oracle中OPTIMIZER_MODE与隐含参数_optimizer_cost_model

1. OPTIMIZER_MODE Parameter

Value Description
ALL_ROWS
The optimizer uses a cost-based approach for all SQL statements in the session regardless of the presence of statistics and optimizes with a goal of best throughput (minimum resource use to complete the entire statement). This is the default value.

FIRST_ROWS_n
The optimizer uses a cost-based approach, regardless of the presence of statistics, and optimizes with a goal of best response time to return the first n number of rows; n can equal 1, 10, 100, or 1000.

FIRST_ROWS
The optimizer uses a mix of cost and heuristics to find a best plan for fast delivery of the first few rows.
Note: Using heuristics sometimes leads the query optimizer to generate a plan with a cost that is significantly larger than the cost of a plan without applying the heuristic. FIRST_ROWS is available for backward compatibility and plan stability; use FIRST_ROWS_n instead.

CHOOSE
This parameter value has been desupported.

RULE
This parameter value has been desupported.

2. _optimizer_cost_model

Oracle 10g has recognized this trend toward CPU-based optimization by providing you with the ability to choose CPU-based or I/O-based costing during SQL optimization (the 10g default is CPU-costing). 
In Oracle10g, system stats are gathered by default, and in Oracle9i the DBA must manually execute the dbms_stat.gather_system_stats package to get CBO statistics.

alter session set "_optimizer_cost_model"=choose; -- default value
alter session set "_optimizer_cost_model"=io;
alter session set "_optimizer_cost_model"=cpu;