Preface
 
    Today,I'm gonna say something what is related with the character set in Oracle database,the scenarios in changing character set from UTF8 to ZHS16GBK and the contra way as well.
    It' not recommended to modify character set in Oracle database because of the risk of lost data especially in changing superset to subset.Even though you can use "internal_use" option to change.
    You need to pay more attention in understanding some limitation in doing that.Data must be export with correct character set before you to really change it.Let's see the details below.
 
Environment
 
Hostname IP Linux Version Oracle Database SID
ogg 192.168.1.20 RHEL 5.7 11.2.0.1 ogg
 
Procedure
 
1. Set database character set from ZHS16GBK to UTF8.
 
Check the character set of database.
 SQL> set line  pages

 SQL> col value for a40

 SQL> select * from nls_database_parameters;

 PARAMETER               VALUE

 ------------------------------ ----------------------------------------

 NLS_LANGUAGE               AMERICAN

 NLS_TERRITORY               AMERICA

 NLS_CURRENCY               $

 NLS_ISO_CURRENCY           AMERICA

 NLS_NUMERIC_CHARACTERS           .,

 NLS_CHARACTERSET           ZHS16GBK

 NLS_CALENDAR               GREGORIAN

 NLS_DATE_FORMAT            DD-MON-RR

 NLS_DATE_LANGUAGE           AMERICAN

 NLS_SORT               BINARY

 NLS_TIME_FORMAT            HH.MI.SSXFF AM

 NLS_TIMESTAMP_FORMAT           DD-MON-RR HH.MI.SSXFF AM

 NLS_TIME_TZ_FORMAT           HH.MI.SSXFF AM TZR

 NLS_TIMESTAMP_TZ_FORMAT        DD-MON-RR HH.MI.SSXFF AM TZR

 NLS_DUAL_CURRENCY           $

 NLS_COMP               BINARY

 NLS_LENGTH_SEMANTICS           BYTE

 NLS_NCHAR_CONV_EXCP           FALSE

 NLS_NCHAR_CHARACTERSET           AL16UTF16

 NLS_RDBMS_VERSION           11.2.0.1.

  rows selected.

 SQL> select userenv('language') from dual;  

 USERENV('LANGUAGE')

 ----------------------------------------------------

 AMERICAN_AMERICA.ZHS16GBK
Create test user and test table.
 SQL> col file_name for a60

 SQL> col tablespace_name for a30

 SQL> select file_name,tablespace_name,bytes// from dba_data_files;

 FILE_NAME                             TABLESPACE_NAME            BYTES//

 ------------------------------------------------------------ ------------------------------ ---------------

 /u01/app/oracle/oradata/ogg/system01.dbf             SYSTEM

 /u01/app/oracle/oradata/ogg/sysaux01.dbf             SYSAUX

 /u01/app/oracle/oradata/ogg/undotbs01.dbf             UNDOTBS1

 /u01/app/oracle/oradata/ogg/zlm01.dbf                 ZLM                    

 SQL> create user zlm identified by zlm default tablespace zlm;

 User created.

 SQL> grant dba to zlm;

 Grant succeeded.

 SQL> conn zlm/zlm

 Connected.

 SQL> create table zlm.test_charset(

     id number,

     name varchar2(),

     primary key(id)

     );

 Table created.

 SQL> insert into zlm.test_charset(id,name) values(,'黎明');

  row created.

 SQL> commit;

 Commit complete.

 SQL> select * from test_charset;

     ID NAME

 ---------- ----------

       黎明

 SQL> select length('黎'),length('明') from dual;

 LENGTH('黎') LENGTH('明')

 ------------ ------------

 SQL> select lengthb('黎'),lengthb('明') from dual;

 LENGTHB('黎') LENGTHB('明')

 ------------- -------------

 //ZHS16GBK character set occupies 2 byte to store one Chinese character here.

 //Be aware of the difference of length() function between MySQL and Oracle.

