about Set Session Time Zone

1. How to check the session time zone ?

The new SESSIONTIMEZONE built-in SQL function returns the value of the current session's time zone.

SQL> SELECT SESSIONTIMEZONE FROM DUAL;

SESSIONTIMEZONE
---------------
+01:00


2. How to set the session time zone ?

The session time zone can be set to:
- O/S local time zone
- Database time zone
- An absolute offset
- A named region

1). The first method consists to use one of the following ALTER SESSION SET TIME_ZONE statements:
SQL> ALTER SESSION SET TIME_ZONE = local;
SQL> ALTER SESSION SET TIME_ZONE = dbtimezone;
SQL> ALTER SESSION SET TIME_ZONE = '-05:00';
SQL> ALTER SESSION SET TIME_ZONE = 'Europe/London';

2). As an alternative, the operating system environment variable ORA_SDTZ can also be used to set the session time zone:
ORA_SDTZ = 'OS_TZ' | 'DB_TZ' | '[+ | -] hh:mm' | 'timezone_region'

Example:
$ ORA_SDTZ='OS_TZ'
$ export ORA_SDTZ

$ ORA_SDTZ='-05:00'
$ export ORA_SDTZ


3. How to check the database time zone ?

The DBTIMEZONE SQL function returns the value of the database time zone.

SQL> SELECT DBTIMEZONE FROM DUAL;

DBTIMEZONE
--------------
Europe/Lisbon

@ Note that the database timezone is also included in both database_properties and props$ views:
@ SQL> SELECT property_name, property_value FROM database_properties WHERE property_name='DBTIMEZONE';
@ and
@ SQL> SELECT name, value$ FROM props$ WHERE name='DBTIMEZONE';
@ Be aware that you should not rely on these views because in case of db time zone
@ change, these views reflect the new db time zone too early: they should reflect
@ it only after database shutdown and restart.


4. How to set the database time zone ?

Note that the database time zone is only relevant for TIMESTAMP WITH LOCAL TIME ZONE columns.

1). At creation time
SQL> CREATE DATABASE ...
SET TIME_ZONE='Europe/London';
If not specified with the CREATE DATABASE statement, the database time zone defaults to the server timezone offset.

2). After database creation, use the ALTER DATABASE SET TIME_ZONE statement and then shut down and restart the database.
SQL> ALTER DATABASE SET TIME_ZONE = '-05:00';
SQL> ALTER DATABASE SET TIME_ZONE = 'Europe/Lisbon';
The change will not take effect until the database is bounced.


5. How to list the valid time zone regions ?

SQL> SELECT * FROM v$timezone_names;

TZNAME TZABBREV
------------------------------ ----------
Pacific/Tahiti LMT
Poland LMT
US/Pacific PST
Europe/Zurich CET
...

The result will depend on which timezone file is currently in use. See Q&A 7-8 for further details.


6. How to retrieve the time zone offset corresponding to a time zone region ?

The new 9i TZ_OFFSET() SQL function returns the time zone offset displacement to the input time zone region.

SQL> SELECT TZ_OFFSET('Europe/London') FROM DUAL;

TZ_OFFS
-------
+01:00

The returned offset depends on the date this statement is executed.
For example, in the 'US/Pacific' time zone, it may return '-07:00' or '-08:00' whether daylight saving is in effect or not.

SQL> SELECT TZ_OFFSET(SESSIONTIMEZONE), TZ_OFFSET(DBTIMEZONE) FROM DUAL;
returns the time zone offset corresponding to the time zone set for session and database.

TZ_OFFS TZ_OFFS
------- -------
+01:00 +00:00


7. How to use a larger set of defined time zones ?

2 different time zone files contain for each zone the offset from UTC, the transition times for daylight savings and abbreviation :

- ORACLE_HOME/oracore/zoneinfo/timezone.dat
This is the default and contains the most commonly used time zones. This is the smallest file.

- ORACLE_HOME/oracore/zoneinfo/timezlrg.dat
This file contains the larger set of defined time zones and can be used if you require time zones that are not defined in the default time zone file.

To enable the use of this larger time zone data file :

1). Shutdown the database

2).1 On UNIX platforms :
set the ORA_TZFILE environment variable to the full pathname of the location for the timezlrg.dat. By default, this should be $ORACLE_HOME/oracore/zoneinfo/timezlrg.dat:

$ setenv ORA_TZFILE $ORACLE_HOME/oracore/zoneinfo/timezlrg.dat

2).2 On Windows systems :
You should add an ORA_TZFILE registry subkey for each of your Oracle Homes with the Windows Registry Editor:
Start -> Run...
Type "regedit", and click "ok"
Add or edit the following registry entry:
HKEY_LOCAL_MACHINE/SOFTWARE/ORACLE/HOMEID/ORA_TZFILE
where ID is the unique number identifying the Oracle home.

Set the ORA_TZFILE to the full pathname of the location for the timezlrg.dat:
By default, the value should be %ORACLE_HOME%/oracore/zoneinfo/timezlrg.dat

3). Restart the database

Once the larger timezlrg.dat is used, it must continue to be used unless the user is sure that none of the non-default zones are used for data that is stored in the database. Also, all databases that share information should use the same time zone data file.

Be aware that you can neither create nore alter these timezone definition files !


8. Which timezone-set is currently used ?

Beside checking the UNIX ORA_TZFILE parameter or the Windows registry subkey, you can easily check whether you are using the large or the default timezone file with the following query:

SQL> SELECT COUNT(*) FROM v$timezone_names;

If the default-smallest timezone region file is in use, it will return:

COUNT(*)
----------
616 (in 9.0.1.x and 9.2.x)

or

COUNT(*)
----------
1250 (in 9.0.1.x and 9.2.x)

if the largest file is in use.


9. Can customers rely on Oracle time zones ?

Timezone.dat and timezlrg.dat are based on information maintained by the U.S. Navy Observatory.

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