In this post, I demonstrate the kind of scenario that will result in an ORA-04091 errors. I then show the "traditional" solution, using a collection defined in a package. Then I demonstrate how to use the compound trigger, added in Oracle Database 11g Release1, to solve the problem much more simply.
All the code shown in this example may be found in this LiveSQL script.
How to Get a Mutating Table Error
I need to implement this rule on my employees table:
Your new salary cannot be more than 25x the lowest salary in the company. Your salary will be automatically set to the maximum allowed, if this rule is broken.So just check to see if that rule is violated, right? Easy enough in PL/SQL, right in a database trigger (see links at bottom of post for discussions on whether or not you should put logic like this in your database triggers):
CREATE OR REPLACE TRIGGER equitable_salary_trg AFTER INSERT OR UPDATE ON employees FOR EACH ROW DECLARE l_max_allowed employees.salary%TYPE; BEGIN SELECT MIN (salary) * 25 INTO l_max_allowed FROM employees; IF l_max_allowed < :NEW.salary THEN UPDATE employees SET salary = l_max_allowed WHERE employee_id = :NEW.employee_id; END IF; END equitable_salary_trg;
Well....maybe not. I execute the following block:
BEGIN UPDATE employees SET salary = 100000 WHERE last_name = 'King'; END;
and I see this error:
ORA-04091: table EMPLOYEES is mutating, trigger/function may not see it ORA-06512: at "EQUITABLE_SALARY_TRG", line 4
OK, we get that, right? I am both selecting from and trying to update the EMPLOYEES table in a row-level trigger. That's the no-no.
Getting Around ORA-04091 with PL/SQL Packages
The solution, conceptually, is simple enough. If I can do task X in the row level trigger, save whatever information I need to perform X on that row in a to-do list (a collection, perhaps?). Then define an AFTER STATEMENT trigger that goes through the to-do list, and executes the desired logic for each row.
The traditional (now, out-of-date) solution is to define a package that contains a collection defined at the package level. Package-level variables have session scope. So I can add information to the collection within the row-level trigger, and it will still be there when I bubble up to the statement-level trigger.
Here's my package specification:
CREATE OR REPLACE PACKAGE equitable_salaries_pkg IS PROCEDURE initialize; PROCEDURE add_employee_info ( employee_id_in IN employees.employee_id%TYPE , salary_in IN employees.salary%TYPE ); PROCEDURE make_equitable; END equitable_salaries_pkg;
Huh. I don't see any collection there. Right. You shouldn't. If you put the collection in the specification, it can be modified by any schema with EXECUTE authority on the package, in whatever way anyone wants to mess with that collection. Well, that's no good. So I "hide" the list in the body and "expose" it through the procedures in the spec.
CREATE OR REPLACE PACKAGE BODY equitable_salaries_pkg IS TYPE id_salary_rt IS RECORD ( employee_id employees.employee_id%TYPE , salary employees.salary%TYPE ); TYPE g_emp_info_t IS TABLE OF id_salary_rt INDEX BY PLS_INTEGER; g_emp_info g_emp_info_t; g_corrections_in_process BOOLEAN := FALSE; PROCEDURE initialize IS BEGIN g_emp_info.DELETE; END initialize; PROCEDURE finished_corrections IS BEGIN g_corrections_in_process := FALSE; END finished_corrections; PROCEDURE starting_corrections IS BEGIN g_corrections_in_process := TRUE; END starting_corrections; FUNCTION corrections_in_process RETURN BOOLEAN IS BEGIN RETURN g_corrections_in_process; END corrections_in_process; PROCEDURE add_employee_info ( employee_id_in IN employees.employee_id%TYPE , salary_in IN employees.salary%TYPE ) IS l_index PLS_INTEGER := g_emp_info.COUNT + 1; BEGIN IF NOT corrections_in_process THEN g_emp_info (l_index).employee_id := employee_id_in; g_emp_info (l_index).salary := salary_in; END IF; END add_employee_info; PROCEDURE make_equitable IS l_max_allowed employees.salary%TYPE; l_index PLS_INTEGER; BEGIN IF NOT corrections_in_process THEN starting_corrections; SELECT MIN (salary) * 25 INTO l_max_allowed FROM employees; WHILE (g_emp_info.COUNT > 0) LOOP l_index := g_emp_info.FIRST; IF l_max_allowed < g_emp_info (l_index).salary THEN UPDATE employees SET salary = l_max_allowed WHERE employee_id = g_emp_info (l_index).employee_id; END IF; g_emp_info.DELETE (g_emp_info.FIRST); END LOOP; finished_corrections; END IF; END make_equitable; END equitable_salaries_pkg;
See? Aren't you glad I wrote that, so you didn't have to? :-) Well, it gets better - as in lots of that code is unnecessary. But before I get to that, let's finish up the old-style approach. We need to rebuild the triggers!
