如何在 oracle PL/SQL 中设置列删除触发器？

Question

I tried to do this:

CREATE OR REPLACE TRIGGER DLL_No_Column_Del
BEFORE DROP ON SCHEMA
BEGIN
 IF ORA_DICT_OBJ_TYPE = 'COLUMN'
 THEN
    INSERT INTO column_del_attempt VALUES
    (
        user,
        SYSDATE,
        ora_dict_obj_name
    );
    RAISE_APPLICATION_ERROR(-20100, 'No column deletion allowed on table ' || ORA_DICT_OBJ_NAME);
 END IF;
END;

Then drop column like that:

ALTER TABLE emp_copy
DROP COLUMN employee_id;

But it doesn't work

Answer 1

Create an autonomous procedure to log the event:

CREATE PROCEDURE log_system_event(
  p_user_name   IN COLUMN_DEL_ATTEMPT.USER_NAME%TYPE,
  p_object_name IN COLUMN_DEL_ATTEMPT.OBJECT_NAME%TYPE,
  p_object_type IN COLUMN_DEL_ATTEMPT.OBJECT_TYPE%TYPE
)
AS
  PRAGMA AUTONOMOUS_TRANSACTION;
BEGIN
  INSERT INTO column_del_attempt (
    user_name,
    datetime,
    object_name,
    object_type
  ) VALUES (
    p_user_name,
    SYSDATE,
    p_object_name,
    p_object_type
  );
  COMMIT;
END log_system_event;
/

Then you can call it from the trigger (on the ALTER event, rather than the DROP event):

CREATE TRIGGER DLL_No_Column_Del
BEFORE ALTER ON SCHEMA
BEGIN
  IF ora_dict_obj_name = 'EMP_COPY' AND ORA_DICT_OBJ_TYPE = 'TABLE' THEN
    log_system_event( login_user, ora_dict_obj_name, ORA_DICT_OBJ_TYPE );
    
    RAISE_APPLICATION_ERROR(-20100, 'No column deletion allowed on table ' || ORA_DICT_OBJ_NAME);
  END IF;
END;
/

Then, if you do:

ALTER TABLE emp_copy DROP COLUMN employee_id;

It will raise the exception:

 ORA-04088: error during execution of trigger 'FIDDLE_SPRKWOBAQONFKBBYMKLX.DLL_NO_COLUMN_DEL' ORA-00604: error occurred at recursive SQL level 1 ORA-20100: No column deletion allowed on table EMP_COPY ORA-06512: at line 5

Then:

SELECT * FROM column_del_attempt;

Outputs:

 USER_NAME | DATETIME | OBJECT_NAME | OBJECT_TYPE:-------------------------- |:-------- |:---------- |:---------- FIDDLE_SPRKWOBAQONFKBBYMKLX |  18-DEC-20 |  EMP_COPY |  TABLE

db<>fiddle here

---

Update

To get the text of the ALTER DDL statement, according to AskTom , you can use the V$OPEN_CURSOR dynamic performance view to get the text of the DDL statement and then you can filter on that:

CREATE PROCEDURE log_system_event(
  p_user_name   IN COLUMN_DEL_ATTEMPT.USER_NAME%TYPE,
  p_object_name IN COLUMN_DEL_ATTEMPT.OBJECT_NAME%TYPE,
  p_object_type IN COLUMN_DEL_ATTEMPT.OBJECT_TYPE%TYPE,
  p_text        IN COLUMN_DEL_ATTEMPT.TEXT%TYPE
)
AS
  PRAGMA AUTONOMOUS_TRANSACTION;
BEGIN
  INSERT INTO column_del_attempt (
    user_name,
    datetime,
    object_name,
    object_type,
    text
  ) VALUES (
    p_user_name,
    SYSDATE,
    p_object_name,
    p_object_type,
    p_text
  );
  COMMIT;
END log_system_event;
/

Then:

CREATE TRIGGER DLL_No_Column_Del
BEFORE ALTER ON SCHEMA
DECLARE
  v_text VARCHAR2(4000);
BEGIN
  SELECT SQL_TEXT
  INTO   v_text
  FROM   V$OPEN_CURSOR;
  
  IF REGEXP_LIKE( v_text, '^ALTER\s+TABLE\s+("?)EMP_COPY\1\s+DROP\s+', 'i') THEN
    log_system_event( login_user, ora_dict_obj_name, ORA_DICT_OBJ_TYPE, v_text );
    
    RAISE_APPLICATION_ERROR(-20100, 'No column deletion allowed on table ' || ORA_DICT_OBJ_NAME);
  END IF;
END;
/

(However, none of db<>fiddle , SQLFiddle nor Oracle's LiveSQL has granted permissions to use the V$OPEN_CURSOR dynamic performance view so I cannot test this.)

Answer 2

Final solution:

CREATE TABLE column_del_attempt (
  user_name   VARCHAR2(30),
  datetime    DATE,
  object_name VARCHAR2(30),
  object_type VARCHAR2(30)
)
/
CREATE OR REPLACE PROCEDURE log_system_event(
  p_user_name   IN COLUMN_DEL_ATTEMPT.USER_NAME%TYPE,
  p_object_name IN COLUMN_DEL_ATTEMPT.OBJECT_NAME%TYPE,
  p_object_type IN COLUMN_DEL_ATTEMPT.OBJECT_TYPE%TYPE
)
AS
  PRAGMA AUTONOMOUS_TRANSACTION;
BEGIN
  INSERT INTO column_del_attempt (
    user_name,
    datetime,
    object_name,
    object_type
  ) VALUES (
    p_user_name,
    SYSDATE,
    p_object_name,
    p_object_type
  );
  COMMIT;
END log_system_event;
/

CREATE OR REPLACE TRIGGER DLL_No_Column_Del
BEFORE ALTER ON SCHEMA
DECLARE
    n NUMBER;
    sql_text_list ora_name_list_t;
    drop_exist BOOLEAN := FALSE;
BEGIN
n := ora_sql_txt(sql_text_list);
FOR i in 1..n LOOP
    IF sql_text_list(i) LIKE '%DROP%' THEN 
        drop_exist := TRUE;
    END IF;
END LOOP;
  IF drop_exist THEN
    log_system_event( login_user, ora_dict_obj_name, ORA_DICT_OBJ_TYPE );
    RAISE_APPLICATION_ERROR(-20100, 'No column deletion allowed on table ' || ORA_DICT_OBJ_NAME);
  END IF;
END;
/
-- for tests:
/*
ALTER TABLE emp_copy ADD birth_date DATE NOT NULL;
ALTER TABLE emp_copy DROP COLUMN birth_date;

SELECT * FROM column_del_attempt;
*/