What is MySQL's adaptive hash index and when is it used?

Adaptive hash index is automatically created by InnoDB for frequently accessed pages.

Characteristics:
• Built automatically on B-tree indexes
• Uses hash function for O(1) lookups
• Only for equality searches
• Cannot be manually created

Benefits:
• Faster point queries
• Reduced CPU usage
• Automatic optimization

Controlled by innodb_adaptive_hash_index parameter.