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6#
发表于 2012-4-19 22:09:06
ODM FINDING:
8i/9i only: Changing the Database Character Set or the Database National Character Set in 8i/9i
If not done already please see first of all this note:
Note:225912.1 Changing the Database Character Set ( NLS_CHARACTERSET )
It contains more general information and links to guides for the most common
characterset changes.
Please note that in 10g and up the "ALTER DATABASE CHARACTER SET" command
is NOT to be used anymore.
The CSALTER script, combined with CSSCAN, is the only way to change the
characterset of a 10g or higher database.
Note:225912.1 Changing the Database Character Set ( NLS_CHARACTERSET )
This note is for 8i/9i ONLY.
PURPOSE
=======
To explain how to change the database character set or national character set
of an existing Oracle8(i) or Oracle9i database without having to recreate
the database.
1. SCOPE & APPLICATION
======================
The method described here is documented in the Oracle 8.1.x and Oracle9i
documentation. It is not documented but it can be used in version 8.0.x.
It does not work in Oracle7.
The database character set is the character set of CHAR, VARCHAR2, LONG,
and CLOB data stored in the database columns, and of SQL and PL/SQL text
stored in the Data Dictionary. The national character set is
the character set of NCHAR, NVARCHAR2, and NCLOB data.
In certain database configurations the CLOB and NCLOB data are stored
in the fixed-width Unicode encoding UCS-2. If you are using CLOB or NCLOB
please make sure you read section "4. HANDLING CLOB AND NCLOB COLUMNS"
below in this document.
Before changing the character set of a database make sure you understand
how Oracle deals with character sets. Before proceeding please refer to
note:158577.1 "NLS_LANG Explained (How Does Client-Server Character
Conversion Work?)". See also note:225912.1 "Changing the Database
Character Set (NLS_CHARACTERSET)" for general discussion about various
methods of migration to a different database character set.
If you are migrating an Oracle Applications instance, read note:124721.1
"Migrating an Applications Installation to a New Character Set" for specific
steps that have to be performed.
If you are migrating from 8.x to 9.x please have a look at
note:140014.1 "ALERT: Oracle8/8i to Oracle9i Using New "AL16UTF16""
and other referenced notes below.
Before using the method described in this note it is essential to do
a full backup of the database and to use the Character Set Scanner utility
to check your data. See the section "2. USING THE CHARACTER SET SCANNER"
below.
Note that changing the database or the national character set as described
in this document does not change the actual character codes, it only
changes the character set declaration. If you want to convert the contents
of the database (character codes) from one character set to another
you must use the Oracle Export and Import utilities. This is needed,
for example, if the source character set is not a binary subset of
the target character set, i.e. if a character exists in the source
and in the target character set but not with the same binary code.
All binary subset-superset relationships between characters sets
recognized by the Oracle Server are listed in note:119164.1
"Changing Database Character Set - Valid Superset Definitions".
Note: The varying width character sets (like UTF8) are not supported as
national character sets in Oracle8(i) (see note:62107.1).
Thus, changing the national character set from a fixed width
character set to a varying width character set is not supported
in Oracle8(i). NCHAR types in Oracle8 and Oracle8i were designed
to support special Oracle specific fixed-width Asian character sets,
that were introduced to provide higher performance processing of
Asian character data. Examples of these character sets are :
JA16EUCFIXED ,JA16SJISFIXED , ZHT32EUCFIXED.
For a definition of varying width character sets see also section
"4. HANDLING CLOB AND NCLOB COLUMNS" below.
WARNING: Do not use any undocumented Oracle7 method to change
the database character set of an Oracle8(i) or Oracle9i
database. This will corrupt the database.
2. USING THE CHARACTER SET SCANNER
==================================
Install Csscan following Note 458122.1 Installing and configuring CSSCAN in 8i and 9i
To verify that your database contains only valid codes, specify the
new database character set in the TOCHAR parameter and/or the new
national character set in the TONCHAR parameter. Specify FULL=Y
to scan the whole database. Set ARRAY and PROCESS parameters
depending on your system's resources to speed up the scanning.
FROMCHAR and FROMNCHAR will default to the original database and
national character sets.
The Character Set Scanner should report only "Changeless" data in
both the Data Dictionary and in application data. If any Convertible
or Exceptional data are reported, the ALTER DATABASE [NATIONAL]
CHARACTER SET statement must NOT be used without further
investigation of the source and type of these data.
