Implement Uniform Lock Ordering to Eliminate Deadlocks

Enforce Consistent Lock Ordering

Write a PostgreSQL query to demonstrate how acquiring locks in a consistent order can prevent deadlocks.

Solution:

-- Begin the transaction.
BEGIN;

-- First, lock TableA and then TableB in the same order.
SELECT * FROM TableA
WHERE id = 1
FOR UPDATE;

SELECT * FROM TableB
WHERE id = 2
FOR UPDATE;

-- Perform necessary updates.
UPDATE TableA SET value = value + 1 WHERE id = 1;
UPDATE TableB SET value = value + 1 WHERE id = 2;

-- Commit the transaction.
COMMIT;

Explanation:

• Purpose of the Query:
• The goal is to ensure that locks are acquired in a predetermined order to prevent deadlock conditions.
• This demonstrates the best practice of consistent lock ordering across transactions.
• Key Components:
• Sequential SELECT FOR UPDATE statements to lock TableA before TableB.
• UPDATE statements that follow the locked order.
• Real-World Application:
• Essential in multi-table transactional operations where consistent lock acquisition can prevent deadlocks.

Notes:

• Always enforce a global order for acquiring locks in concurrent transactions.
• Consistent ordering reduces the likelihood of cyclic dependencies.

For more Practice: Solve these Related Problems:

• Write a PostgreSQL query to lock rows in TableA and then TableB in that order using SELECT FOR UPDATE to enforce consistent lock ordering.
• Write a PostgreSQL query to update two related tables, ensuring that Table1 is always locked before Table2 in the transaction.
• Write a PostgreSQL query to lock the Customers table followed by the Orders table using FOR UPDATE in a consistent order to avoid deadlocks.
• Write a PostgreSQL query to perform two updates in a transaction that first locks the Departments table and then the Employees table to enforce a consistent lock sequence.

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Previous PostgreSQL Exercise: Simulate a Deadlock Scenario.
Next PostgreSQL Exercise: Acquire an Advisory Lock.