ORA-04091 Mutating Table error during AFTER INSERT Trigger - oracle

Question

I have the following AFTER INSERT trigger on a table called DM_USER_ROLE
create or replace TRIGGER "DM_USER_ROLE_T1"
AFTER
insert on "DM_USER_ROLE"
for each row
DECLARE
v_cert_enrolment_id number;
v_user_role_id number;
begin
v_cert_enrolment_id := "DM_CERTIFICATION_ENROLMEN_SEQ".nextval;
v_user_role_id := :new.USER_ROLE_ID;
/*
When a user is assigned a role, we create an enrolment record
in DM Certification record linked to this user/role combination.
We also insert into the DM_COURSE_ENROLMENT table the courses
associated with the certfication
*/
--FIRST AN ENROLMENT RECORD IS CREATED IN DM_CERTIFICATION_ENROLMENT
INSERT INTO DM_CERTIFICATION_ENROLMENT
(CERTIFICATION_ENROLMENT_ID, ALLOCATED_DT, DEADLINE_DATE, STATUS, USER_ROLE_ID)
VALUES
(
v_cert_enrolment_id,
trunc(sysdate),
trunc(sysdate) + 60,
'Enrolled',
v_user_role_id
);
--COURSES LINKED TO THE CERTIFICATION ARE INSERTED INTO DM_COURSE_ENROLMENT
INSERT INTO DM_COURSE_ENROLMENT
(
CERTIFICATION_ENROLMENT_ID,
COURSE_ID,
ALLOCATED_DT,
DEADLINE_DT,
STATUS
)
SELECT v_cert_enrolment_id,
COURSE.COURSE_ID,
trunc(sysdate),
trunc(sysdate) + 60,
'Enrolled'
FROM DM_CERTIFICATION_COURSE COURSE
WHERE CERTIFICATION_ID =
(
SELECT C.CERTIFICATION_ID FROM
DM_CERTIFICATION_ENROLMENT A,
DM_USER_ROLE B,
DM_ROLE_CERTIFICATION C
WHERE
A.USER_ROLE_ID = B.USER_ROLE_ID
AND
B.ROLE_ID = C.ROLE_ID
AND
A.CERTIFICATION_ENROLMENT_ID = v_cert_enrolment_id
);
EXCEPTION
WHEN NO_DATA_FOUND
THEN
DBMS_OUTPUT.PUT_LINE(TO_CHAR(SQLERRM(-20299)));
WHEN OTHERS THEN
DBMS_OUTPUT.PUT_LINE(SUBSTR(SQLERRM, 1, 2000));
end;
I need to populate 2 separate tables when an insert happens in this table, and I thought AFTER INSERT triggers avoided issues with mutating tables?
I am not sure what is causing it, perhaps the read in the second INSERT statement from DM_USER_ROLE, which is where this trigger is initiated...but I was under the impression AFTER INSERTs were safe to avoid mutations, as the update has already happened.
Error is:
ORA-04091: table AZLEARN_BACKUP.DM_USER_ROLE is mutating,
trigger/function may not see it
The first insert happens, the second one does not.
This article led me to believe AFTER triggers were safe.
http://www.dba-oracle.com/t_avoiding_mutating_table_error.htm
-------UPDATE---------------
I changed it to do row by row insert using two parameterised cursors and it worked...still not sure what the error was:
create or replace TRIGGER "DM_USER_ROLE_T1"
AFTER
insert on "DM_USER_ROLE"
for each row
DECLARE
v_cert_enrolment_id number;
v_user_role_id number;
v_role_id number;
v_certification_id number;
cursor certs_for_role(p_role_id number) is
select * from DM_ROLE_CERTIFICATION where ROLE_ID = p_role_id;
r_certs_for_role certs_for_role%rowtype;
cursor courses_for_certs(p_cert_id number) is
select * from DM_CERTIFICATION_COURSE where CERTIFICATION_ID = p_cert_id;
r_courses_for_certs courses_for_certs%rowtype;
begin
v_user_role_id := :new.USER_ROLE_ID;
v_role_id := :new.ROLE_ID;
open certs_for_role(v_role_id);
loop
fetch certs_for_role into r_certs_for_role;
exit when certs_for_role%notfound;
v_cert_enrolment_id := "DM_CERTIFICATION_ENROLMEN_SEQ".nextval;
INSERT INTO DM_CERTIFICATION_ENROLMENT
(CERTIFICATION_ENROLMENT_ID, ALLOCATED_DT, DEADLINE_DATE, STATUS, USER_ROLE_ID, CERTIFICATION_ID)
VALUES
(
v_cert_enrolment_id,
trunc(sysdate),
trunc(sysdate) + 60,
'Enrolled',
v_user_role_id,
r_certs_for_role.CERTIFICATION_ID
);
open courses_for_certs(r_certs_for_role.CERTIFICATION_ID);
loop
fetch courses_for_certs into r_courses_for_certs;
exit when courses_for_certs%notfound;
INSERT INTO DM_COURSE_ENROLMENT
(
CERTIFICATION_ENROLMENT_ID,
COURSE_ID,
ALLOCATED_DT,
DEADLINE_DT,
STATUS
)
VALUES
(
v_cert_enrolment_id,
r_courses_for_certs.COURSE_ID,
trunc(sysdate),
trunc(sysdate) + 60,
'Enrolled'
);
end loop;
close courses_for_certs;
end loop;
close certs_for_role;
EXCEPTION
WHEN NO_DATA_FOUND
THEN
DBMS_OUTPUT.PUT_LINE(TO_CHAR(SQLERRM(-20299)));
WHEN OTHERS THEN
DBMS_OUTPUT.PUT_LINE(SUBSTR(SQLERRM, 1, 2000));
end;

