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.
1 SQL> set line 120 pages 9999 2 SQL> col value for a40 3 SQL> select * from nls_database_parameters; 4 5 PARAMETER VALUE 6 ------------------------------ ---------------------------------------- 7 NLS_LANGUAGE AMERICAN 8 NLS_TERRITORY AMERICA 9 NLS_CURRENCY $ 10 NLS_ISO_CURRENCY AMERICA 11 NLS_NUMERIC_CHARACTERS ., 12 NLS_CHARACTERSET ZHS16GBK 13 NLS_CALENDAR GREGORIAN 14 NLS_DATE_FORMAT DD-MON-RR 15 NLS_DATE_LANGUAGE AMERICAN 16 NLS_SORT BINARY 17 NLS_TIME_FORMAT HH.MI.SSXFF AM 18 NLS_TIMESTAMP_FORMAT DD-MON-RR HH.MI.SSXFF AM 19 NLS_TIME_TZ_FORMAT HH.MI.SSXFF AM TZR 20 NLS_TIMESTAMP_TZ_FORMAT DD-MON-RR HH.MI.SSXFF AM TZR 21 NLS_DUAL_CURRENCY $ 22 NLS_COMP BINARY 23 NLS_LENGTH_SEMANTICS BYTE 24 NLS_NCHAR_CONV_EXCP FALSE 25 NLS_NCHAR_CHARACTERSET AL16UTF16 26 NLS_RDBMS_VERSION 11.2.0.1.0 27 28 20 rows selected. 29 30 SQL> select userenv('language') from dual; 31 32 USERENV('LANGUAGE') 33 ---------------------------------------------------- 34 AMERICAN_AMERICA.ZHS16GBK
Create test user and test table.
1 SQL> col file_name for a60 2 SQL> col tablespace_name for a30 3 SQL> select file_name,tablespace_name,bytes/1024/1024 from dba_data_files; 4 5 FILE_NAME TABLESPACE_NAME BYTES/1024/1024 6 ------------------------------------------------------------ ------------------------------ --------------- 7 /u01/app/oracle/oradata/ogg/system01.dbf SYSTEM 700 8 /u01/app/oracle/oradata/ogg/sysaux01.dbf SYSAUX 600 9 /u01/app/oracle/oradata/ogg/undotbs01.dbf UNDOTBS1 325 10 /u01/app/oracle/oradata/ogg/zlm01.dbf ZLM 100 11 12 SQL> create user zlm identified by zlm default tablespace zlm; 13 14 User created. 15 16 SQL> grant dba to zlm; 17 18 Grant succeeded. 19 SQL> conn zlm/zlm 20 Connected. 21 SQL> create table zlm.test_charset( 22 2 id number, 23 3 name varchar2(10), 24 4 primary key(id) 25 5 ); 26 27 Table created. 28 29 SQL> insert into zlm.test_charset(id,name) values(1,'黎明'); 30 31 1 row created. 32 33 SQL> commit; 34 35 Commit complete. 36 37 SQL> select * from test_charset; 38 39 ID NAME 40 ---------- ---------- 41 1 黎明 42 43 SQL> select length('黎'),length('明') from dual; 44 45 LENGTH('黎') LENGTH('明') 46 ------------ ------------ 47 1 1 48 49 SQL> select lengthb('黎'),lengthb('明') from dual; 50 51 LENGTHB('黎') LENGTHB('明') 52 ------------- ------------- 53 2 2 54 55 //ZHS16GBK character set occupies 2 byte to store one Chinese character here. 56 //Be aware of the difference of length() function between MySQL and Oracle. 57 //In MySQL,length() returns bytes instead of characters.
Export the test table with expdp.
