The ORACLE_DATAPUMP access driver has the following restrictions:
Exporting and importing of external tables with encrypted columns is not supported.
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 data type 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.
Handling of byte-order marks during a load: In an external table load for which the data file character set is UTF8 or UTF16, it is not possible to suppress checking for byte-order marks. Suppression of byte-order mark checking is necessary only if the beginning of the data file contains binary data that matches the byte-order mark encoding. (It is possible to suppress byte-order mark checking with SQL*Loader loads.) Note that checking for a byte-order mark does not mean that a byte-order mark must be present in the data file. If no byte-order mark is present, then the byte order of the server platform is used.
The external tables feature does not support the use of the backslash (\) escape character within strings. See "Use of the Backslash Escape Character".
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, then it must be enclosed in double quotation marks.