源库ZHS16BGK，汉字在数据库存放的时候占用两个字节
目标库UTF8，汉字在数据库里存放的时候占用三个字节
由于字符集不同，导致现在数据库impdp的时候有些表的字段长度不够，出现ORA-12899: value too large for column的错误。
解决办法：更改数据库字符集为GBK
SHUTDOWN IMMEDIATE;
STARTUP MOUNT EXCLUSIVE;
ALTER SYSTEM ENABLE RESTRICTED SESSION;
ALTER SYSTEM SET JOB_QUEUE_PROCESSES=0;
ALTER SYSTEM SET AQ_TM_PROCESSES=0;
ALTER DATABASE OPEN;
ALTER DATABASE NATIONAL CHARACTER SET AL16UTF16; 更改国际字符集
STARTUP MOUNT EXCLUSIVE;
ALTER SYSTEM ENABLE RESTRICTED SESSION;
ALTER SYSTEM SET JOB_QUEUE_PROCESSES=0;
ALTER SYSTEM SET AQ_TM_PROCESSES=0;
ALTER DATABASE OPEN;
ALTER DATABASE NATIONAL CHARACTER SET AL16UTF16; 更改国际字符集
ALTER DATABASE CHARACTER SET ZHS16GBK; 更改本地字符集
SHUTDOWN IMMEDIATE;
STARTUP ;
如果错误信息，请对照一下信息
如果提示：
ORA-12717: Cannot issue ALTER DATABASE NATIONAL CHARACTER SET when NCLOB, NCHAR
or NVARCHAR2 data exists
or NVARCHAR2 data exists
则更改使用以下语句:
ALTER DATABASE NATIONAL CHARACTER SET INTERNAL_USE AL16UTF16;
如果提示：
ORA-12712: new character set must be a superset of old character set
则更改使用以下语句:
ALTER DATABASE character set INTERNAL_USE ZHS16GBK;
如果提示：
ORA-12721: operation cannot execute when other sessions are active
则更改使用以下语句:
SHUTDOWN IMMEDIATE;
STARTUP MOUNT EXCLUSIVE;
STARTUP MOUNT EXCLUSIVE;
如果提示：
ORA-01109: database not open
则更改使用以下语句:
ALTER DATABASE OPEN;
如果提示：
ORA-12719: operation requires database is in RESTRICTED mode
则更改使用以下语句:
ALTER SYSTEM ENABLE RESTRICTED SESSION;