What are MySQL's invisible indexes and their use cases?

Invisible indexes are not used by the optimizer but maintained by DML operations.

Use cases:
• Test index effectiveness before dropping
• Gradual index deployment
• A/B testing query performance

Example:
CREATE INDEX idx_email ON users(email) INVISIBLE;
ALTER INDEX idx_email VISIBLE;

Query: SELECT * FROM users WHERE email = 'test@example.com';
-- Won't use invisible index unless forced