Explanation:
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). By setting the OPTIMIZER_MODE into ALL_ROWS you will minimize TOTAL response time. The CBO is a statistics-driven optimizer, and mode ALL_ROWS, FIRST_ROWS, FIRST_ROWS_n, CHOOSE are CBO settings.
Incorrect Answers:
1: The rule-based optimizer mode does not use table or index statistics at all. When RULE is specified, statistics are not used. In this case total response time will not be minimal.
3: By setting the OPTIMIZER_MODE into FIRST_ROWS you will minimize response time for first row.
4: By setting the OPTIMIZER_MODE into FIRST_ROWS_n you will minimize response time for first N rows.
OCP: Oracle 9i Performance Tuning Study Guide, Joseph C. Johnson, p. 99-101
Chapter 3: SQL Application Tuning and Design

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