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.

If the optimizer uses the cost-based approach for a SQL statement, and if some
tables accessed by the statement have no statistics, then the optimizer uses
internal information, such as the number of data blocks allocated to these
tables, to estimate other statistics for these tables.