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Mutating Table Exceptions

Mutating table exceptions occur when we try to reference the triggering table in a query from within row-level trigger code. In this article I'll present examples of how a mutating table exception might occur and simple methods to get round it.

Test Schema

The following schema objects are necessary to run the code in this article.

CREATE TABLE tab1 (
  id           NUMBER(10) NOT NULL,
  description  VARCHAR2(50) NOT NULL
);

ALTER TABLE tab1 ADD (
  CONSTRAINT tab1_pk PRIMARY KEY (id)
);

CREATE SEQUENCE tab1_seq;


CREATE TABLE tab1_audit (
  id            NUMBER(10) NOT NULL,
  action        VARCHAR2(10) NOT NULL,
  tab1_id       NUMBER(10),
  record_count  NUMBER(10),
  created_time  TIMESTAMP
);

ALTER TABLE tab1_audit ADD (
  CONSTRAINT tab1_audit_pk PRIMARY KEY (id)
);

ALTER TABLE tab1_audit ADD (
  CONSTRAINT tab1_audit_tab1_fk FOREIGN KEY (tab1_id)
  REFERENCES tab1(id)
);

CREATE SEQUENCE tab1_audit_seq;

Mutating Table Demonstration

Let's assume we need to audit the actions on the parent table and for some reason, this involves querying the triggering table. We can demonstrate this with the following package and trigger.

We place all our trigger code into a package as follows.

CREATE OR REPLACE PACKAGE trigger_api AS

PROCEDURE tab1_row_change (p_id      IN  tab1.id%TYPE,
                           p_action  IN  VARCHAR2);

END trigger_api;
/
SHOW ERRORS


CREATE OR REPLACE PACKAGE BODY trigger_api AS

PROCEDURE tab1_row_change (p_id      IN  tab1.id%TYPE,
                           p_action  IN  VARCHAR2) IS
  l_count  NUMBER(10) := 0;
BEGIN
  SELECT COUNT(*)
  INTO   l_count
  FROM   tab1;

  INSERT INTO tab1_audit (id, action, tab1_id, record_count, created_time)
  VALUES (tab1_audit_seq.NEXTVAL, p_action, p_id, l_count, SYSTIMESTAMP);
END tab1_row_change;

END trigger_api;
/
SHOW ERRORS

Next we create the row-level trigger itself to catch any changes to the table.

CREATE OR REPLACE TRIGGER tab1_ariu_trg
AFTER INSERT OR UPDATE ON tab1
FOR EACH ROW
BEGIN
  IF inserting THEN
    trigger_api.tab1_row_change(p_id => :new.id, p_action => 'INSERT');
  ELSE
    trigger_api.tab1_row_change(p_id => :new.id, p_action => 'UPDATE');
  END IF;
END;
/
SHOW ERRORS

If we try to insert into the TAB1 table we might expect the insert to complete and the audit record to be created but as you can see below this is not the case.

SQL> INSERT INTO tab1 (id, description) VALUES (tab1_seq.NEXTVAL, 'ONE');
INSERT INTO tab1 (id, description) VALUES (tab1_seq.NEXTVAL, 'ONE')
                                           *
ERROR at line 1:
ORA-04091: table TEST.TAB1 is mutating, trigger/function may not see it
ORA-06512: at "TEST.TRIGGER_API", line 7
ORA-06512: at "TEST.TAB1_ARIU_TRG", line 3
ORA-04088: error during execution of trigger 'TEST.TAB1_ARIU_TRG'

SQL>

Solution 1 (Collection in Package Variable)

We can get round this issue by using a combination of row-level and statement-level triggers. First we alter the TRIGGER_API package to store any data passed by the row-level trigger in a PL/SQL table. We also add a new statement-level procedure to process each of the rows in the PL/SQL table.

CREATE OR REPLACE PACKAGE trigger_api AS

PROCEDURE tab1_row_change (p_id      IN  tab1.id%TYPE,
                           p_action  IN  VARCHAR2);

PROCEDURE tab1_statement_change;

END trigger_api;
/
SHOW ERRORS


CREATE OR REPLACE PACKAGE BODY trigger_api AS

TYPE t_change_rec IS RECORD (
  id      tab1.id%TYPE,
  action  tab1_audit.action%TYPE
);

TYPE t_change_tab IS TABLE OF t_change_rec;
g_change_tab  t_change_tab := t_change_tab();

PROCEDURE tab1_row_change (p_id      IN  tab1.id%TYPE,
                           p_action  IN  VARCHAR2) IS
BEGIN
  g_change_tab.extend;
  g_change_tab(g_change_tab.last).id     := p_id;
  g_change_tab(g_change_tab.last).action := p_action;
END tab1_row_change;

PROCEDURE tab1_statement_change IS
  l_count  NUMBER(10);
BEGIN
  FOR i IN g_change_tab.first .. g_change_tab.last LOOP
    SELECT COUNT(*)
    INTO   l_count
    FROM   tab1;

    INSERT INTO tab1_audit (id, action, tab1_id, record_count, created_time)
    VALUES (tab1_audit_seq.NEXTVAL, g_change_tab(i).action, g_change_tab(i).id, l_count, SYSTIMESTAMP);
  END LOOP;
  g_change_tab.delete;
END tab1_statement_change;

END trigger_api;
/
SHOW ERRORS

Our existing row-level trigger is fine, but we need to create a statement-level trigger to call our new procedure.

