External Table Restrictions【每日一译】--1227

This section lists what the external tables feature does not do and also describes some

processing restrictions.

这部份列出了外部表的一些不能做的功能和另外描述了一些处理的限制

■ Exporting and importing of external tables with encrypted columns is not
supported.
■ An external table does not describe any data that is stored in the database.
■ An external table does not describe how data is stored in the external source. This
is the function of the access parameters.
■ Column processing: By default, the external tables feature fetches all columns
defined for an external table. This guarantees a consistent result set for all queries.
However, for performance reasons you can decide to process only the referenced
columns of an external table, thus minimizing the amount of data conversion and
data handling required to execute a query. In this case, a row that is rejected
because a column in the row causes a datatype conversion error will not get
rejected in a different query if the query does not reference that column. You can
change this column-processing behavior with the ALTER TABLE command.
■ An external table cannot load data into a LONG column.
■ When identifiers (for example, column or table names) are specified in the external
table access parameters, certain values are considered to be reserved words by the
access parameter parser. If a reserved word is used as an identifier, it must be

enclosed in double quotation marks.

#导出和导入外部对于包含加密的列是不支持的

#一个外部表不能描述任何存在于数据库中的任何数据

#一个外部表不能描述外部源数据是怎样存储的。这个是ACCESS参数的功能。

#列的处理：默认，外部表功能获取所有定义在外部表的列。这确保了一个致性的结果集

对于所有的查询。但，从性能角度讲你可以决定处理那些仅仅有关关系的外部表的列，这样可以

最小化数据转换的数量和数据要求被处理的数量对于一个查询来讲。有种情况，一行被拒绝的因为

在一行的中列导致数据类型转换的错误将不会被拒绝在不同的查询如果这个查询没有关联到那个列。

你可以改变这个变处理列的行为通过ALTER TABLE 命令。

#一个外部表不能导入数据到一个LONG列。

#当标识符（比如，列或者是表名）在外部表的ACCESS参数被指定时，某些值被认为是预留关键字通

过ACCESS 参数分析器。如果一个预留关键字当作一个标识符使用，它必须内嵌在双引号里。