关于表空间自动扩展的功能一直比较模糊，这次有了一个比较清晰的认识。

1 自动扩展局限于数据文件大小；如maxsize 设置为100M，那么自动扩展达到100M之后，即使AUTOEXTEND ON也没有意义；

2 自动扩展是针对TABLESPACE的，不是针对DATABASE的；

3 NEXT 5M MAXSIZE 100M.表示TABLESPACE达到100%之后，tablespace自动扩展5M，再到100%时再扩展5M.直到100M为止。

SQL> conn / as sysdba;
已连接。

---查看tablespace的剩余空间
SQL> SELECT SUM(BYTES) FROM DBA_FREE_SPACE WHERE TABLESPACE_NAME='MELODY';

SUM(BYTES)                                                                     
----------                                                                     
   2031616      
----关闭tablespace的自动扩展功能                                                              

SQL> alter database datafile 'd:\melody.dbf' autoextend off;

数据库已更改。
---查看自动扩展功能是否打开

SQL> select tablespace_name,file_name,autoextensible from dba_data_files where tablespace_name = 'MELODY';

TABLESPACE_NAME                                                                
------------------------------                                                 
FILE_NAME                                                                      
--------------------------------------------------------------------------------
AUT                                                                            
---                                                                            
MELODY                                                                         
D:\MELODY.DBF                                                                  
NO                                                                             
-- 打开melody自动扩展的功能，扩展大小为5M,D:\MELODY.DBF文件大小最大为20M                                                                            

SQL> ALTER DATABASE DATAFILE 'D:\MELODY.DBF' AUTOEXTEND ON NEXT 5M MAXSIZE 20M;

数据库已更改。
--查看TABLESPACE自动功能是否打开
SQL> select tablespace_name,file_name,autoextensible from dba_data_files where tablespace_name = 'MELODY';

TABLESPACE_NAME                                                                
------------------------------                                                 
FILE_NAME                                                                      
--------------------------------------------------------------------------------
AUT                                                                            
---                                                                            
MELODY                                                                         
D:\MELODY.DBF                                                                  
YES