2020年OCP考试071真题解析

Which three statements are true about external tables?

A) They can be temporary tables.

B) They can be used in queries containing joins.

C) They can be used in queries containing sorts.

D) Their metadata is stored in the database.

E) DML statements can modify them

F) They can be indexed.

Oracle Database allows you read-only access to data in external tables. External tables are defined as tables that do not reside in the database, and can be in any format for which an access driver is provided. By providing the database with metadata describing an external table, the database is able to expose the data in the external table as if it were data residing in a regular database table. The external data can be queried directly and in parallel using SQL.

Oracle数据库允许您以只读方式（read-only）访问外部表（external tables）中的数据。 外部表是不驻留在oracle数据库中的表，但却可以采用oracle提供的驱动程序访问的任何格式的表。 通过为oracle数据库提供描述外部表的元数据，oracle数据库可以象访问oralce常规数据库表一样访问外部表中的数据。外部数据可以被直接查询也可以使用SQL语句查询。

You can, for example, select, join, or sort external table data. You can also create views and synonyms for external tables. However, no DML operations (

UPDATE

,

INSERT

, or

DELETE

) are possible, and no indexes can be created, on external tables.

例如，您对外部表数据进行select，join或 order by 排序。还可以为外部表创建视图view和同义词synonym。 然而不能在外部表上进行DML操作（UPDATE，INSERT或DELETE），也不能创建索引（INDEX）。

External tables also provide a framework to unload the result of an arbitrary

SELECT

statement into a platform-independent Oracle-proprietary format that can be used by Oracle Data Pump.

外部表还提供了一个框架，可将任意SELECT语句的结果卸载为独立于平台的Oracle专有格式，Oracle Data Pump可以使用该格式。

Note:

The DBMS_STATS package can be used for gathering statistics for external tables. The ANALYZE statement is not supported for gathering statistics for external tables.

注意：

1.DBMS_STATS包可用于收集外部表的统计信息。

2. ANALYZE语句不可以用来收集外部表的统计信息。

The means of defining the metadata for external tables is through the CREATE TABLE...ORGANIZATION EXTERNAL statement. This external table definition can be thought of as a view that allows running any SQL query against external data without requiring that the external data first be loaded into the database. An access driver is the actual mechanism used to read the external data in the table. When you use external tables to unload data, the metadata is automatically created based on the datatypes in the SELECT statement.

定义外部表元数据的方法是通过CREATE TABLE ... ORGANIZATION EXTERNAL语句。可以将这个外部表定义视为一个视图，该视图允许对外部数据运行任何SQL查询，而无需先将外部数据加载到数据库中。访问驱动程序是用于读取表中外部数据的实际机制。使用外部表卸载数据时，将根据SELECT语句中的数据类型自动创建元数据。

Oracle Database provides two access drivers for external tables. The default access driver is ORACLE_LOADER, which allows the reading of data from external files using the Oracle loader technology. The ORACLE_LOADER access driver provides data mapping capabilities which are a subset of the control file syntax of SQL*Loader utility. The second access driver, ORACLE_DATAPUMP, lets you unload data--that is, read data from the database and insert it into an external table, represented by one or more external files--and then reload it into an Oracle Database.

Oracle数据库为外部表提供了两个访问驱动程序(ORACLE_LOADER与ORACLE_DATAPUMP)。

默认的访问驱动程序是ORACLE_LOADER，它允许使用Oracle加载程序技术从外部文件读取数据。 ORACLE_LOADER访问驱动程序提供数据映射功能，这些功能是SQL * Loader实用程序的控制文件语法的子集。第二个访问驱动程序ORACLE_DATAPUMP允许您卸载数据（即，从数据库中读取数据并将其插入由一个或多个外部文件表示的外部表中），然后将其重新加载到Oracle数据库中。

The Oracle Database external tables feature provides a valuable means for performing basic extraction, transformation, and loading (ETL) tasks that are common for data warehousing.

Oracle数据库外部表功能提供了一种宝贵的手段，可以执行数据仓库中常见的基本提取，转换和加载（ETL）任务。

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