 //In MySQL,length() returns bytes instead of characters.
Export the test table with expdp.
 SQL> select *from dba_directories;

 OWNER                   DIRECTORY_NAME

 ------------------------------ ------------------------------

 DIRECTORY_PATH

 --------------------------------------------------------------------------------

 SYS                   ORACLE_OCM_CONFIG_DIR

 /u01/app/oracle/product/11.2./db_1/ccr/state

 SYS                   DATA_PUMP_DIR

 /u01/app/oracle/admin/ogg/dpdump/

 SYS                   XMLDIR

 /u01/app/oracle/product/11.2./db_1/rdbms/xml

 SQL> create directory datapump as '/u01/';

 Directory created.

 SQL> grant read,write on directory datapump to zlm;

 grant read,write on directory datapump to zlm

                                           *

 ERROR at line :

 ORA-: you may not GRANT/REVOKE privileges to/from yourself

 SQL> conn / as sysdba

 Connected.

 SQL> grant read,write on directory datapump to zlm;

 Grant succeeded.

 SQL> exit

 Disconnected from Oracle Database 11g Enterprise Edition Release 11.2.0.1. - 64bit Production

 With the Partitioning option

 [oracle@ogg ~]$ expdp zlm/zlm directory=datapump dumpfile=zlm_datapump_`date +%Y%m%d`.dmp logfile=zlm_datapump_`date +%Y%m%d`.log schemas=zlm

 Export: Release 11.2.0.1. - Production on Wed Jul  :: 

 Copyright (c) , , Oracle and/or its affiliates.  All rights reserved.

 Connected to: Oracle Database 11g Enterprise Edition Release 11.2.0.1. - 64bit Production

 With the Partitioning option

 Starting "ZLM"."SYS_EXPORT_SCHEMA_01":  zlm/******** directory=datapump dumpfile=zlm_datapump_20180711.dmp logfile=zlm_datapump_20180711.log schemas=zlm

 Estimate in progress using BLOCKS method...

 Processing object type SCHEMA_EXPORT/TABLE/TABLE_DATA

 Total estimation using BLOCKS method: 64 KB

 Processing object type SCHEMA_EXPORT/USER

 Processing object type SCHEMA_EXPORT/SYSTEM_GRANT

 Processing object type SCHEMA_EXPORT/ROLE_GRANT

 Processing object type SCHEMA_EXPORT/DEFAULT_ROLE

 Processing object type SCHEMA_EXPORT/PRE_SCHEMA/PROCACT_SCHEMA

 Processing object type SCHEMA_EXPORT/TABLE/TABLE

 Processing object type SCHEMA_EXPORT/TABLE/INDEX/INDEX

 Processing object type SCHEMA_EXPORT/TABLE/CONSTRAINT/CONSTRAINT

 Processing object type SCHEMA_EXPORT/TABLE/INDEX/STATISTICS/INDEX_STATISTICS

 Processing object type SCHEMA_EXPORT/TABLE/COMMENT

 . . exported "ZLM"."TEST_CHARSET"                        5.414 KB       1 rows

 Master table "ZLM"."SYS_EXPORT_SCHEMA_01" successfully loaded/unloaded

 ******************************************************************************

 Dump file set for ZLM.SYS_EXPORT_SCHEMA_01 is:

   /u01/zlm_datapump_20180711.dmp

 Job "ZLM"."SYS_EXPORT_SCHEMA_01" successfully completed at 12:51:31

 [oracle@ogg ~]$ cd /u01

 [oracle@ogg u01]$ ls -l

 total 200

 drwxrwxr-x  4 oracle oinstall   4096 May 19  2017 app

 drwxrwxr-x  3 oracle oinstall   4096 Mar 29 15:40 arch

 drwxrwxr-x 14 oracle oinstall   4096 Feb  7 17:01 ogg

 -rw-r-----  1 oracle oinstall 184320 Jul 11 12:51 zlm_datapump_20180711.dmp

 -rw-r--r--  1 oracle oinstall   1500 Jul 11 12:51 zlm_datapump_20180711.log

 [oracle@ogg u01]$
Modify the character set into "UTF8".
 SQL> alter database character set utf8;

 alter database character set utf8

 *

 ERROR at line :

 ORA-: new character set must be a superset of old character set

 //Notice!This means that UTF8 is not the superset of ZHS16GBK.

 SQL> alter database character set internal_use utf8;

 alter database character set internal_use utf8

 *

 ERROR at line :

 ORA-: operation requires database is in RESTRICTED mode

 SQL> shutdown immediate

 ORA-: insufficient privileges

 SQL> conn / as sysdba

 Connected.

