好处：
When a table is redefined online, it is accessible to both queries and DML during much of the redefinition process.
内部机制：
Typically, the table is locked in the exclusive mode only during
a very small window
that is independent of the size of the table and complexity of the redefinition, and that is completely transparent to users.
However, if there are many concurrent DML operations during redefinition, then a longer wait might be necessary before the table can be locked. Due to the wait, more changes are committed to the original table, which increases the refresh time and the amount of time that the table is locked.
使用前提：
Online table redefinition
requires an amount of free space that is approximately equivalent to the space used by the table being redefined.
More space may be required if new columns are added.
具体方法：
You can perform online table redefinition with the Enterprise Manager Reorganize Objects wizard or with the
DBMS_REDEFINITION
package.
