting the INSTANCE parameter lets you access another instance as if you were connected to your own instance. INSTANCE is a session parameter only, not an initialization parameter. In a Real Application Clusters (RAC) environment, each RAC instance retains static or dynamic ownership of disk space for optimal DML performance based on the setting of this parameter.

ISOLATION_LEVEL

Syntax:

ISOLATION_LEVEL = {SERIALIZABLE | READ COMMITTED} 

The ISOLATION_LEVEL parameter specifies how transactions containing database modifications are handled. ISOLATION_LEVEL is a session parameter only, not an initialization parameter.

• SERIALIZABLE indicates that transactions in the session use the serializable transaction isolation mode as specified in SQL92. That is, if a serializable transaction attempts to execute a DML statement that updates rows currently being updated by another uncommitted transaction at the start of the serializable transaction, then the DML statement fails. A serializable transaction can see its own updates.

• READ COMMITTED indicates that transactions in the session will use the default Oracle Database transaction behavior. That is, if the transaction contains DML that requires row locks held by another transaction, then the DML statement will wait until the row locks are released.

PLSQL_DEBUG

In earlier releases, PLSQL_DEBUG was a session parameter only. It is now an initialization parameter. Please refer to PLSQL_DEBUG in the Oracle Database Reference.

SKIP_UNUSABLE_INDEXES

In earlier releases, SKIP_UNUSABLE_INDEXES was a session parameter only. It is now an initialization parameter. Please refer to SKIP_UNUSABLE_INDEXES in the Oracle Database Reference.

SQL_TRACE

In earlier releases, SQL_TRACE was a session parameter only. It is now an initialization parameter. Please refer to SQL_TRACE in the Oracle Database Reference.

TIME_ZONE

Syntax:

TIME_ZONE =  '[+ | -] hh:mm' 
             | LOCAL 
             | DBTIMEZONE 
             | 'time_zone_region'

The TIME_ZONE parameter specifies the default local time zone offset or region name for the current SQL session. TIME_ZONE is a session parameter only, not an initialization parameter. To determine the time zone of the current session, query the built-in function SESSIONTIMEZONE (see SESSIONTIMEZONE).

• Specify a format mask ('[+|-]hh:mm') indicating the hours and minutes before or after UTC (Coordinated Universal Time--formerly Greenwich Mean Time). The valid range for hh:mm is -12:00 to +14:00.

• Specify LOCAL to set the default local time zone offset of the current SQL session to the original default local time zone offset that was established when the current SQL session was started.

• Specify DBTIMEZONE to set the current session time zone to match the value set for the database time zone. If you specify this setting, then the DBTIMEZONE function will return the database time zone as a UTC offset or a time zone region, depending on how the database time zone has been set.

• Specify a valid time_zone_region. To see a listing of valid region names, query the TZNAME column of the V$TIMEZONE_NAMES dynamic performance view. If you specify this setting, then the SESSIONTIMEZONE function will return the region name.

USE_PRIVATE_OUTLINES

Syntax:

USE_PRIVATE_OUTLINES = { TRUE | FALSE | category_name }

The USE_PRIVATE_OUTLINES parameter lets you control the use of private outlines. When this parameter is enabled and an outlined SQL statement is issued, the optimizer retrieves the outline from the session private area rather than the public area used when USE_STORED_OUTLINES is enabled. If no outline exists in the session private area, then the optimizer will not use an outline to compile the statement. USE_PRIVATE_OUTLINES is not an initialization parameter.

• TRUE causes the optimizer to use private outlines stored in the DEFAULT category when compiling requests.

• FALSE specifies that the optimizer should not use stored private outlines. This is the default. If USE_STORED_OUTLINES is enabled, then the optimizer will use stored public outlines.

• category_name causes the optimizer to use outlines stored in the category_name category when compiling requests.

Restriction on USE_PRIVATE_OUTLINES You cannot enable this parameter if USE_STORED_OUTLINES is enabled.

USE_STORED_OUTLINES

Syntax:

USE_STORED_OUTLINES = { TRUE | FALSE | category_name }

The USE_STORED_OUTLINES parameter determines whether the optimizer will use stored public outlines to generate execution plans. USE_STORED_OUTLINES is not an initialization parameter.

• TRUE causes the optimizer to use outlines stored in the DEFAULT category when compiling requests.

• FALSE specifies that the optimizer should not use stored outlines. This is the default.

