Explain PostgreSQL LISTEN/NOTIFY mechanism.

LISTEN/NOTIFY provides asynchronous communication between database sessions.

How it works:
• Sessions LISTEN to channels
• Other sessions send NOTIFY messages
• Listeners receive notifications asynchronously

Example:

-- Session 1: Listen for notifications
LISTEN order_updates;

-- Session 2: Send notification
NOTIFY order_updates, 'New order #123';

-- Session 1 receives the notification

Use Cases:
• Real-time updates
• Cache invalidation
• Event-driven architecture