CREATE OR REPLACE TRIGGER tab1_asiu_trg
AFTER INSERT OR UPDATE ON tab1
BEGIN
  trigger_api.tab1_statement_change;
END;
/
SHOW ERRORS

The TAB1 inserts/updates will now work without mutation errors.

SQL> INSERT INTO tab1 (id, description) VALUES (tab1_seq.NEXTVAL, 'ONE');

1 row created.

SQL> INSERT INTO tab1 (id, description) VALUES (tab1_seq.NEXTVAL, 'TWO');

1 row created.

SQL> UPDATE tab1 SET description = description;

2 rows updated.

SQL> SELECT * FROM tab1;

        ID DESCRIPTION
---------- -----------
         2 ONE
         3 TWO

2 rows selected.

SQL> COLUMN created_time FORMAT A30
SQL> SELECT * FROM tab1_audit;

        ID ACTION        TAB1_ID RECORD_COUNT CREATED_TIME
---------- ---------- ---------- ------------ ------------------------------
         1 INSERT              2            1 03-DEC-03 14.42.47.515589
         2 INSERT              3            2 03-DEC-03 14.42.47.600550
         3 UPDATE              2            2 03-DEC-03 14.42.49.178678
         4 UPDATE              3            2 03-DEC-03 14.42.49.179655

4 rows selected.

SQL>

There is a flaw in this solution. Sometimes Oracle chooses to rollback changes and rerun the DML. If this happens, the contents of the PL/SQL table will not be cleaned up because it is not transactional. For this reason it is better to use the temporary table approach.

Note. The introduction of Compound Triggers in Oracle 11g Release 1 makes solving mutating table errors much simpler as a single trigger definition can perform most of the actions.

Solution 2 (Global Temporary Table)

An alternative strategy is to use a Global Temporary Table (GTT) in place of the collection. This is a better solution as the contents of the global temporary table are transactions, and so work as expected for automatic reruns of DML. First we need to create the temporary table to hold the data.

CREATE GLOBAL TEMPORARY TABLE tab1_mods (
  id      NUMBER(10),
  action  VARCHAR2(10)
) ON COMMIT DELETE ROWS;

Next, we recreate the package body to use the global temporary table in place of the collection.

CREATE OR REPLACE PACKAGE BODY trigger_api AS

PROCEDURE tab1_row_change (p_id      IN  tab1.id%TYPE,
                           p_action  IN  VARCHAR2) IS
BEGIN
  INSERT INTO tab1_mods (id, action) VALUES (p_id, p_action);
END tab1_row_change;

PROCEDURE tab1_statement_change IS
  l_count  NUMBER(10);
BEGIN
  FOR i IN (SELECT * FROM tab1_mods) LOOP
    SELECT COUNT(*)
    INTO   l_count
    FROM   tab1;

    INSERT INTO tab1_audit (id, action, tab1_id, record_count, created_time)
    VALUES (tab1_audit_seq.NEXTVAL, i.action, i.id, l_count, SYSTIMESTAMP);
  END LOOP;
  DELETE FROM tab1_mods;
END tab1_statement_change;

END trigger_api;
/
SHOW ERRORS

Once again, the inserts and updates work as expected.

SQL> INSERT INTO tab1 (id, description) VALUES (tab1_seq.NEXTVAL, 'THREE');

1 row created.

SQL> INSERT INTO tab1 (id, description) VALUES (tab1_seq.NEXTVAL, 'FOUR');

1 row created.

SQL> UPDATE tab1 SET description = description;

2 rows updated.

SQL> SELECT * FROM tab1;

        ID DESCRIPTION
---------- -----------
         2 ONE
         3 TWO
         4 THREE
         5 FOUR

4 rows selected.

SQL> COLUMN created_time FORMAT A30
SQL> SELECT * FROM tab1_audit;

        ID ACTION        TAB1_ID RECORD_COUNT CREATED_TIME
---------- ---------- ---------- ------------ ------------------------------
         1 INSERT              2            1 23-NOV-2011 13:24:48.300250
         2 INSERT              3            2 23-NOV-2011 13:24:54.744229
         3 UPDATE              2            2 23-NOV-2011 13:25:01.170393
         4 UPDATE              3            2 23-NOV-2011 13:25:01.170837
         5 INSERT              4            3 23-NOV-2011 13:27:03.765868
         6 INSERT              5            4 23-NOV-2011 13:27:10.651991
         7 UPDATE              2            4 23-NOV-2011 13:27:18.097429
         8 UPDATE              3            4 23-NOV-2011 13:27:18.097957
         9 UPDATE              4            4 23-NOV-2011 13:27:18.098176
        10 UPDATE              5            4 23-NOV-2011 13:27:18.098801

10 rows selected.

SQL>

For more information see:

Hope this helps. Regards Tim...

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