1 SQL> select *from dba_directories; 2 3 OWNER DIRECTORY_NAME 4 ------------------------------ ------------------------------ 5 DIRECTORY_PATH 6 -------------------------------------------------------------------------------- 7 SYS ORACLE_OCM_CONFIG_DIR 8 /u01/app/oracle/product/11.2.0/db_1/ccr/state 9 10 SYS DATA_PUMP_DIR 11 /u01/app/oracle/admin/ogg/dpdump/ 12 13 SYS XMLDIR 14 /u01/app/oracle/product/11.2.0/db_1/rdbms/xml 15 16 17 SQL> create directory datapump as '/u01/'; 18 19 Directory created. 20 21 SQL> grant read,write on directory datapump to zlm; 22 grant read,write on directory datapump to zlm 23 * 24 ERROR at line 1: 25 ORA-01749: you may not GRANT/REVOKE privileges to/from yourself 26 27 28 SQL> conn / as sysdba 29 Connected. 30 SQL> grant read,write on directory datapump to zlm; 31 32 Grant succeeded. 33 34 SQL> exit 35 Disconnected from Oracle Database 11g Enterprise Edition Release 11.2.0.1.0 - 64bit Production 36 With the Partitioning option 37 [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 38 39 Export: Release 11.2.0.1.0 - Production on Wed Jul 11 12:51:06 2018 40 41 Copyright (c) 1982, 2009, Oracle and/or its affiliates. All rights reserved. 42 43 Connected to: Oracle Database 11g Enterprise Edition Release 11.2.0.1.0 - 64bit Production 44 With the Partitioning option 45 Starting "ZLM"."SYS_EXPORT_SCHEMA_01": zlm/******** directory=datapump dumpfile=zlm_datapump_20180711.dmp logfile=zlm_datapump_20180711.log schemas=zlm 46 Estimate in progress using BLOCKS method... 47 Processing object type SCHEMA_EXPORT/TABLE/TABLE_DATA 48 Total estimation using BLOCKS method: 64 KB 49 Processing object type SCHEMA_EXPORT/USER 50 Processing object type SCHEMA_EXPORT/SYSTEM_GRANT 51 Processing object type SCHEMA_EXPORT/ROLE_GRANT 52 Processing object type SCHEMA_EXPORT/DEFAULT_ROLE 53 Processing object type SCHEMA_EXPORT/PRE_SCHEMA/PROCACT_SCHEMA 54 Processing object type SCHEMA_EXPORT/TABLE/TABLE 55 Processing object type SCHEMA_EXPORT/TABLE/INDEX/INDEX 56 Processing object type SCHEMA_EXPORT/TABLE/CONSTRAINT/CONSTRAINT 57 Processing object type SCHEMA_EXPORT/TABLE/INDEX/STATISTICS/INDEX_STATISTICS 58 Processing object type SCHEMA_EXPORT/TABLE/COMMENT 59 . . exported "ZLM"."TEST_CHARSET" 5.414 KB 1 rows 60 Master table "ZLM"."SYS_EXPORT_SCHEMA_01" successfully loaded/unloaded 61 ****************************************************************************** 62 Dump file set for ZLM.SYS_EXPORT_SCHEMA_01 is: 63 /u01/zlm_datapump_20180711.dmp 64 Job "ZLM"."SYS_EXPORT_SCHEMA_01" successfully completed at 12:51:31 65 66 [oracle@ogg ~]$ cd /u01 67 [oracle@ogg u01]$ ls -l 68 total 200 69 drwxrwxr-x 4 oracle oinstall 4096 May 19 2017 app 70 drwxrwxr-x 3 oracle oinstall 4096 Mar 29 15:40 arch 71 drwxrwxr-x 14 oracle oinstall 4096 Feb 7 17:01 ogg 72 -rw-r----- 1 oracle oinstall 184320 Jul 11 12:51 zlm_datapump_20180711.dmp 73 -rw-r--r-- 1 oracle oinstall 1500 Jul 11 12:51 zlm_datapump_20180711.log 74 [oracle@ogg u01]$
Modify the character set into "UTF8".
