Stop Dirty Reads with Proper Transaction Isolation

Using Isolation Level to Prevent Dirty Reads

Write a PostgreSQL query to demonstrate that setting an isolation level prevents dirty reads by updating and then selecting data in a transaction.

Solution:

-- Begin a transaction with READ COMMITTED isolation to prevent dirty reads.
BEGIN;
SET TRANSACTION ISOLATION LEVEL READ COMMITTED;

-- Update the status of an order.
UPDATE Orders
SET status = 'Processing'
WHERE order_id = 150;

-- Immediately select the updated order.
SELECT order_id, status FROM Orders WHERE order_id = 150;

-- Commit the transaction.
COMMIT;

Explanation:

• Purpose of the Query:
• The goal is to ensure that the transaction does not read uncommitted changes from other transactions.
• This demonstrates prevention of dirty reads using an appropriate isolation level.
• Key Components:
• The UPDATE and SELECT operations performed within the same transaction.
• SET TRANSACTION ISOLATION LEVEL READ COMMITTED; ensures only committed data is visible.
• Real-World Application:
• Critical in order processing systems where reading uncommitted data can lead to inaccurate information.

Notes:

• READ COMMITTED isolation prevents dirty reads by design.
• Other isolation levels provide further guarantees on data consistency.

For more Practice: Solve these Related Problems:

• Write a PostgreSQL query to start a transaction with READ COMMITTED isolation, update a record in the Orders table, and immediately select the updated record to verify no dirty reads occur.
• Write a PostgreSQL query to begin a transaction with READ COMMITTED isolation and update a record in the Sessions table, then perform a SELECT to confirm only committed data is visible.
• Write a PostgreSQL query to use READ COMMITTED isolation in a transaction that updates the status in the Tasks table and then reads the same record for verification.
• Write a PostgreSQL query to demonstrate that under READ COMMITTED isolation, concurrent uncommitted changes from other transactions are not visible in a SELECT query.

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