Oracle的ALTER TABLE语句其实可以直接修改：

SQL> CREATE TABLE T
  2  (ID NUMBER, 
  3  CONTENTS LONG);

Table created.

SQL> INSERT INTO T
  2  SELECT ROWNUM, OWNER || OBJECT_NAME || OBJECT_TYPE
  3  FROM ALL_OBJECTS;

75327 rows created.

SQL> COMMIT;

Commit complete.

SQL> ALTER TABLE T MODIFY CONTENTS CLOB;

Table altered.

看上去似乎很简单，不过Oracle并不是简单的将列的定义换成CLOB，而是生成了一个临时列，将数据保存，然后删除原LONG列，其实操作类似上一篇文章最后给出的步骤，不过Oracle进行了封装，因此效率更高，不需要通过关联来实现了。

SQL> DROP TABLE T PURGE;

Table dropped.

SQL> CREATE TABLE T
  2  (ID NUMBER, 
  3  CONTENTS LONG);

Table created.

SQL> INSERT INTO T
  2  SELECT ROWNUM, OWNER || OBJECT_NAME || OBJECT_TYPE
  3  FROM ALL_OBJECTS;

75327 rows created.

SQL> COMMIT;

Commit complete.

SQL> SET TIMING ON
SQL> ALTER TABLE T MODIFY CONTENTS CLOB;

Table altered.

Elapsed: 00:00:00.65
SQL> SET TIMING OFF
SQL> DROP TABLE T PURGE;

Table dropped.

SQL> CREATE TABLE T
  2  (ID NUMBER, 
  3  CONTENTS LONG);

Table created.

SQL> INSERT INTO T
  2  SELECT ROWNUM, OWNER || OBJECT_NAME || OBJECT_TYPE
  3  FROM ALL_OBJECTS, USER_DB_LINKS;

527289 rows created.

SQL> COMMIT;

Commit complete.

SQL> SET TIMING ON
SQL> ALTER TABLE T MODIFY CONTENTS CLOB;

Table altered.

Elapsed: 00:00:04.49
SQL> SET TIMING OFF
SQL> DROP TABLE T PURGE;

Table dropped.

SQL> CREATE TABLE T
  2  (ID NUMBER, 
  3  CONTENTS LONG);

Table created.

SQL> INSERT INTO T
  2  SELECT ROWNUM, OWNER || OBJECT_NAME || OBJECT_TYPE
  3  FROM ALL_OBJECTS, USER_DB_LINKS, USER_DB_LINKS;

3691023 rows created.

SQL> COMMIT;

Commit complete.

SQL> SET TIMING ON
SQL> ALTER TABLE T MODIFY CONTENTS CLOB;

Table altered.

Elapsed: 00:00:37.10

可以看到，随着数据量的增加，ALTER TABLE所花费的时间也线性增长，说明这个操作并不是简单的修改数据字典，而是对数据进行转换。

不过这个操作的效率比第一种方法要高不少，对于7*24要求不是非常高的绝大多数系统都是满足要求的，毕竟一个SQL就可以完成转换还是非常方便的。