Error while selecting from view that references external table

Question

Can someone explain why the error near the bottom of the code below is occuring? If USER1 grants SELECT on the external table to USER2, then USER2 can select from the view without any problems; however, I want to avoid giving USER2 access to all of the columns in the external table. (I only want to give USER2 access to two of the four columns.)
SQL> CONNECT sys AS SYSDBA
Connected as SYS@ as sysdba
SQL> CREATE USER user1 IDENTIFIED BY user1
User created.
SQL> CREATE USER user2 IDENTIFIED BY user2
User created.
SQL> GRANT CONNECT, CREATE TABLE, CREATE VIEW TO user1
Grant complete.
SQL> GRANT CONNECT TO user2
Grant complete.
SQL> GRANT READ, WRITE ON DIRECTORY EXT_DATA_DIR TO user1, user2
Grant complete.
SQL> CONNECT user1/user1
Connected as USER1@
SQL> CREATE TABLE emp_xt
  emp_id     NUMBER,
  first_name VARCHAR2(30),
  last_name  VARCHAR2(30),
  phone      VARCHAR2(15)
ORGANIZATION EXTERNAL
  TYPE ORACLE_LOADER
  DEFAULT DIRECTORY EXT_DATA_DIR
  ACCESS PARAMETERS
    RECORDS DELIMITED BY NEWLINE
    FIELDS TERMINATED BY ','
    OPTIONALLY ENCLOSED BY '"'           
  LOCATION ('emp.txt')
REJECT LIMIT 0
Table created.
SQL> SELECT COUNT(1) FROM emp_xt
  COUNT(1)
         4
1 row selected.
SQL> CREATE OR REPLACE VIEW emp_xt_view AS SELECT first_name, last_name FROM emp_xt;
View created.
SQL> SELECT COUNT(1) FROM emp_xt_view
  COUNT(1)
         4
1 row selected.
SQL> GRANT SELECT ON emp_xt_view TO user2
Grant complete.
SQL> CONNECT user2/user2
Connected as USER2@
SQL> SELECT COUNT(1) from user1.emp_xt_view
SELECT COUNT(1) from user1.emp_xt_view
Error at line 0
ORA-29913: error in executing ODCIEXTTABLEOPEN callout
ORA-29400: data cartridge error
ORA-04043: object "USER1"."EMP_XT" does not exist
SQL> CONNECT user1/user1
Connected as USER1@
SQL> GRANT SELECT ON user1.emp_xt TO user2
Grant complete.
SQL> CONNECT user2/user2
Connected as USER2@
SQL> SELECT COUNT(1) from user1.emp_xt_view
  COUNT(1)
         4
1 row selected.
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Answer 1

user503699 wrote:
user1983440 wrote:
Can someone explain why the error near the bottom of the code below is occuring? If USER1 grants SELECT on the external table to USER2, then USER2 can select from the view without any problems; however, I want to avoid giving USER2 access to all of the columns in the external table. (I only want to give USER2 access to two of the four columns.)As you have demonstrated, I guess the view approach only works for database tables. External tables are actually files on the file system. Even through OS, it is not possible to grant READ/WRITE access to only part of the file. The access is for entire file. An "External Table" is just a "wrapper" provided by oracle (using data cartridge) to allow user to be able to access the file as a "table". So it can definitely not do something that underlying OS can not do.
p.s. In fact, oracle does not even allow to edit data in external tables using SQL.Why not just make a second external table (only including the 2 columns you need) and grant select directly on that to the user. I know you say "views only" but there's an exception to every rule ... just because it's called a table doesn't make it so. You could argue an external table is nothing more than a fancy view.
Worst case, make a view on top of the second external table.
CREATE TABLE emp_xt_less_information
  first_name VARCHAR2(30),
  last_name  VARCHAR2(30)
ORGANIZATION EXTERNAL
  TYPE ORACLE_LOADER
  DEFAULT DIRECTORY EXT_DATA_DIR
  ACCESS PARAMETERS
    RECORDS DELIMITED BY NEWLINE
    FIELDS TERMINATED BY ','
    OPTIONALLY ENCLOSED BY '"'   
         emp_id      number,
         first_name  char,
         last_name   char,
         phone       char
  LOCATION ('emp.txt')
REJECT LIMIT 0
{code}
Should do it, but my syntax may be off a touch ....