ORA-04091: table name is mutating - when trigger from child table wants to update parent table - sql

Question

I have 2 simple tables:
CREATE TABLE ORDERS
( ORDER_KEY number(10) NOT NULL,
ORDER_NR varchar2(50) NOT NULL,
LAST_UPDATE DATE,
CONSTRAINT ORDERS_PK PRIMARY KEY (ORDER_KEY)
);
CREATE TABLE ORDER_POSITIONS
( ORDER_POS_KEY number(10) NOT NULL,
ORDER_POS_NR number(10),
ORDER_POS_DESCRIPTION varchar2(50),
ORDER_KEY NUMBER(10) NOT NULL,
CONSTRAINT ORDER_POSITIONS_PK PRIMARY KEY (ORDER_POS_KEY),
CONSTRAINT ORDERS_FK
FOREIGN KEY (ORDER_KEY)
REFERENCES ORDERS(ORDER_KEY)
ON DELETE CASCADE
);
On the table ORDER_POSITIONS I created a trigger which should update the column LAST_UPDATE whenever a position is deleted.
CREATE OR REPLACE TRIGGER TGAUDIT_ORDER_POS
AFTER DELETE
ON ORDER_POSITIONS
FOR EACH ROW
DECLARE
BEGIN
UPDATE ORDERS O SET O.LAST_UPDATE = SYSDATE WHERE O.ORDER_KEY = :OLD.ORDER_KEY;
END;
If I delete a position from the table ORDER_POSITION, everything is working fine (the column LAST_UPDATE is updated).
However, if I want to delete an order, all its positions are gonna be deleted, too (via CASCADE DELETE). At this moment also the trigger on the table ORDER_POSITIONS is being raised and it wants to update the column of the table which is currently being deleted - ORDERS. Obviously I get here the error : ORA-04091 Table ORDERS is mutating.
Is there a way to get it solved?

Answer 1

I solved it finally using a compound trigger:
CREATE OR REPLACE TRIGGER TGAUDIT_ORDER_POS
FOR DELETE ON ORDER_POSITIONS
COMPOUND TRIGGER
TYPE parent_key_type IS TABLE OF ORDERS.ORDER_KEY%TYPE;
parent_keys parent_key_type := parent_key_type();
AFTER EACH ROW IS BEGIN
IF DELETING THEN
BEGIN
parent_keys.extend;
parent_keys(parent_keys.last) := :old.ORDER_KEY;
END;
END IF;
END AFTER EACH ROW;
AFTER STATEMENT IS BEGIN
FOR i IN 1..parent_keys.count LOOP
UPDATE DEVART_TEST.ORDERS O SET O.LAST_UPDATE = SYSDATE WHERE O.ORDER_KEY = parent_keys(i);
END LOOP;
END AFTER STATEMENT;
END;
UPDATE: Another option would be to catch this specific exception within the regular trigger.
CREATE OR REPLACE TRIGGER TGAUDIT_ORDER_POS
AFTER DELETE
ON ORDER_POSITIONS
FOR EACH ROW
DECLARE
TABLE_MUTATING EXCEPTION;
PRAGMA EXCEPTION_INIT(TABLE_MUTATING, -4091 );
BEGIN
UPDATE ORDERS O SET O.LAST_UPDATE = SYSDATE WHERE O.ORDER_KEY = :OLD.ORDER_KEY;
EXCEPTION
WHEN TABLE_MUTATING THEN
NULL; -- suppress
END;

Answer 2

I would recreate the foreign-key constraint without the ON DELETE CASCADE clause and delete all order positions before deleting an order. That way you avoid the mutating table error by not having two tables mutating at the same time.