1 SQL> alter database character set utf8; 2 alter database character set utf8 3 * 4 ERROR at line 1: 5 ORA-12712: new character set must be a superset of old character set 6 7 //Notice!This means that UTF8 is not the superset of ZHS16GBK. 8 9 SQL> alter database character set internal_use utf8; 10 alter database character set internal_use utf8 11 * 12 ERROR at line 1: 13 ORA-12719: operation requires database is in RESTRICTED mode 14 15 16 SQL> shutdown immediate 17 ORA-01031: insufficient privileges 18 SQL> conn / as sysdba 19 Connected. 20 SQL> shutdown immediate 21 Database closed. 22 Database dismounted. 23 ORACLE instance shut down. 24 SQL> startup mount exclusive; 25 ORACLE instance started. 26 27 Total System Global Area 835104768 bytes 28 Fixed Size 2217952 bytes 29 Variable Size 562038816 bytes 30 Database Buffers 268435456 bytes 31 Redo Buffers 2412544 bytes 32 Database mounted. 33 SQL> alter system enable restricted session; 34 35 System altered. 36 37 SQL> alter database open; 38 39 Database altered. 40 41 SQL> alter database character set internal_use utf8; 42 alter database character set internal_use utf8 43 * 44 ERROR at line 1: 45 ORA-12721: operation cannot execute when other sessions are active 46 47 //Exit the other sessions who is active. 48 49 SQL> alter database character set internal_use utf8; 50 51 Database altered. 52 53 SQL>
Check the new database character set.
1 SQL> set line 120 pages 9999 2 SQL> col value for a40 3 SQL> select * from nls_database_parameters; 4 5 PARAMETER VALUE 6 ------------------------------------------------------------ ---------------------------------------- 7 NLS_LANGUAGE AMERICAN 8 NLS_TERRITORY AMERICA 9 NLS_CURRENCY $ 10 NLS_ISO_CURRENCY AMERICA 11 NLS_NUMERIC_CHARACTERS ., 12 NLS_CHARACTERSET UTF8 13 NLS_CALENDAR GREGORIAN 14 NLS_DATE_FORMAT DD-MON-RR 15 NLS_DATE_LANGUAGE AMERICAN 16 NLS_SORT BINARY 17 NLS_TIME_FORMAT HH.MI.SSXFF AM 18 NLS_TIMESTAMP_FORMAT DD-MON-RR HH.MI.SSXFF AM 19 NLS_TIME_TZ_FORMAT HH.MI.SSXFF AM TZR 20 NLS_TIMESTAMP_TZ_FORMAT DD-MON-RR HH.MI.SSXFF AM TZR 21 NLS_DUAL_CURRENCY $ 22 NLS_COMP BINARY 23 NLS_LENGTH_SEMANTICS BYTE 24 NLS_NCHAR_CONV_EXCP FALSE 25 NLS_NCHAR_CHARACTERSET AL16UTF16 26 NLS_RDBMS_VERSION 11.2.0.1.0 27 28 20 rows selected. 29 30 SQL> select userenv('language') from dual; 31 32 USERENV('LANGUAGE') 33 -------------------------------------------------------------------------------------------------------- 34 AMERICAN_AMERICA.UTF8 35 36 SQL>
Check the original data in test table.
1 SQL> conn zlm/zlm 2 Connected. 3 SQL> select * from test_charset; 4 5 ID NAME 6 ---------- -------------------- 7 1 ?？ 8 9 //My client character set of Xshell is still GBK now,change it to utf8 and check again. 10 11 SQL> select * from test_charset; 12 13 ID NAME 14 ---------- -------------------- 15 1 ?£¿ 16 17 //The NLS_LANG variable is still GBK，modify the oracle profile to correct the character set to UTF8. 18 19 SQL> ! 20 [oracle@ogg u01]$ cat ~/.bash_profile|grep NLS_LANG 21 export NLS_LANG=AMERICAN_AMERICA.ZHS16GBK 22 [oracle@ogg u01]$ vi ~/.bash_profile 23 [oracle@ogg u01]$ cat ~/.bash_profile|grep NLS_LANG 24 export NLS_LANG=AMERICAN_AMERICA.UTF8 25 [oracle@ogg u01]$ . ~/.bash_profile 26 [oracle@ogg u01]$ echo $NLS_LANG 27 AMERICAN_AMERICA.UTF8 28 [oracle@ogg u01]$ sqlplus zlm/zlm 29 30 SQL*Plus: Release 11.2.0.1.0 Production on Wed Jul 11 15:11:55 2018 31 32 Copyright (c) 1982, 2009, Oracle. All rights reserved. 33 34 35 Connected to: 36 Oracle Database 11g Enterprise Edition Release 11.2.0.1.0 - 64bit Production 37 With the Partitioning option 38 39 SQL> select * from test_charset; 40 41 ID NAME 42 ---------- ---------- 43 1 hķ 44 45 //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.