• category_name causes the optimizer to use outlines stored in the category_name category when compiling requests.

Restriction on USED_STORED_OUTLINES You cannot enable this parameter if USE_PRIVATE_OUTLINES is enabled.

Examples

Enabling Parallel DML: Example Issue the following statement to enable parallel DML mode for the current session:

ALTER SESSION ENABLE PARALLEL DML;

Forcing a Distributed Transaction: Example The following transaction inserts an employee record into the employees table on the database identified by the database link remote and deletes an employee record from the employees table on the database identified by local:

ALTER SESSION
   ADVISE COMMIT; 

INSERT INTO employees@remote
   VALUES (8002, 'Juan', 'Fernandez', 'juanf@hr.com', NULL, 
   TO_DATE('04-OCT-1992', 'DD-MON-YYYY'), 'SA_CLERK', 3000, 
   NULL, 121, 20); 

ALTER SESSION
   ADVISE ROLLBACK; 

DELETE FROM employees@local
   WHERE employee_id = 8002; 

COMMIT;  

This transaction has two ALTER SESSION statements with the ADVISE clause. If the transaction becomes in doubt, then remote is sent the advice 'COMMIT' by virtue of the first ALTER SESSION statement and local is sent the advice 'ROLLBACK' by virtue of the second statement.

Closing a Database Link: Example This statement updates the jobs table on the local database using a database link, commits the transaction, and explicitly closes the database link:

UPDATE jobs@local SET min_salary = 3000
   WHERE job_id = 'SH_CLERK';

COMMIT; 

ALTER SESSION
   CLOSE DATABASE LINK local;

Changing the Date Format Dynamically: Example The following statement dynamically changes the default date format for your session to 'YYYY MM DD-HH24:MI:SS':

ALTER SESSION 
   SET NLS_DATE_FORMAT = 'YYYY MM DD HH24:MI:SS';

Oracle Database uses the new default date format:

SELECT TO_CHAR(SYSDATE) Today
   FROM DUAL; 

TODAY 
------------------- 
2001 04 12 12:30:38

Changing the Date Language Dynamically: Example The following statement changes the language for date format elements to French:

ALTER SESSION 
   SET NLS_DATE_LANGUAGE = French;

SELECT TO_CHAR(SYSDATE, 'Day DD Month YYYY') Today
   FROM DUAL; 

TODAY 
--------------------------- 
Jeudi    12 Avril     2001

Changing the ISO Currency: Example The following statement dynamically changes the ISO currency symbol to the ISO currency symbol for the territory America:

ALTER SESSION
   SET NLS_ISO_CURRENCY = America; 

SELECT TO_CHAR( SUM(salary), 'C999G999D99') Total
   FROM employees; 

TOTAL
------------------
     USD694,900.00

Changing the Decimal Character and Group Separator: Example The following statement dynamically changes the decimal character to comma (,) and the group separator to period (.):

ALTER SESSION SET NLS_NUMERIC_CHARACTERS = ',.' ;

Oracle Database returns these new characters when you use their number format elements:

ALTER SESSION SET NLS_CURRENCY = 'FF';

SELECT TO_CHAR( SUM(salary), 'L999G999D99') Total FROM employees;

TOTAL
---------------------
         FF694.900,00

Changing the NLS Currency: Example The following statement dynamically changes the local currency symbol to 'DM':

ALTER SESSION
   SET NLS_CURRENCY = 'DM'; 

SELECT TO_CHAR( SUM(salary), 'L999G999D99') Total
   FROM employees; 

TOTAL
---------------------
         DM694.900,00

Changing the NLS Language: Example The following statement dynamically changes to French the language in which error messages are displayed:

ALTER SESSION
   SET NLS_LANGUAGE = FRENCH; 

Session modifiee.

SELECT * FROM DMP;

ORA-00942: Table ou vue inexistante

Changing the Linguistic Sort Sequence: Example The following statement dynamically changes the linguistic sort sequence to Spanish:

ALTER SESSION
   SET NLS_SORT = XSpanish; 

Oracle Database sorts character values based on their position in the Spanish linguistic sort sequence.

Enabling SQL Trace: Example To enable the SQL trace facility for your session, issue the following statement:

ALTER SESSION 
   SET SQL_TRACE = TRUE; 

Enabling Query Rewrite: Example This statement enables query rewrite in the current session for all materialized views that have not been explicitly disabled:

ALTER SESSION SET QUERY_REWRITE_ENABLED = TRUE;