Prevent Delays by Using NOWAIT in Row Locking

Use NOWAIT to Avoid Lock Waits

Write a PostgreSQL query to lock a row using SELECT FOR UPDATE with the NOWAIT option to avoid waiting if the row is already locked.

Solution:

-- Attempt to lock the employee row with ID 101 without waiting.
SELECT * FROM Employees -- Specify the table to query.
WHERE employee_id = 101
FOR UPDATE NOWAIT; -- Do not wait if the row is locked.

Explanation:

• Purpose of the Query:
• The goal is to immediately return an error instead of waiting for the lock if the row is already held by another transaction.
• This demonstrates how NOWAIT can be used to prevent long lock wait times.
• Key Components:
• FOR UPDATE NOWAIT : Combines row locking with a non-waiting mechanism.
• WHERE employee_id = 101 : Identifies the specific row for locking.
• Real-World Application:
• Useful in high-concurrency environments where waiting for locks is not acceptable and prompt error handling is needed.

Notes:

• If the row is locked, the query will raise an error immediately.
• Applications must handle the error gracefully, possibly by retrying later.

For more Practice: Solve these Related Problems:

• Write a PostgreSQL query to lock a specific row in the Sessions table using SELECT FOR UPDATE NOWAIT where session_id = 300.
• Write a PostgreSQL query to lock a record in the Orders table using NOWAIT to immediately raise an error if the row is locked, filtering by order_id = 789.
• Write a PostgreSQL query to attempt locking a customer record in the Customers table with NOWAIT, where customer_id = 150.
• Write a PostgreSQL query to lock a row in the Products table using SELECT FOR UPDATE NOWAIT when product_id = 250.

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