1 SQL> show user; 2 USER is "ZLM" 3 SQL> conn / as sysdba 4 Connected. 5 SQL> drop user zlm cascade; 6 drop user zlm cascade 7 * 8 ERROR at line 1: 9 ORA-01940: cannot drop a user that is currently connected 10 11 12 SQL> select sid,serial#,status from v$session where username='ZLM'; 13 14 SID SERIAL# STATUS 15 ---------- ---------- -------- 16 1 7 INACTIVE 17 18 SQL> alter system kill session '1,7'; 19 20 System altered. 21 22 SQL> select sid,serial#,status from v$session where username='ZLM'; 23 24 SID SERIAL# STATUS 25 ---------- ---------- -------- 26 1 7 KILLED 27 28 SQL> drop user zlm cascade; 29 30 User dropped. 31 32 SQL> alter user system identified by 'Passw0rd'; 33 alter user system identified by 'Passw0rd' 34 * 35 ERROR at line 1: 36 ORA-00988: missing or invalid password(s) 37 38 39 SQL> alter user system identified by Passw0rd; 40 41 User altered. 42 43 SQL> ! 44 [oracle@ogg u01]$ ls -l 45 total 200 46 drwxrwxr-x 4 oracle oinstall 4096 May 19 2017 app 47 drwxrwxr-x 3 oracle oinstall 4096 Jul 11 13:00 arch 48 drwxrwxr-x 14 oracle oinstall 4096 Feb 7 17:01 ogg 49 -rw-r----- 1 oracle oinstall 184320 Jul 11 12:51 zlm_datapump_20180711.dmp 50 -rw-r--r-- 1 oracle oinstall 1500 Jul 11 12:51 zlm_datapump_20180711.log 51 [oracle@ogg u01]$ impdp system/Passw0rd directory=datapump dumpfile=zlm_datapump_20180711.dmp 52 53 Import: Release 11.2.0.1.0 - Production on Wed Jul 11 15:32:53 2018 54 55 Copyright (c) 1982, 2009, Oracle and/or its affiliates. All rights reserved. 56 57 Connected to: Oracle Database 11g Enterprise Edition Release 11.2.0.1.0 - 64bit Production 58 With the Partitioning option 59 Master table "SYSTEM"."SYS_IMPORT_FULL_01" successfully loaded/unloaded 60 Starting "SYSTEM"."SYS_IMPORT_FULL_01": system/******** directory=datapump dumpfile=zlm_datapump_20180711.dmp 61 Processing object type SCHEMA_EXPORT/USER 62 Processing object type SCHEMA_EXPORT/SYSTEM_GRANT 63 Processing object type SCHEMA_EXPORT/ROLE_GRANT 64 Processing object type SCHEMA_EXPORT/DEFAULT_ROLE 65 Processing object type SCHEMA_EXPORT/PRE_SCHEMA/PROCACT_SCHEMA 66 Processing object type SCHEMA_EXPORT/TABLE/TABLE 67 Processing object type SCHEMA_EXPORT/TABLE/TABLE_DATA 68 . . imported "ZLM"."TEST_CHARSET" 5.414 KB 1 rows 69 Processing object type SCHEMA_EXPORT/TABLE/CONSTRAINT/CONSTRAINT 70 Job "SYSTEM"."SYS_IMPORT_FULL_01" successfully completed at 15:32:56 71 72 [oracle@ogg u01]$
Check the data in user zlm again.
