The query optimizer makes cost-based decisions to determine:

• Which index (if any) to use on each table in a query (see About the optimizer's choice of access path)
• The join order (see About the optimizer's choice of join order)
• The join strategy (see About the optimizer's choice of join strategy)
• Whether to avoid additional sorting (see About the optimizer's choice of sort avoidance)
• Automatic lock escalation (see About the system's selection of lock granularity)
• Whether to use bulk fetch (see About the optimizer's selection of bulk fetch)