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A mutating table error is the most common runtime error for database triggers. This type of error occurs when a row-level database trigger (or an object called from within the trigger) violates one of the following several rules for SQL statements:
Following, youll find an example of triggers that attempt to violate these rules. Youll also find a way to write triggers that dont break these rules.
In its simplest incarnation, this error is a reference to the triggers associated table by the trigger itself. The table is in a state of flux because an operation is in progress, so the trigger cant read from the table. Consider the trigger in Listing 7.5, which attempts to read from the SCHEDULED_CLASSES table when a row is created or modified.
Listing 7.5 A trigger that causes a mutating table error.
CREATE OR REPLACE TRIGGER SCHEDULED_CLASSES_ARU AFTER UPDATE OF course_time, course_location ON SCHEDULED CLASSES FOR EACH ROW DECLARE xLOCATION_CONFLICT EXCEPTION; BEGIN SELECT 1 INTO nClassConflicts FROM SCHEDULED_CLASSES WHERE course_number = :new.course_number AND course_location = :new.course_location AND course_time = :new.course_time; RAISE xLOCATION_CONFLICT; EXCEPTION WHEN NO_DATA_FOUND THEN NULL; END SCHEDULED_CLASSES_ARU; /
This trigger is designed to verify that there are no scheduling conflicts for the class based on time or location. The trigger fails because it must reference its own associated table, SCHEDULED_CLASSES, to determine if there is a conflict.
An Exception To This Rule
The before row INSERT trigger and the after row INSERT trigger are the only types of triggers that can read from or modify a triggers base table. This may only be done when the INSERT statement on the triggers associated table generated only one new row of data. Consider the trigger in Listing 7.6.
Listing 7.6 A trigger that can read from its associated table.
CREATE OR REPLACE TRIGGER CLASSES_ARI AFTER INSERT ON CLASSES FOR EACH ROW DECLARE xLOCATION_CONFLICT EXCEPTION; BEGIN SELECT 1 INTO nClassConflicts FROM CLASSES WHERE course_number = :new.course_number AND course_location = :new.course_location AND course_time = :new.course_time; RAISE xLOCATION_CONFLICT; EXCEPTION WHEN NO_DATA_FOUND THEN NULL; END CLASSES_ARI; /
This trigger will not cause a mutating table error as long as only a single class is inserted into the CLASSES table at one time. Its possible to insert multiple rows of data using a multiple INSERT statement like this one:
INSERT INTO CLASSES SELECT * FROM IMPORTED_CLASSES;
Even if this statement returns only one row, its still a multiple INSERT statement, and a mutating table error will result.
Tables that have a foreign key to the triggers associated table will generate a mutating table error if you attempt to read from or modify any of that tables indexed columns.
The trigger in Listing 7.7 is designed to prevent a courses location and time from being changed if at least one student is scheduled for the course. However, in order to do so, it must make a reference to the SCHEDULED_CLASSES table to determine which students are already enrolled in the course. This causes a mutating table error because the act of updating the primary key of the CLASSES table impacts the foreign key from the SCHEDULED_CLASSES table.
Listing 7.7 Referencing a foreign key column in another table.
CREATE OR REPLACE TRIGGER CLASSES_BU BEFORE UPDATE OF course_time, course_location ON CLASSES DECLARE nTotalStudents number; xCLASS_HAS_STUDENTS EXCEPTION; BEGIN SELECT count (course_number) INTO nTotalStudents FROM SCHEDULED_CLASSES WHERE course_number = :new.course_number; IF (nTotalStudents > :new.maximum_enrollment) THEN RAISE xCLASS_HAS_STUDENTS; END IF; END CLASSES_BU /
A cascading delete occurs when a table with a foreign key reference to another table is given the ON DELETE CASCADE constraint. This constraint instructs Oracle to always delete the child rows of deleted parent rows. The use of this constraint is extremely common because it prevents application developers from writing code that leaves leftover children hanging around to muck up the works.
Unfortunately, this can cause problems with database triggers that fire on child tables due to a delete on parent tables. If the DELETE trigger attempts to reference the parent table, a mutating table error occurs. The same situation occurs if a DELETE trigger on a parent attempts to reference the child table.
This type of mutating table error occurs quite frequently in both statement-level and row-level database triggers, and there is no acceptable workaround using either type of trigger. The only real option is to not use the ON DELETE CASCADE constraint.
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