Answer 1

The reason is simply you cannot select from table DM_USER_ROLE where the ROW LEVEL trigger is based on.
In your first solution you have a
SELECT ...
FROM DM_USER_ROLE ...
This is not allowed. Your second trigger does not select table DM_USER_ROLE thus it is working.
The advise in linked page is correct but misleading when they state 'Use an "after" or "instead of" trigger' - It should be more precisely 'Use an "after statement" or "instead of" trigger' . Oracle provides triggers based on following actions:
DML statements (INSERT, UPDATE, DELETE) on a particular table or view
DDL statements (CREATE or ALTER primarily)
Database events, such as logon/logoff, errors, or startup/shutdown
You have a DML trigger which can have different timing points:
Before the triggering statement executes
After the triggering statement executes
Before each row that the triggering statement affects
After each row that the triggering statement affects
Compound Trigger -> this one combines the four triggers listed above
INSTEAD OF Trigger (only for views)
Many people miss the difference between a statement level trigger and a row level trigger.
Row Level Triggers have keyword FOR EACH ROW and run for each row like the keyword implies. If you skip the FOR EACH ROW keyword, then the trigger is executed only once for each statement, no matter how many rows are affected by your INSERT/UPDATE/DELETE statement.

Answer 2

The most likely cause of a mutating table error is the misuse of triggers. Here is a typical example:
1.you insert a row in table A
2.a trigger on table A (for each row) executes a query on table A, for example to compute a summary column
3.Oracle throws an ORA-04091: table A is mutating, trigger/function may not see it
This is an expected and normal behaviour, Oracle wants to protect you from yourself since Oracle guarantees:
•(i) that each statement is atomic (i.e will either fail or succeed
completely)
•(ii) that each statement sees a consistent view of the data
Now in your trigger, when you do the second insert, it's doing a join on the DM_USER_ROLE table to fetch the records and that's the reason you face
ORA-04091: table AZLEARN_BACKUP.DM_USER_ROLE is mutating,
trigger/function may not see it