PLSQL Table keeps mutating - plsql

Question

I'm trying to figure out why my table keeps mutating even though I have already created another package to maintain in memory a copy of the rows in the jobs table.
This is my emp_pkg that handles all the code for the employee, i will only be adding the procedure that i'm using in this package so it will be short:
CREATE OR REPLACE PACKAGE BODY emp_pkg
IS
PROCEDURE SET_SALARY(pjobid jobs.job_id%type,
psal jobs.min_salary%type)
IS
BEGIN
FOR i IN (SELECT * FROM jobs)
LOOP
UPDATE employees
SET salary = psal
WHERE job_id = pjobid;
END LOOP;
END SET_SALARY;
END emp_pkg;
Then I created a trigger on the jobs table that invokes the set_salary procedure on my emp_pkg package when the minimum salary in the jobs table is updated for a specified job id:
CREATE OR REPLACE TRIGGER upd_minsalary_trg
AFTER UPDATE OF min_salary ON jobs
FOR EACH ROW
WHEN (new.min_salary != old.min_salary)
BEGIN
emp_pkg.set_salary(:old.job_id, :new.min_salary);
END upd_minsalary_trg;
/
Now, I tested it by running the code below with the following errors as expected:
UPDATE jobs
SET min_salary = min_salary + 1000
WHERE job_id = 'IT_PROG';
--ORA-04091: table SQL_WLALHQOYXPNRGAIYBLVJOQTEB.JOBS is mutating, trigger/function may not see it
Now like I said before, to fix this i have created another package to maintain in memory a copy of the rows in the jobs table:
CREATE OR REPLACE PACKAGE BODY jobs_pkg
IS
TYPE jobs_tab_type IS TABLE OF jobs%rowtype INDEX BY jobs.job_id%type;
jobstab jobs_tab_type;
PROCEDURE initialize
IS
BEGIN
FOR rec IN (SELECT * FROM jobs)
LOOP
jobstab(rec.job_id) := rec;
END LOOP;
END;
FUNCTION get_minsalary(p_jobid VARCHAR2)
RETURN NUMBER
IS
vmin_salary jobs.min_salary%type;
BEGIN
SELECT min_salary
INTO vmin_salary
FROM jobs
WHERE job_id = jobstab(p_jobid).job_id;
RETURN vmin_salary;
END get_minsalary;
FUNCTION get_maxsalary(p_jobid VARCHAR2)
RETURN NUMBER
IS vmax_salary jobs.max_salary%type;
BEGIN
SELECT max_salary
INTO vmax_salary
FROM jobs
WHERE job_id = jobstab(p_jobid).job_id;
RETURN vmax_salary;
END get_maxsalary;
PROCEDURE set_minsalary(p_jobid VARCHAR2, pmin_salary NUMBER)
IS
BEGIN
UPDATE jobs
SET min_salary = pmin_salary
WHERE job_id = jobstab(p_jobid).job_id;
END set_minsalary;
PROCEDURE set_maxsalary(p_jobid VARCHAR2, pmax_salary NUMBER)
IS
BEGIN
UPDATE jobs
SET max_salary = pmax_salary
WHERE job_id = jobstab(p_jobid).job_id;
END set_maxsalary;
END jobs_pkg;
Then I implemented another trigger for before inserting or updating on the jobs table that invoke my jobs_pkg.initialize procedure on the jobs package to ensure that the package state is current before the DML operations are performed:
CREATE OR REPLACE TRIGGER init_jobpkg_trg
BEFORE UPDATE OR INSERT ON jobs
FOR EACH ROW
BEGIN
jobs_pkg.initialize;
END init_jobpkg_trg;
Now the problem is when I tried to run the code below it still mutates:
UPDATE jobs
SET min_salary = min_salary + 1000
WHERE job_id = 'IT_PROG';
I keep searching on why it still mutates but can't seem to find the problem.
Any help will be appreciated.

Answer 1

Triggers are a very powerful tool, but there are a couple of limitations, one of which is that you can't select from the table the trigger is on (the dreaded ORA-04091). This limitation makes total sense. There are plenty of articles on google explaining why this error occurs.
There are 2 ways to solve it.
Option 1: avoid the SELECT ... FROM jobs in the row level trigger
CREATE OR REPLACE TRIGGER JOBS_BIU
AFTER UPDATE OF min_salary ON jobs
FOR EACH ROW
WHEN (new.min_salary != old.min_salary)
BEGIN
UPDATE employees SET salary = :new.min_salary WHERE job_id = :new.job_id;
END;
/
Open 2: Do not use a row level trigger but a statement level trigger. This will fire after the statement has completed:
CREATE OR REPLACE TRIGGER JOBS_BIU
AFTER UPDATE OF min_salary ON jobs
BEGIN
FOR r IN (SELECT * FROM jobs) LOOP
UPDATE employees
SET salary = r.min_salary
WHERE job_id = r.job_id;
END LOOP;
END;
/