1. Before getting started, make sure no one is going to muck with those rows. And make sure the package-based collection is empty.
CREATE OR REPLACE TRIGGER equitable_salaries_bstrg before INSERT OR UPDATE ON employees BEGIN LOCK TABLE employees IN EXCLUSIVE MODE; equitable_salaries_pkg.initialize; END;
2. For each insert or update to employees, add the necessary information to the to-do list.
CREATE OR REPLACE TRIGGER equitable_salaries_rtrg AFTER INSERT OR UPDATE OF salary ON employees FOR EACH ROW BEGIN equitable_salaries_pkg.add_employee_info (:NEW.employee_id, :NEW.salary); END;
3. Create a statement-level trigger to apply the rule.
CREATE OR REPLACE TRIGGER equitable_salaries_astrg AFTER INSERT OR UPDATE ON employees BEGIN equitable_salaries_pkg.make_equitable; END;
And now the update statement will work without raising any ORA-04091 errors!
BEGIN UPDATE employees SET salary = 100000 WHERE last_name = 'King'; ROLLBACK; END; add_employee_info: 100-100000 add_employee_info: 156-100000 make_equitable max allowed 52500 make_equitable emp id and salary: 100-100000
Yep. That's a lot of code to write and deal with to get around this problem. So several years back, the PL/SQL team decided to make things easier for their users with....
The Compound DML Trigger
Straight from the doc: A compound DML trigger created on a table or editioning view can fire at multiple timing points. Each timing point section has its own executable part and optional exception-handling part, but all of these parts can access a common PL/SQL state. The common state is established when the triggering statement starts and is destroyed when the triggering statement completes, even when the triggering statement causes an error. Two common uses of compound triggers are: (1) To accumulate rows destined for a second table so that you can periodically bulk-insert them; (2) To avoid the mutating-table error (ORA-04091).
The compound trigger more allows you to define variables which persist through the execution of the steps defined in the compound trigger. And that's the aspect of this feature that makes things so much easier when it comes to mutable table errors.
Using this feature, I can combine all the different trigger events and code, plus they share scope like the subprograms of a package body. So I declare a variable in the compound trigger and reference it in both trigger events. Take a look:
CREATE OR REPLACE TRIGGER equitable_salary_trg FOR UPDATE OR INSERT ON employees COMPOUND TRIGGER TYPE id_salary_rt IS RECORD ( employee_id employees.employee_id%TYPE , salary employees.salary%TYPE ); TYPE row_level_info_t IS TABLE OF id_salary_rt INDEX BY PLS_INTEGER; g_row_level_info row_level_info_t; AFTER EACH ROW IS BEGIN g_row_level_info (g_row_level_info.COUNT + 1).employee_id := :NEW.employee_id; g_row_level_info (g_row_level_info.COUNT).salary := :NEW.salary; END AFTER EACH ROW; AFTER STATEMENT IS l_max_allowed employees.salary%TYPE; BEGIN SELECT MIN (salary) * 25 INTO l_max_allowed FROM employees; FOR indx IN 1 .. g_row_level_info.COUNT LOOP IF l_max_allowed < g_row_level_info (indx).salary THEN UPDATE employees SET salary = l_max_allowed WHERE employee_id = g_row_level_info (indx).employee_id; END IF; END LOOP; END AFTER STATEMENT; END equitable_salary_trg;
Much simpler - all relevant code in one place.
More reliable - you don't have to worry about managing the session-persistent collection.
Less code - always a nice thing, as long as the "less code" is also understandable and easy to maintain.
You might also find these resources helpful:
ORACLE-BASE: Trigger Enhancements in Oracle Database 11g Release 1
ORACLE-BASE: Should you use triggers at all? (Facts, Thoughts and Opinions)
Toon Koopelars: Triggers Considered Harmful, Considered Harmful