Confio Software

Recently, I was assisting one of our customers of Ignite for Oracle trying to diagnose sessions waiting on the "enq: TM - contention" event.  The blocked sessions were executing simple INSERT statements similar to:

INSERT INTO supplier VALUES (:1, :2, :3);

Waits on "enq: TM - contention" indicate there are unindexed foreign key constraints.  Reviewing the SUPPLIER table, we found a foreign key constraint referencing the PRODUCT table that did not have an associated index.  This was also confirmed by the Top Objects feature of Ignite for Oracle because all the time was associated with the PRODUCT table.  We added the index on the column referencing the PRODUCT table and the problem was solved.

Cause

After using Ignite for Oracle's locking feature to find the blocking sessions, we found the real culprit.  Periodically, as the company reviewed its vendor list, they "cleaned up" the SUPPLIER table several times a week.  As a result, rows from the SUPPLIER table were deleted.  Those delete statements were then cascading to the PRODUCT table and taking out TM locks on it.

Reproducing the Problem

This problem has a simple fix, but I wanted to understand more about why this happens.  So I reproduced the issue to see what happens under the covers. I first created a subset of the tables from this customer and loaded them with sample data.

CREATE TABLE supplier
    (     supplier_id     number(10)     not null,
        supplier_name     varchar2(50)     not null,
        contact_name     varchar2(50),    
        CONSTRAINT supplier_pk PRIMARY KEY (supplier_id)
    );

INSERT INTO supplier VALUES (1, 'Supplier 1', 'Contact 1');
INSERT INTO supplier VALUES (2, 'Supplier 2', 'Contact 2');
COMMIT;

CREATE TABLE product
    (     product_id     number(10)     not null,
        product_name    varchar2(50)    not null,
        supplier_id     number(10)     not null,
        CONSTRAINT fk_supplier
          FOREIGN KEY (supplier_id)
         REFERENCES supplier(supplier_id)
         ON DELETE CASCADE
    );

INSERT INTO product VALUES (1, 'Product 1', 1);
INSERT INTO product VALUES (2, 'Product 2', 1);
INSERT INTO product VALUES (3, 'Product 3', 2);
COMMIT;

I then executed statements similar to what we found at this customer:

User 1: DELETE supplier WHERE supplier_id = 1;
User 2: DELETE supplier WHERE supplier_id = 2;
User 3: INSERT INTO supplier VALUES (5, 'Supplier 5', 'Contact 5');

Similar to the customer's experience, User 1 and User 2 hung waiting on "enq: TM - contention".  Reviewing information from V$SESSION I found the following:

SELECT l.sid, s.blocking_session blocker, s.event, l.type, l.lmode, l.request, o.object_name, o.object_type
FROM v$lock l, dba_objects o, v$session s
WHERE UPPER(s.username) = UPPER('&User')
AND   l.id1        = o.object_id (+)
AND   l.sid        = s.sid
ORDER BY sid, type;

 

SID	BLOCK	EVENT	TYPE	MODE	REQ	OBJECT	OBJECT_TYPE
42		SQL*Net message from client	TM	3	0	PRODUCT	TABLE
42		SQL*Net message from client	TM	3	0	SUPPLIER	TABLE
42		SQL*Net message from client	TX	6	0		TABLE
54	42	enq: TM - contention	TM	3	0	SUPPLIER	TABLE
54	42	enq: TM - contention	TM	0	5	PRODUCT	TABLE
83	54	enq: TM - contention	TM	3	0	SUPPLIER	TABLE
83	54	enq: TM - contention	TM	0	2	PRODUCT	TABLE

Following along with the solution we used for our customer, we added an index for the foreign key constraint on the SUPPLIER table back to the PRODUCT table:

CREATE INDEX fk_supplier ON product (supplier_id);

When we ran the test case again everything worked fine.  There were no exclusive locks acquired and hence no hanging.  Oracle takes out exclusive locks on the child table, the PRODUCT table in our example, when a foreign key constraint is not indexed.

Query to Find Unindexed Foreign Key Constraints

Now that we know unindexed foreign key constraints can cause severe problems, here is a script that I use to find them for a specific user (this can easily be tailored to search all schemas):

SELECT * FROM (
   SELECT c.table_name, cc.column_name, cc.position column_position
   FROM   user_constraints c, user_cons_columns cc
   WHERE  c.constraint_name = cc.constraint_name
   AND    c.constraint_type = 'R'
   MINUS
   SELECT i.table_name, ic.column_name, ic.column_position
   FROM   user_indexes i, user_ind_columns ic
   WHERE  i.index_name = ic.index_name
   )
ORDER BY table_name, column_position;

Dean Richards (deanrichards@confio.com)

Confio Software