External tables access data in external sources as if it were in a table in the database.
You can connect to the database and create metadata for the external table using DDL.
The DDL for an external table consists of two parts: one part that describes the Oracle
column types, and another part (the access parameters) that describes the mapping of
the external data to the Oracle data columns.

An external table does not describe any data that is stored in the database. Nor does it
describe how data is stored in the external source. Instead, it describes how the
external table layer needs to present the data to the server. It is the responsibility of the
access driver and the external table layer to do the necessary transformations required
on the data in the datafile so that it matches the external table definition.

External tables are read only; therefore, no DML operations are possible, and no index
can be created on them.

外部表
1. 用户可以使用外部表,访问位于数据库外部的数据源的数据.用户连接到数据库后可以使用 DDL 语句创建外部表的元数据
这种 DDL 语句由两部分构成：一部分描述外部表各列的数据类型,另一部分数据访问参数描述外部数据与 Oracle 数据列的映射方式.
2. 外部表规定了外部表层将外部数据展现给数据库服务器的格式.而外部表层和数据访问驱动的职责
是将外部数据文件内的数据进行适当转换,使之符合外部表的定义.
3. 外部表是只读的 , 也不能在上面创建索引

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