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Solving Oracle enq: TM Contention Wait Events: A Case Study

Dean Richards, Sr. DBA Confio Software

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Case study: Customer experienceing enq: TM waits

• Sessions blocked on customer’s database• enq: TM – contention wait events• Executing simple INSERT statements similar to:

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

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What’s going on?

• Reviewed the SUPPLIER table– Found a foreign key constraint referencing the

PRODUCT table that didn’t have an associated index

• Confirmed by reviewing Top Objects in Ignite– All the wait time was associated with the

PRODUCT table

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How did we fix it?

• Added index on the column referencing the PRODUCT table

• Found the blocking sessions using Ignite—what was happening?– Periodically, the company reviewed its vendor list

and "cleaned up” SUPPLIER table– As a result, rows from SUPPLIER table were

deleted– Those DELETE statements then cascaded to the

PRODUCT table and took out TM locks on it

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What’s really happening under the covers?

• Reproduced customer’s issue to see what happens under the covers– Created a subset of the tables from this customer and loaded them

with sample data

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What’s really happening under the covers?

• Reproduced statements similar to original situation:

• User 1 and User 2 hung with enq: TM – contention wait events

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What’s really happening under the covers?

• Reviewed V$SESSION:

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Reproducing the solution

• Added an index for the foreign key constraint on the SUPPLIER table back to the PRODUCT table

• Re-ran the test case and everything worked fine – No exclusive locks acquired, 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

• Conclusion: unindexed foreign key constraints can cause severe problems!

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Sample query to find unindexed foreign key constraints

SELECT * FROM ( SELECT c.table_name, cc.column_name,

cc.position column_position FROM user_constraints c, user_cons_columns ccWHERE c.constraint_name = cc.constraint_nameAND 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;

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