In situations in which the ALTER DATABASE [NATIONAL] CHARACTER SET
statement is used to repair an incorrect database character set
declaration rather than to simply migrate to a new wider character set,
you may be advised by Oracle Support Services analysts to execute
the statement even if Exceptional data are reported.
For more information see also note:225912.1 "Changing the Database
Character Set (NLS_CHARACTERSET)".
3. CHANGING THE DATABASE OR THE NATIONAL CHARACTER SET
======================================================
Oracle8(i) introduces a new documented method of changing the database
and national character sets. The method uses two SQL statements, which
are described in the Oracle8i National Language Support Guide:
ALTER DATABASE [<db_name>] CHARACTER SET <new_character_set>
ALTER DATABASE [<db_name>] NATIONAL CHARACTER SET <new_NCHAR_character_set>
The database name is optional. The character set name should be specified
without quotes, for example:
ALTER DATABASE CHARACTER SET WE8ISO8859P1
To change the database character set perform the following steps.
Note that some of them have been erroneously omitted from the Oracle8i
documentation:
1. Use the Character Set Scanner utility to verify that your database
contains only valid character codes -- see "2. USING THE CHARACTER SET
SCANNER" above.
2. If you go from a 7/8bit characterset to varying width character set
(like UTF8, AL32UTF8, JA16EUC, JA16SJIS, ZHT16BIG5, ZHS16GBK, KO16KSC5601...)
then prepare the CLOB columns for the character set change
-- see "4. HANDLING CLOB AND NCLOB COLUMNS" below.
Omitting this step can lead to corrupted CLOB/NCLOB values
in the database.
If SYS.JOB$, SYS.METASTYLESHEET or SYS.RULE$ have convertible please
see note 258904.1 Solving Convertible data in data dictionary objects
when changing the NLS_CHARACTERSET for the actions that need to be taken.
3. Make sure the parallel_server parameter in INIT.ORA is set to false
or it is not set at all.
If you are using RAC see
Note:221646.1 Changing the Character Set for a RAC Database Fails with an ORA-12720 Error
4. For Oracle 9 and up, make sure you are connected "AS SYSDBA" in sqlplus.
For Oracle 8/8i, make sure you are connected as INTERNAL in svrmgrl.
Then follow these steps:
SHUTDOWN IMMEDIATE;
-- make sure there is a database backup you can rely on, or create one
STARTUP MOUNT;
ALTER SYSTEM ENABLE RESTRICTED SESSION;
ALTER SYSTEM SET JOB_QUEUE_PROCESSES=0;
ALTER SYSTEM SET AQ_TM_PROCESSES=0;
ALTER DATABASE OPEN;
ALTER DATABASE CHARACTER SET <new_character_set>;
-- a alter database takes typically only a few minutes or less,
-- it depends on the number of columns in the database, not the
-- amount of data.
SHUTDOWN;
-- If you use Oracle8 then also do:
STARTUP RESTRICT;
SHUTDOWN;
The extra restart/shutdown is necessary in Oracle8(i) because of a SGA
initialization bug which is fixed in Oracle9i.
5. Restore the parallel_server parameter in INIT.ORA, if necessary.
6. Restart the database:
STARTUP;
7. If necessary, restore CLOB columns -- see "4. HANDLING CLOB AND NCLOB
COLUMNS" below.
To change the national character set replace the ALTER DATABASE CHARACTER SET
statement with ALTER DATABASE NATIONAL CHARACTER SET. You can issue both
statements together if you wish.
Error Conditions
----------------
A number of error conditions may be reported when trying to change the database
or national character set.
In Oracle8(i) the ALTER DATABASE [NATIONAL] CHARACTER SET statement will return:
ORA-01679: database must be mounted EXCLUSIVE and not open to activate
- if you do not enable restricted session
- if you startup the instance in PARALLEL/SHARED mode
- if you do not set the number of queue processes to 0
- if you do not set the number of AQ time manager processes to 0
- if anybody is logged in apart from you.
This error message is misleading. The command requires the database to be
open but only one session, the one executing the command, is allowed.
