oracle 12c以前如果字符串长度超过4000，必须使用blob或者clob类型。12c开始支持超过4000的字符串长度，提高一些应用的灵活性，
达到32K，避免一些字段定义为clob，blob类型，提高处理能力。
但是12c默认的方式不支持大于4000的字符串长度，必须经过一些步骤升级完成，自己测试如下：
SYS@test> @ver
BANNER CON_ID
-------------------------------------------------------------------------------- ----------
Oracle Database 12c Enterprise Edition Release 12.1.0.1.0 - 64bit Production 0
SYS@test> show parameter max_string_size
NAME TYPE VALUE
---------------- ------- -----------
max_string_size string STANDARD
SYS@test> alter system set max_string_size='EXTENDED' ;
alter system set max_string_size='EXTENDED'
*
ERROR at line 1:
ORA-02097: parameter cannot be modified because specified value is invalid
ORA-02095: specified initialization parameter cannot be modified--不能在线修改这个参数。
SYS@test> alter system set max_string_size='EXTENDED' scope=spfile ;
System altered.
--修改spfile的参数文件。shutdown immediate，进入升级模式startup upgrade;，调用@?/rdbms/admin/utl32k.sql。
SYS@test> shutdown immediate
Database closed.
Database dismounted.
ORACLE instance shut down.
SYS@test> startup upgrade;
ORACLE instance started.
Total System Global Area 1670221824 bytes
Fixed Size 2403352 bytes
Variable Size 1006633960 bytes
Database Buffers 654311424 bytes
Redo Buffers 6873088 bytes
Database mounted.
Database opened.
SYS@test> @?/rdbms/admin/utl32k.sql...
--如果存在pdb库，还必须升级PDB$SEED，以及PDB数据库，不然会出现如下提示，在启动时。
ORA-01092: ORACLE instance terminated. Disconnection forced
ORA-14696: MAX_STRING_SIZE migration is incomplete for pluggable database PDB$SEED
Process ID: 1192
Session ID: 355 Serial number: 5
SYS@test> startup upgrade
ORACLE instance started.
Total System Global Area 1670221824 bytes
Fixed Size 2403352 bytes
Variable Size 1006633960 bytes
Database Buffers 654311424 bytes
Redo Buffers 6873088 bytes
Database mounted.
Database opened.
SYS@test> ALTER SESSION SET CONTAINER = PDB$SEED;
Session altered.
SYS@test> @?/rdbms/admin/utl32k.sql
...
SYS@test> ALTER SESSION SET CONTAINER = TEST01p;
Session altered.
SYS@test> alter pluggable database test01p open upgrade;
Pluggable database altered.
SYS@test> @?/rdbms/admin/utl32k.sql
...
--再重新启动数据库，OK。
SYS@test> startup
ORACLE instance started.
Total System Global Area 1670221824 bytes
Fixed Size 2403352 bytes
Variable Size 1023411176 bytes
Database Buffers 637534208 bytes
Redo Buffers 6873088 bytes
Database mounted.
Database opened.
SYS@test> alter pluggable database test01p open ;
Pluggable database altered.
==以scott用户test01p服务：
SCOTT@test01p> create table t1 (id number,text varchar2(32767));
Table created.
SCOTT@test01p> insert into t1 values (1,lpad('a',32767,'b'));
1 row created.
--OK，现在可以使用32K的字符串，实际上是32767的长度。
SCOTT@test01p> create table t2 (id number,text varchar2(32768));
create table t1 (id number,text varchar2(32768))
*
ERROR at line 1:
ORA-00910: specified length too long for its datatype