 SQL> shutdown immediate

 Database closed.

 Database dismounted.

 ORACLE instance shut down.

 SQL> startup mount exclusive;

 ORACLE instance started.

 Total System Global Area   bytes

 Fixed Size             bytes

 Variable Size           bytes

 Database Buffers       bytes

 Redo Buffers             bytes

 Database mounted.

 SQL> alter system enable restricted session;

 System altered.

 SQL> alter database open;                 

 Database altered.

 SQL> alter database character set internal_use utf8;

 alter database character set internal_use utf8

 *

 ERROR at line :

 ORA-: operation cannot execute when other sessions are active

 //Exit the other sessions who are active.

 SQL> alter database character set internal_use utf8;

 Database altered.

 SQL>
Check the new database character set.
 SQL> set line  pages

 SQL> col value for a40

 SQL> select * from nls_database_parameters;

 PARAMETER                             VALUE

 ------------------------------------------------------------ ----------------------------------------

 NLS_LANGUAGE                             AMERICAN

 NLS_TERRITORY                             AMERICA

 NLS_CURRENCY                             $

 NLS_ISO_CURRENCY                         AMERICA

 NLS_NUMERIC_CHARACTERS                         .,

 NLS_CHARACTERSET                         UTF8

 NLS_CALENDAR                             GREGORIAN

 NLS_DATE_FORMAT                          DD-MON-RR

 NLS_DATE_LANGUAGE                         AMERICAN

 NLS_SORT                             BINARY

 NLS_TIME_FORMAT                          HH.MI.SSXFF AM

 NLS_TIMESTAMP_FORMAT                         DD-MON-RR HH.MI.SSXFF AM

 NLS_TIME_TZ_FORMAT                         HH.MI.SSXFF AM TZR

 NLS_TIMESTAMP_TZ_FORMAT                      DD-MON-RR HH.MI.SSXFF AM TZR

 NLS_DUAL_CURRENCY                         $

 NLS_COMP                             BINARY

 NLS_LENGTH_SEMANTICS                         BYTE

 NLS_NCHAR_CONV_EXCP                         FALSE

 NLS_NCHAR_CHARACTERSET                         AL16UTF16

 NLS_RDBMS_VERSION                         11.2.0.1.

  rows selected.

 SQL> select userenv('language') from dual; 

 USERENV('LANGUAGE')

 --------------------------------------------------------------------------------------------------------

 AMERICAN_AMERICA.UTF8

 SQL>
Check the original data in test table.
 SQL> conn zlm/zlm

 Connected.

 SQL> select * from test_charset;

     ID NAME

 ---------- --------------------

       ?？

 //My client character set of Xshell is still GBK now,change it to utf8 and check again.

 SQL> select * from test_charset;

     ID NAME

 ---------- --------------------

       ?£¿

 //The NLS_LANG variable is still GBK，modify the oracle profile to correct the character set to UTF8.

 SQL> !

 [oracle@ogg u01]$ cat ~/.bash_profile|grep NLS_LANG

 export NLS_LANG=AMERICAN_AMERICA.ZHS16GBK

 [oracle@ogg u01]$ vi ~/.bash_profile

 [oracle@ogg u01]$ cat ~/.bash_profile|grep NLS_LANG

 export NLS_LANG=AMERICAN_AMERICA.UTF8

 [oracle@ogg u01]$ . ~/.bash_profile

 [oracle@ogg u01]$ echo $NLS_LANG

 AMERICAN_AMERICA.UTF8

 [oracle@ogg u01]$ sqlplus zlm/zlm

 SQL*Plus: Release 11.2.0.1. Production on Wed Jul  :: 

 Copyright (c) , , Oracle.  All rights reserved.

 Connected to:

 Oracle Database 11g Enterprise Edition Release 11.2.0.1. - 64bit Production

 With the Partitioning option

 SQL> select * from test_charset;                               

     ID NAME

 ---------- ----------

           hķ

 //It's still messy code here after i've unified the character set with server,client,NLS_LANG.
Drop test user and import it into database with the previous dump file.
 SQL> show user;

 USER is "ZLM"

 SQL> conn / as sysdba

 Connected.