For the above error conditions Oracle9i will report one of the errors:
ORA-12719: operation requires database is in RESTRICTED mode
ORA-12720: operation requires database is in EXCLUSIVE mode
ORA-12721: operation cannot execute when other sessions are active
Oracle9i can also report:
ORA-12718: operation requires connection as SYS
if you are not connect as SYS (INTERNAL, "/ AS SYSDBA").
If the specified new character set name is not recognized, Oracle will report
one of the errors:
ORA-24329: invalid character set identifier
ORA-12714: invalid national character set specified
ORA-12715: invalid character set specified
The ALTER DATABASE [NATIONAL] CHARACTER SET command will only work if
the old character set is considered a binary subset of the new character set.
Oracle Server 8.0.3 to 8.1.5 recognizes US7ASCII as the binary subset of
all ASCII-based character sets. It also treats each character set as
a binary subset of itself. No other combinations are recognized.
Newer Oracle Server versions recognize additional subset/superset
combinations, which are listed in note:119164.1.
If the old character set is not recognized as a binary subset of
the new character set, the ALTER DATABASE [NATIONAL] CHARACTER SET
statement will return:
- in Oracle 8.1.5 and above:
ORA-12712: new character set must be a superset of old character set
- in Oracle 8.0.5 and 8.0.6:
ORA-12710: new character set must be a superset of old character set
- in Oracle 8.0.3 and 8.0.4:
ORA-24329: invalid character set identifier
You will also get these errors if you try to change the characterset of a
US7ASCII database that was started without a (correct) ORA_NLSxx parameter.
See Note:77442.1
It may be necessary to switch off the superset check to allow changes
between formally incompatible character sets to solve certain character set
problems or to speed up migration of huge databases. Oracle Support Services
may pass the necessary information to customers after verifying the safety
of the change for the customers' environments.
If in Oracle9i an ALTER DATABASE NATIONAL CHARACTER SET is issued and
there are N-type colums who contain data then this error is returned:
ORA-12717:Cannot ALTER DATABASE NATIONAL CHARACTER SET when NCLOB data exists
The error only speaks about Nclob but Nchar and Nvarchar2 are also checked.
4. HANDLING CLOB AND NCLOB COLUMNS
==================================
Background
----------
In a fixed width character set codes of all characters have the same
number of bytes. Fixed width character sets are: all single-byte
character sets and those multibyte character sets which have names
ending with 'FIXED'. In Oracle9i the character set AL16UTF16 is also
fixed width.
In a varying width character set codes of different characters may
have different number of bytes. All multibyte character sets except
those with names ending with FIXED (and except Oracle9i AL16UTF16
character set) are varying width.
Single-byte character sets are character sets with names of the form
xxx7yyyyyy and xxx8yyyyyy. Each character code of a single-byte
character set occupies exactly one byte. Multibyte character sets
are all other character sets (including UTF8). Some -- usually most
-- character codes of a multibyte character set occupy more than
one byte.
CLOB values in a database whose database character set is fixed width
are stored in this character set.
CLOB values in an Oracle 8.0.x database whose database character set
is varying width are not allowed. They have to be NULL.
CLOB values in an Oracle >= 8.1.5 database whose database character set
is varying width are stored in the fixed width Unicode UCS-2 encoding.
The same holds for NCLOB values and the national character set.
The UCS-2 storage format of character LOB values, as implemented
in Oracle8i, ensures that calculation of character positions in
LOB values is fast. Finding the byte offset of a character stored
in a varying width character set would require reading the whole LOB
value up to this character (possibly 4GB). In the fixed width
character sets the byte offsets are simply character offsets
multiplied by the number of bytes in a character code.
In UCS-2 byte offsets are simply twice the character offsets.
As the Unicode character set contains all characters defined in any other
Oracle character set, there is no data loss when a CLOB/NCLOB value
is converted to UCS-2 from the character set in which it was provided
by a client program (usually the NLS_LANG character set).
CLOB Values and the Database Character Set Change
-------------------------------------------------
In Oracle 8.0.x CLOB values are invalid in varying width character sets.
Thus you must delete all CLOB column values before changing the database
character set to a varying width character set.
In Oracle 8.1.5 and later CLOB values are valid in varying width
character sets but they are converted to Unicode UCS-2 before being
stored. But UCS-2 encoding is not a binary superset of any other
Oracle character set. Even codes of the basic ASCII characters are
different, e.g. single-byte code for "A"=0x41 becomes two-byte
code 0x0041. This implies that even if the new varying width
character set is a binary superset of the old fixed width
character set and thus VARCHAR2/LONG character codes remain valid,
the fixed width character codes in CLOB values will not longer
be valid in UCS-2.
