What are PostgreSQL advisory locks and when to use them?

Advisory locks are application-level locks that don't lock actual database objects.

Types:
• Session-level locks
• Transaction-level locks
• Shared and exclusive modes

Use Cases:
• Preventing duplicate processing
• Coordinating batch jobs
• Application-level synchronization

Example:

-- Acquire exclusive lock
SELECT pg_advisory_lock(12345);

-- Try to acquire lock (non-blocking)
SELECT pg_try_advisory_lock(12345);

-- Release lock
SELECT pg_advisory_unlock(12345);