ORACLE外部表总结
外部表介绍
ORACLE外部表用来存取数据库以外的文本文件（Text File）或ORACLE专属格式文 件。因此，建立外部表时不会产生段、区、数据块等存储结构，只有与表相关的定义放在数据字典中。外部表，顾名思义，存储在数据库外面的表。当存取时才能从 ORACLE专属格式文件中取得数据，外部表仅供查询，不能对外部表的内容进行修改（INSERT、UPDATE、DELETE操作）。不能对外部表建立 索引。因为创建索引就意味着要存在对应的索引记录。而外部表其实在没有存储在数据库中。故在外部是无法建立索引的。如果硬要建立的话，则系统会提示“操作 在外部组织表上不受支持”的错误提示。
Notice: 外部表是ORACLE 9i后引入的。
外部表特征
（1） 位于文件系统之中（一定要在数据库服务器中，而不是其它网络路径），按一定格式分割，例如@#$等，文本文件或者其他类型的表可以作为外部表。
（2） 对外部表的访问可以通过SQL语句来完成，而不需要先将外部表中的数据装载进数据库中。
（3） 外部数据表都是只读的，因此在外部表不能够执行DML操作，也不能创建索引。
（4） ANALYZE语句不支持采集外部表的统计数据，应该使用DMBS_STATS包来采集外部表的统计数据。
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