 SQL> drop user zlm cascade;

 drop user zlm cascade

 *

 ERROR at line :

 ORA-: cannot drop a user that is currently connected

 SQL> select sid,serial#,status from v$session where username='ZLM';

        SID    SERIAL# STATUS

 ---------- ---------- --------

               INACTIVE

 SQL> alter system kill session '1,7';

 System altered.

 SQL> select sid,serial#,status from v$session where username='ZLM';

        SID    SERIAL# STATUS

 ---------- ---------- --------

               KILLED

 SQL> drop user zlm cascade;

 User dropped.

 SQL> alter user system identified by 'Passw0rd';

 alter user system identified by 'Passw0rd'

                                 *

 ERROR at line :

 ORA-: missing or invalid password(s)

 SQL> alter user system identified by Passw0rd;

 User altered.

 SQL> !

 [oracle@ogg u01]$ ls -l

 total

 drwxrwxr-x   oracle oinstall    May    app

 drwxrwxr-x   oracle oinstall    Jul  : arch

 drwxrwxr-x  oracle oinstall    Feb   : ogg

 -rw-r-----   oracle oinstall  Jul  : zlm_datapump_20180711.dmp

 -rw-r--r--   oracle oinstall    Jul  : zlm_datapump_20180711.log

 [oracle@ogg u01]$ impdp system/Passw0rd directory=datapump dumpfile=zlm_datapump_20180711.dmp

 Import: Release 11.2.0.1. - Production on Wed Jul  :: 

 Copyright (c) , , Oracle and/or its affiliates.  All rights reserved.

 Connected to: Oracle Database 11g Enterprise Edition Release 11.2.0.1. - 64bit Production

 With the Partitioning option

 Master table "SYSTEM"."SYS_IMPORT_FULL_01" successfully loaded/unloaded

 Starting "SYSTEM"."SYS_IMPORT_FULL_01":  system/******** directory=datapump dumpfile=zlm_datapump_20180711.dmp

 Processing object type SCHEMA_EXPORT/USER

 Processing object type SCHEMA_EXPORT/SYSTEM_GRANT

 Processing object type SCHEMA_EXPORT/ROLE_GRANT

 Processing object type SCHEMA_EXPORT/DEFAULT_ROLE

 Processing object type SCHEMA_EXPORT/PRE_SCHEMA/PROCACT_SCHEMA

 Processing object type SCHEMA_EXPORT/TABLE/TABLE

 Processing object type SCHEMA_EXPORT/TABLE/TABLE_DATA

 . . imported "ZLM"."TEST_CHARSET"                        5.414 KB       1 rows

 Processing object type SCHEMA_EXPORT/TABLE/CONSTRAINT/CONSTRAINT

 Job "SYSTEM"."SYS_IMPORT_FULL_01" successfully completed at 15:32:56

 [oracle@ogg u01]$
Check the data in user zlm again.
 [oracle@ogg u01]$ exit

 exit

 SQL> conn zlm/zlm

 ERROR:

 ORA-: your session has been killed

 Connected.

 SQL> exit

 Disconnected from Oracle Database 11g Enterprise Edition Release 11.2.0.1. - 64bit Production

 With the Partitioning option

 [oracle@ogg u01]$ sqlplus zlm/zlm

 SQL*Plus: Release 11.2.0.1. Production on Wed Jul  :: 

 Copyright (c) , , Oracle.  All rights reserved.

 Connected to:

 Oracle Database 11g Enterprise Edition Release 11.2.0.1. - 64bit Production

 With the Partitioning option

 SQL> select * from test_charset;

     ID NAME

 ---------- --------------------

       hķ

 SQL> select userenv('language') from dual; 

 USERENV('LANGUAGE')

 --------------------------------------------------------------------------------

 AMERICAN_AMERICA.UTF8

 SQL> exit

 Disconnected from Oracle Database 11g Enterprise Edition Release 11.2.0.1. - 64bit Production

 With the Partitioning option

 [oracle@ogg u01]$ echo $NLS_LANG

 AMERICAN_AMERICA.ZHS16GBK //The NLS_LANG is not equal with the character set in database.