1 [oracle@ogg u01]$ exit 2 exit 3 4 SQL> conn zlm/zlm 5 ERROR: 6 ORA-00028: your session has been killed 7 8 9 Connected. 10 SQL> exit 11 Disconnected from Oracle Database 11g Enterprise Edition Release 11.2.0.1.0 - 64bit Production 12 With the Partitioning option 13 [oracle@ogg u01]$ sqlplus zlm/zlm 14 15 SQL*Plus: Release 11.2.0.1.0 Production on Wed Jul 11 15:33:48 2018 16 17 Copyright (c) 1982, 2009, Oracle. All rights reserved. 18 19 20 Connected to: 21 Oracle Database 11g Enterprise Edition Release 11.2.0.1.0 - 64bit Production 22 With the Partitioning option 23 24 SQL> select * from test_charset; 25 26 ID NAME 27 ---------- -------------------- 28 1 hķ 29 30 SQL> select userenv('language') from dual; 31 32 USERENV('LANGUAGE') 33 -------------------------------------------------------------------------------- 34 AMERICAN_AMERICA.UTF8 35 36 SQL> exit 37 Disconnected from Oracle Database 11g Enterprise Edition Release 11.2.0.1.0 - 64bit Production 38 With the Partitioning option 39 [oracle@ogg u01]$ echo $NLS_LANG 40 AMERICAN_AMERICA.ZHS16GBK //The NLS_LANG is not equal with the character set in database. 41 [oracle@ogg u01]$ sed -i 's/ZHS16GBK/UTF8/g' ~/.bash_profile 42 [oracle@ogg u01]$ cat ~/.bash_profile|grep NLS_LANG 43 export NLS_LANG=AMERICAN_AMERICA.UTF8 44 [oracle@ogg u01]$ . ~/.bash_profile 45 [oracle@ogg u01]$ echo $NLS_LANG 46 AMERICAN_AMERICA.UTF8 47 [oracle@ogg u01]$ sqlplus zlm/zlm 48 49 SQL*Plus: Release 11.2.0.1.0 Production on Wed Jul 11 15:53:12 2018 50 51 Copyright (c) 1982, 2009, Oracle. All rights reserved. 52 53 54 Connected to: 55 Oracle Database 11g Enterprise Edition Release 11.2.0.1.0 - 64bit Production 56 With the Partitioning option 57 58 SQL> select * from test_charset; 59 60 ID NAME 61 ---------- ---------- 62 1 黎明 63 64 SQL> select length('黎'),length('明') from dual; 65 66 LENGTH('黎') LENGTH('明') 67 ------------ ------------ 68 1 1 69 70 SQL> select lengthb('黎'),lengthb('明') from dual; 71 72 LENGTHB('黎') LENGTHB('明') 73 ------------- ------------- 74 3 3 75 76 SQL> 77 78 //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.
1 SQL> insert into test_charset values(2,'上海'); 2 3 1 row created. 4 5 SQL> commit; 6 7 Commit complete. 8 9 SQL> select * from test_charset; 10 11 ID NAME 12 ---------- ---------- 13 1 黎明 14 2 上海 15 16 SQL> select length('上'),length('海') from dual; 17 18 LENGTH('上') LENGTH('海') 19 ------------ ------------ 20 1 1 21 22 SQL> select lengthb('上'),lengthb('海') from dual; 23 24 LENGTHB('上') LENGTHB('海') 25 ------------- ------------- 26 3 3 27 28 SQL> alter database character set ZHS16GBK; 29 alter database character set ZHS16GBK 30 * 31 ERROR at line 1: 32 ORA-12712: new character set must be a superset of old character set 33 34 35 SQL> alter database character set internal_use ZHS16GBK; 36 alter database character set internal_use ZHS16GBK 37 * 38 ERROR at line 1: 39 ORA-12719: operation requires database is in RESTRICTED mode 40 41 42 SQL> shutdown immediate 43 Database closed. 44 Database dismounted. 45 ORACLE instance shut down. 46 SQL> startup restrict 47 ORACLE instance started. 48 49 Total System Global Area 835104768 bytes 50 Fixed Size 2217952 bytes 51 Variable Size 562038816 bytes 52 Database Buffers 268435456 bytes 53 Redo Buffers 2412544 bytes 54 Database mounted. 55 Database opened. 56 SQL> alter database character set internal_use ZHS16GBK; 57 58 Database altered. 59 60 SQL> select userenv('language') from dual; 61 62 USERENV('LANGUAGE') 63 -------------------------------------------------------------------------------- 64 AMERICAN_AMERICA.ZHS16GBK 65 66 SQL>
Modify the character set of NLS_LANG togeter with that in client tool such as Xshell in my environment.
