One thing is sure that the default value 0 and 100 for OPTIMIZER_INDEX_CACHING and OPTIMIZER_INDEX_COST_ADJ respectively are set for Data Warehouse System. If your system is OLTP, then these 2 parameters values should be changed. I would test the system with different values before choosing the correct values. Setting 90 to OPTIMIZER_INDEX_CACHING and 10 (or 15) to OPTIMIZER_INDEX_COST_ADJ will perform good for OLTP system. If you collect system statistics, then these 2 parameters play less importance role in deciding the execution plan.