 [oracle@ogg u01]$ sed -i 's/ZHS16GBK/UTF8/g' ~/.bash_profile

 [oracle@ogg u01]$ cat ~/.bash_profile|grep NLS_LANG

 export NLS_LANG=AMERICAN_AMERICA.UTF8

 [oracle@ogg u01]$ . ~/.bash_profile

 [oracle@ogg u01]$ echo $NLS_LANG

 AMERICAN_AMERICA.UTF8

 [oracle@ogg u01]$ sqlplus  zlm/zlm

 SQL*Plus: Release 11.2.0.1. Production on Wed Jul  :: 

 Copyright (c) , , Oracle.  All rights reserved.

 Connected to:

 Oracle Database 11g Enterprise Edition Release 11.2.0.1. - 64bit Production

 With the Partitioning option

 SQL> select * from test_charset;

     ID NAME

 ---------- ----------

           黎明

 SQL> select length('黎'),length('明') from dual;

 LENGTH('黎') LENGTH('明')

 ------------ ------------

 SQL> select lengthb('黎'),lengthb('明') from dual;

 LENGTHB('黎') LENGTHB('明')

 ------------- -------------

 SQL> 

 //The character set of database has been changed to UTF8 now and the data is either equal with the orgiginal data.
2. Set database character set from UTF8 to ZHS16GBK.
 
Insert a test record into test_charset table and shutdown instance to change character set again.
 SQL> insert into test_charset values(,'上海');

  row created.

 SQL> commit;

 Commit complete.

 SQL> select * from test_charset;

     ID NAME

 ---------- ----------

           黎明

           上海

 SQL> select length('上'),length('海') from dual;

 LENGTH('上') LENGTH('海')

 ------------ ------------

 SQL> select lengthb('上'),lengthb('海') from dual;

 LENGTHB('上') LENGTHB('海')

 ------------- -------------

 SQL> alter database character set ZHS16GBK;

 alter database character set ZHS16GBK

 *

 ERROR at line :

 ORA-: new character set must be a superset of old character set

 SQL> alter database character set internal_use ZHS16GBK;

 alter database character set internal_use ZHS16GBK

 *

 ERROR at line :

 ORA-: operation requires database is in RESTRICTED mode

 SQL> shutdown immediate

 Database closed.

 Database dismounted.

 ORACLE instance shut down.

 SQL> startup restrict

 ORACLE instance started.

 Total System Global Area   bytes

 Fixed Size             bytes

 Variable Size           bytes

 Database Buffers       bytes

 Redo Buffers             bytes

 Database mounted.

 Database opened.

 SQL> alter database character set internal_use ZHS16GBK;

 Database altered.

 SQL> select userenv('language') from dual;

 USERENV('LANGUAGE')

 --------------------------------------------------------------------------------

 AMERICAN_AMERICA.ZHS16GBK

 SQL>
Modify the character set of NLS_LANG togeter with that in client tool such as Xshell in my environment.
 SQL> !

 [oracle@ogg u01]$ echo $NLS_LANG

 AMERICAN_AMERICA.UTF8

 [oracle@ogg u01]$ sed -i 's/UTF8/ZHS16GBK/g' ~/.bash_profile

 [oracle@ogg u01]$ echo $NLS_LANG

 AMERICAN_AMERICA.UTF8

 [oracle@ogg u01]$ . ~/.bash_profile

 [oracle@ogg u01]$ echo $NLS_LANG

 AMERICAN_AMERICA.ZHS16GBK

 [oracle@ogg u01]$ exit

 exit

 SQL> select * from test_charset;

 select * from test_charset

               *

 ERROR at line :

 ORA-: table or view does not exist

 SQL> conn zlm/zlm

 Connected.

 SQL> select * from test_charset;

     ID NAME

 ---------- ------------------------------

           姒涘孩妲

           娑撳﹥鎹

 SQL> 

 //It's messy code now only if you've exported the data with datapump before you change the character set.Unfortunetely,I didn't to that this time.The characters in the test table test_charset has been destroyed.Can it come back again then?
Change it back to utf8 and check what will happen then?
 SQL> alter database character set utf8;

 alter database character set utf8

 *

 ERROR at line :

 ORA-: new character set must be a superset of old character set

 SQL> alter database character set internal_use utf8;

 alter database character set internal_use utf8

 *

 ERROR at line :

 ORA-: operation requires connection as SYS //It needs sys role privilege to finish the work.