Answer 2

Thanks to Koen, I got an idea on how to fix my problem so here it is:
First I have updated my jobs package and eliminated all SELECT statement that was being used:
This is my code before:
CREATE OR REPLACE PACKAGE BODY jobs_pkg
IS
TYPE jobs_tab_type IS TABLE OF jobs%rowtype INDEX BY jobs.job_id%type;
jobstab jobs_tab_type;
PROCEDURE initialize
IS
BEGIN
FOR rec IN (SELECT * FROM jobs)
LOOP
jobstab(rec.job_id) := rec;
END LOOP;
END;
FUNCTION get_minsalary(p_jobid VARCHAR2)
RETURN NUMBER
IS
vmin_salary jobs.min_salary%type;
BEGIN
SELECT min_salary
INTO vmin_salary
FROM jobs
WHERE job_id = jobstab(p_jobid).job_id;
RETURN vmin_salary;
END get_minsalary;
FUNCTION get_maxsalary(p_jobid VARCHAR2)
RETURN NUMBER
IS vmax_salary jobs.max_salary%type;
BEGIN
SELECT max_salary
INTO vmax_salary
FROM jobs
WHERE job_id = jobstab(p_jobid).job_id;
RETURN vmax_salary;
END get_maxsalary;
PROCEDURE set_minsalary(p_jobid VARCHAR2, pmin_salary NUMBER)
IS
BEGIN
UPDATE jobs
SET min_salary = pmin_salary
WHERE job_id = jobstab(p_jobid).job_id;
END set_minsalary;
PROCEDURE set_maxsalary(p_jobid VARCHAR2, pmax_salary NUMBER)
IS
BEGIN
UPDATE jobs
SET max_salary = pmax_salary
WHERE job_id = jobstab(p_jobid).job_id;
END set_maxsalary;
END jobs_pkg;
c. Copy the CHECK_SALARY procedure from Practice 9, Exercise 1a, and modify the code by replacing the query on the JOBS table with statements to set the local minsal and maxsal variables with values from the JOBS_PKG data by calling the appropriate GET_*SALARY functions. This step should eliminate the mutating trigger exception.
CREATE OR REPLACE PROCEDURE check_salary (pjobid employees.job_id%type, psal employees.salary%type)
IS
vminsalary jobs.min_salary%type;
vmaxsalary jobs.max_salary%type;
BEGIN
vminsalary := jobs_pkg.get_minsalary(pjobid);
vmaxsalary := jobs_pkg.get_maxsalary(pjobid);
END check_salary;
and this is my code now:
CREATE OR REPLACE PACKAGE BODY jobs_pkg
IS
TYPE jobs_tab_type IS TABLE OF jobs%rowtype INDEX BY jobs.job_id%type;
jobstab jobs_tab_type;
PROCEDURE initialize
IS
BEGIN
FOR rec IN (SELECT * FROM jobs)
LOOP
jobstab(rec.job_id) := rec;
END LOOP;
END;
FUNCTION get_minsalary(p_jobid VARCHAR2)
RETURN NUMBER
IS
BEGIN
RETURN jobstab(p_jobid).min_salary;
END get_minsalary;
FUNCTION get_maxsalary(p_jobid VARCHAR2)
RETURN NUMBER
IS
BEGIN
RETURN jobstab(p_jobid).max_salary;
END get_maxsalary;
PROCEDURE set_minsalary(p_jobid VARCHAR2, pmin_salary NUMBER)
IS
BEGIN
jobstab(p_jobid).min_salary := pmin_salary;
END set_minsalary;
PROCEDURE set_maxsalary(p_jobid VARCHAR2, pmax_salary NUMBER)
IS
BEGIN
jobstab(p_jobid).max_salary := pmax_salary;
END set_maxsalary;
END jobs_pkg;
Then I removed the for each row from my initializer procedure that is found on my jobs package:
From this:
CREATE OR REPLACE TRIGGER init_jobpkg_trg
BEFORE UPDATE OR INSERT ON jobs
FOR EACH ROW
BEGIN
jobs_pkg.initialize;
END init_jobpkg_trg;
To this:
CREATE OR REPLACE TRIGGER init_jobpkg_trg
BEFORE INSERT OR UPDATE ON jobs
BEGIN
DBMS_OUTPUT.PUT_LINE('-----Initializing-----');
jobs_pkg.initialize;
END init_jobpkg_trg;
After testing of updaing the min_salary of the jobs table, it all now works :)