Wednesday, 7 June 2017

Solving Oracle enq: TM Contention Wait Events

Solving Oracle enq: TM Contention Wait Events

Recently, I was assisting one of our customers 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 SolarWinds Database Performance Analyzer (DPA) 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.
Finding the root cause of the enq: TM – contention wait event
After using DPA 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 a typical problem that leads to this wait
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'); 

( 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) 
INSERT INTO product VALUES (1, 'Product 1', 1); 
INSERT INTO product VALUES (2, 'Product 2', 1); 
INSERT INTO product VALUES (3, 'Product 3', 2); 
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;
42SQL*Net message from clientTM30PRODUCTTABLE
42SQL*Net message from clientTM30SUPPLIERTABLE
42SQL*Net message from clientTX60TABLE
5442enq: TM – contentionTM30SUPPLIERTABLE
5442enq: TM – contentionTM05PRODUCTTABLE
8354enq: TM – contentionTM30SUPPLIERTABLE
8354enq: TM – contentionTM02PRODUCTTABLE
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.
Sample 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 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'
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;


  1. This post looks an awful lot like this one

    1. it seems to be shamefully copied from the logicalread article.


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