As mentioned above, the ALTER DATABASE [NATIONAL] CHARACTER SET
statement does not change character codes. Thus, before changing
a fixed width database character set to a varying width character set
(like UTF8) in Oracle 8.1.5 or later, you first have to export
all tables containing non-NULL CLOB columns, then truncate these
tables, then change the database character set and, finally, import
the tables back to the database. The import step will perform
the required conversion.
If you omit the steps above, the character set change will succeed
in Oracle8(i) (Oracle9i disallows the change in such situation)
and the CLOBs may appear to be correctly legible but as their
encoding is incorrect, they will cause problems in further
operations. For example, CREATE TABLE AS SELECT will not correctly
copy such CLOB columns. Also, after installation of the 8.1.7.3
server patchset the CLOB columns will not longer be legible.
LONG columns are always stored in the database character set and
thus they behave like CHAR/VARCHAR2 in respect to the character set
change. BLOBs and BFILEs are binary raw datatypes and their
processing does not depend on any Oracle character set setting.
NCLOB Values and the National Character Set Change
--------------------------------------------------
The above discussion about changing the database character set
and exporting and importing CLOB values is theoretically applicable
to the change of the national character set and to NCLOB values.
But as varying width character sets are not supported as national
character sets in Oracle8(i), changing the national character set
from a fixed width character set to a varying width character set
is not supported at all.
Preparing CLOB Columns for the Character Set Change
---------------------------------------------------
Take a backup of the database.
If using Advanced Replication or deferred transactions functionality,
make sure that there are no outstanding deferred transactions with
CLOB parameters, i.e. DEFLOB view must have no rows with non-NULL
CLOB_COL column; to make sure that replication environment remains
consistent use only recommended methods of purging deferred
transaction queue, preferably quiescing the replication environment.
Then:
- If changing the database character set from a fixed width
character set to a varying with character set in Oracle 8.0.x,
set all CLOB column values to NULL -- you are not allowed
to use CLOB columns after the character set change.
- If changing the database character set from a fixed width
character set to a varying width character set in Oracle 8.1.5
or later, perform table-level export of all tables containing
CLOB columns, including SYSTEM's tables. Set NLS_LANG
to the old database character set for the Export utility.
Then truncate these tables.
Restoring CLOB Columns after the Character Set Change
-----------------------------------------------------
In Oracle 8.1.5 or later, after changing the character set as described
above (steps 3. to 6.), restore CLOB columns exported in step 2.
by importing them back into the database. Set NLS_LANG to the old
database character set for the Import utility to avoid IMP-16 errors
and data loss.
RELATED DOCUMENTS:
==================
Note: 62107.1 The National Character Set in Oracle8
Note:276914.1 The National Character Set in Oracle 9i and 10g
Note:119164.1 Changing Database Character set - Valid Superset definitions
Note:118242.1 ALERT: Changing the Database or National Character Set
Can Corrupt LOB Values
Note.158577.1 NLS_LANG Explained (How Does Client-Server Character Conversion Work?)
Note:140014.1 ALERT: Oracle8/8i to Oracle9i/10g using New "AL16UTF16"
Note:159657.1 Complete Upgrade Checklist for Manual Upgrades from 8.X / 9.0.1 to Oracle9i (incl. 9.2)
Note:124721.1 Migrating an Applications Installation to a New Character Set
Oracle8i National Language Support Guide
Oracle8i Release 3 (8.1.7) Readme
- Section 18.12 "Restricted ALTER DATABASE CHARACTER SET Command Support
(CLOB and NCLOB)"
Oracle8i Documentation Addendum, Release 3 (8.1.7)
- Chapter 3 "New Character Set Scanner Utility"
Oracle8i Application Developer's Guide - Large Objects (LOBs), Release 2
- Chapter 2 "Basic Components"
Oracle8 Application Developer's Guide, Release 8.0
- Chapter 6 "Large Objects (LOBs)", Section "Introduction to LOBs"
Oracle9i Globalization Guide, Release 1 (9.0.1)
Oracle9i Database Globalization Guide, Release 2 (9.2) |
|
发表于 2012-4-19 11:30:59
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