When you create an external table, you specify its type. Each type of external table has
its own access driver that provides access parameters unique to that type of external
table. The access driver ensures that data from the data source is processed so that it
matches the definition of the external table.
In the context of external tables, loading data refers to the act of reading data from an
external table and loading it into a table in the database. Unloading data refers to the
act of reading data from a table in the database and inserting it into an external table.
The default type for external tables is ORACLE_LOADER, which lets you read table data
from an external table and load it into a database. Oracle also provides the ORACLE_DATAPUMP
type, which lets you unload data (that is, read data from a table in the
database and insert it into an external table) and then reload it into an Oracle database.
The definition of an external table is kept separately from the description of the data in
the data source. This means that:
■ The source file can contain more or fewer fields than there are columns in the
■ The datatypes for fields in the data source can be different from the columns in the
1. 数据访问驱动将对外部数据源的数据进行处理使之符合外部表的定义 .
2. 外部表的数据加载是指从外部表中读取数据并加载到数据库表中 .
3. 外部表的默认类型是ORACLE_LOADER，用户可以使用这种类型从外部表中读取数据并加载到数据库中 .
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