 SQL> show user;

 USER is "ZLM"

 SQL> conn / as sysdba

 Connected.

 SQL> alter database character set internal_use utf8;

 Database altered.

 SQL> select * from zlm.test_charset;

     ID NAME

 ---------- ----------

           榛庢槑

           涓婃捣

 SQL> select userenv('language') from dual;

 USERENV('LANGUAGE')

 ----------------------------------------------------

 AMERICAN_AMERICA.UTF8 //Character set of database server is UTF8 now.

 SQL> !

 [oracle@ogg u01]$ echo $NLS_LANG

 AMERICAN_AMERICA.UTF8 //NLS_LANG is UTF8,too.

 [oracle@ogg u01]$ 

 //Character set of client tool has been changed to UTF8 as well.But the data cannot show the correct characters anymore.Have they been really destroyed?
Notice,only the user who has sys role privelge  can login instance after using "restrict" mode.See below.
 [oracle@ogg ~]$ sqlplus / as sysdba

 SQL*Plus: Release 11.2.0.1. Production on Wed Jul  :: 

 Copyright (c) , , Oracle.  All rights reserved.

 Connected to:

 Oracle Database 11g Enterprise Edition Release 11.2.0.1. - 64bit Production

 With the Partitioning option

 SQL> create user aaron8219 identified by aaron8219;  

 User created.

 SQL> grant session,connect to aaron8219;

 grant session,connect to aaron8219

       *

 ERROR at line :

 ORA-: role 'SESSION' does not exist

 SQL> grant connect,resource to aaron8219;

 Grant succeeded.

 SQL> conn aaron8219/aaron8219

 ERROR:

 ORA-: ORACLE only available to users with RESTRICTED SESSION privilege

 Warning: You are no longer connected to ORACLE.

 SQL>

Supplemented on July 12

 SQL> select userenv('language') from dual;

 USERENV('LANGUAGE')

 --------------------------------------------------------------------------------------------------------

 AMERICAN_AMERICA.UTF8

 SQL> select * from zlm.test_charset;

     ID NAME

 ---------- --------------------

       hķ

       ʏº£

 SQL> !

 [oracle@ogg ~]$ echo $NLS_LANG

 AMERICAN_AMERICA.ZHS16GBK

 [oracle@ogg ~]$ sed -i 's/ZHS16GBK/UTF8/g' .bash_profile

 [oracle@ogg ~]$ cat .bash_profile|grep NLS

 export NLS_LANG=AMERICAN_AMERICA.UTF8

 [oracle@ogg ~]$ . .bash_profile

 [oracle@ogg ~]$ sqlplus / as sysdba

 SQL*Plus: Release 11.2.0.1. Production on Thu Jul  :: 

 Copyright (c) , , Oracle.  All rights reserved.

 Connected to:

 Oracle Database 11g Enterprise Edition Release 11.2.0.1. - 64bit Production

 With the Partitioning option

 SQL> select * from zlm.test_charset;

     ID NAME

 ---------- ----------

           黎明

           上海

 SQL> select userenv('language') from dual;

 USERENV('LANGUAGE')

 ----------------------------------------------------

 AMERICAN_AMERICA.UTF8

 SQL> select lengthb('上'),lengthb('海') from dual;

 LENGTHB('上') LENGTHB('海')

 ------------- -------------

 //Change the character set of client tool to utf8,then the messy code came back again.

 SQL> select * from zlm.test_charset;  

     ID NAME

 ---------- ----------

           榛庢槑

           涓婃捣

 SQL>

 //Actually,the reason why messy code occurs yesterday is due to the discordance between the character set of database,NLS_LANG,client tool.

 //Although it proves that the data haven't been destroyed but there's a risk of mismatch of data if you change character set without backing up your whole data.
Summary
  • "internal_use" can help you to change character set without the constraint of subset to superset.
  • Only sys privilege users can login instance while database is running in restrict mode.
  • Be aware of ZHS16GBK is not subset of UTF8.It may baffle us most people.
  • It's not recommeded to change character set in oracle database which contains Chinese characters.
  • Don't forget to backup your data(logically) with datapump before you change the character set.
  • Make your  character set equal in client tool,NLS_LANG,db server anytime.This is the most important thing to keep in mind.
 

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