首先看官方文档上的解释：
Managing External Tables
Oracle Database allows you read-only access to data in external tables. External tables are definedas tables that do not reside in the database, and can be in any format forwhich an access driver is provided. By providing the database with metadatadescribing an external table, the database is able to expose the data in theexternal table as if it were data residing in a regular database table. Theexternal data can be queried directly and in parallel using SQL.
You can, for example, select, join, or sort externaltable 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.
External tables also provide a framework to unload theresult of an arbitrary SELECT statement into aplatform-independent Oracle-proprietary format that can be used by Oracle DataPump.
实验测试：
1、 创建一个外部表
（1）元数据：
[oracle@mydb ~]$more prod_master.dat
6,6,6,6,6,600
7,7,7,7,7,700
（2） 建目录
sys@OCM> create directory dir_gyj as '/home/oracle'; Directory created. sys@OCM> grant read,write on directory dir_gyj to gyj; Grant succeeded.
(3)创建外部表
gyj@OCM> CREATE TABLE ext_gyj_costs
2 (
3 "CHANNEL_ID" NUMBER,
4 "PROD_ID" NUMBER,
5 "PROMO_ID" NUMBER,
6 "TIME_ID" NUMBER,
7 "UNIT_COST" NUMBER,
8 "UNIT_PRICE" NUMBER
9 )
10 ORGANIZATION external
11 (
12 TYPE oracle_loader
13 DEFAULT DIRECTORY dir_gyj
14 ACCESS PARAMETERS
15 (
16 FIELDS TERMINATED BY "," LDRTRIM
17 REJECT ROWS WITH ALL NULL FIELDS
18 (
19 "CHANNEL_ID" CHAR(255)
20 TERMINATED BY ",",
21 "PROD_ID" CHAR(255)
22 TERMINATED BY ",",
23 "PROMO_ID" CHAR(255)
24 TERMINATED BY ",",
25 "TIME_ID" CHAR(255)
26 TERMINATED BY ",",
27 "UNIT_COST" CHAR(255)
28 TERMINATED BY ",",
29 "UNIT_PRICE" CHAR(255)
30 TERMINATED BY ","
31 )
32 )
33 location
34 (
35 'prod_master.dat'
36 )
37 )REJECT LIMIT UNLIMITED; Table created. gyj@OCM> select * from ext_gyj_costs; CHANNEL_ID PROD_ID PROMO_ID TIME_ID UNIT_COST UNIT_PRICE
---------- ---------- ---------- ---------- ---------- ----------
6 6 6 6 6 600
7 7 7 7 7 700
2、 对外部表做DML,操作如下，说明外部表不支持DML
gyj@OCM> Update ext_gyj_costs set UNIT_PRICE=800 where channel_id=6;
Update ext_gyj_costs set UNIT_PRICE=800 where channel_id=6
*
ERROR at line 1:
ORA-30657: operation not supported on external organized table gyj@OCM> delete from ext_gyj_costs where channel_id=6;
delete from ext_gyj_costs where channel_id=6
*
ERROR at line 1:
ORA-30657: operation not supported on external organized table
3、 在外部表上建视图和同义词
gyj@OCM> Create view v_cost as select * from ext_gyj_costs; View created. gyj@OCM> select * from v_cost; CHANNEL_ID PROD_ID PROMO_ID TIME_ID UNIT_COST UNIT_PRICE
---------- ---------- ---------- ---------- ---------- ----------
6 6 6 6 6 600
7 7 7 7 7 700 gyj@OCM> create synonym s_cost for ext_gyj_costs; Synonym created. gyj@OCM> select * from s_cost; CHANNEL_ID PROD_ID PROMO_ID TIME_ID UNIT_COST UNIT_PRICE
---------- ---------- ---------- ---------- ---------- ----------
6 6 6 6 6 600
7 7 7 7 7 700
4、 对外部表建索引，不支操作
gyj@OCM> create index idx_chaannel_id on ext_gyj_costs(channel_id);
create index idx_chaannel_id on ext_gyj_costs(channel_id)
*
ERROR at line 1:
ORA-30657: operation not supported on external organized table
总结：外部表除了可以select(查询)、建视图，同义词，其它操作如DML,建索引等都不支持。
正确答案：C