1 SQL> ! 2 [oracle@ogg u01]$ echo $NLS_LANG 3 AMERICAN_AMERICA.UTF8 4 [oracle@ogg u01]$ sed -i 's/UTF8/ZHS16GBK/g' ~/.bash_profile 5 [oracle@ogg u01]$ echo $NLS_LANG 6 AMERICAN_AMERICA.UTF8 7 [oracle@ogg u01]$ . ~/.bash_profile 8 [oracle@ogg u01]$ echo $NLS_LANG 9 AMERICAN_AMERICA.ZHS16GBK 10 [oracle@ogg u01]$ exit 11 exit 12 13 SQL> select * from test_charset; 14 select * from test_charset 15 * 16 ERROR at line 1: 17 ORA-00942: table or view does not exist 18 19 20 SQL> conn zlm/zlm 21 Connected. 22 SQL> select * from test_charset; 23 24 ID NAME 25 ---------- ------------------------------ 26 1 姒涘孩妲 27 2 娑撳﹥鎹 28 29 SQL> 30 31 //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?
1 SQL> alter database character set utf8; 2 alter database character set utf8 3 * 4 ERROR at line 1: 5 ORA-12712: new character set must be a superset of old character set 6 7 8 SQL> alter database character set internal_use utf8; 9 alter database character set internal_use utf8 10 * 11 ERROR at line 1: 12 ORA-12718: operation requires connection as SYS //It needs sys role privilege to finish the work. 13 14 15 SQL> show user; 16 USER is "ZLM" 17 SQL> conn / as sysdba 18 Connected. 19 SQL> alter database character set internal_use utf8; 20 21 Database altered. 22 23 SQL> select * from zlm.test_charset; 24 25 ID NAME 26 ---------- ---------- 27 1 榛庢槑 28 2 涓婃捣 29 30 SQL> select userenv('language') from dual; 31 32 USERENV('LANGUAGE') 33 ---------------------------------------------------- 34 AMERICAN_AMERICA.UTF8 //Character set of database server has been UTF8 35 36 SQL> ! 37 [oracle@ogg u01]$ echo $NLS_LANG 38 AMERICAN_AMERICA.UTF8 //NLS_LANG has been UTF8. 39 [oracle@ogg u01]$ 40 41 //Character set of client tool has been changed to UTF8 as well.But the data cannot show the correct characters anymore.Yes,they've really destroyed.
Notice,only the user who has sys role privelge can login instance after using "restrict" mode.See below.
1 [oracle@ogg ~]$ sqlplus / as sysdba 2 3 SQL*Plus: Release 11.2.0.1.0 Production on Wed Jul 11 17:02:30 2018 4 5 Copyright (c) 1982, 2009, Oracle. All rights reserved. 6 7 8 Connected to: 9 Oracle Database 11g Enterprise Edition Release 11.2.0.1.0 - 64bit Production 10 With the Partitioning option 11 12 SQL> create user aaron8219 identified by aaron8219; 13 14 User created. 15 16 SQL> grant session,connect to aaron8219; 17 grant session,connect to aaron8219 18 * 19 ERROR at line 1: 20 ORA-01919: role 'SESSION' does not exist 21 22 23 SQL> grant connect,resource to aaron8219; 24 25 Grant succeeded. 26 27 SQL> conn aaron8219/aaron8219 28 ERROR: 29 ORA-01035: ORACLE only available to users with RESTRICTED SESSION privilege 30 31 32 Warning: You are no longer connected to ORACLE. 33 SQL>
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.