SQL> desc t_seq;####查看表结构
 Name                                      Null?    Type
 ----------------------------------------- -------- ----------------------------
 A                                                  CHAR(10)  ###注意是char,且为16个宽度
 B                                                  NUMBER(38)

#####用plsql批量插入100000要记录,利用序列为主键生成方式

SQL>   1  declare
  2  v_i int;
  3  begin
  4  for v_i in 1..100000 loop
  5  insert into t_seq values(seq_1.nextval,v_i);
  6  commit;
  7  end loop;
  8* end;

PL/SQL procedure successfully completed.

SQL> select segment_name,bytes,bytes/1024/1024 mb from user_segments where segment_name='T_SEQ';###10w记录占用空间为2m

SEGMENT_NAME
--------------------------------------------------------------------------------
     BYTES         MB
---------- ----------
T_SEQ
   3145728          3

SQL> alter table t_seq modify a char(5);######不能把数据类型从大变成小，只能从小变成大
alter table t_seq modify a char(5)
                         *
ERROR at line 1:
ORA-01441: cannot decrease column length because some value is too big

SQL> alter table t_seq modify a char(16);#####加大a列宽度，从10到16

Table altered.

SQL> select segment_name,bytes,bytes/1024/1024 mb from user_segments where segment_name='T_SEQ';#####这个表占用空间增大到4m，足足加了1m

SEGMENT_NAME
--------------------------------------------------------------------------------
     BYTES         MB
---------- ----------
T_SEQ
   4194304          4

小结：

   1,了解业务，选择适用表的列数据类型，科学节省存储空间

   2,学习oracle各种列（不同数据类型）所占用的空间的方法，结合业务，估算存储空间，为选用存储及分区作好准备