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Level 2

Probably yes,

It looks like comment - it informs database how process query

for ex.

SELECT /*+ LEADING(e2 e1) USE_NL(e1) INDEX(e1 emp_emp_id_pk)

  USE_MERGE(j) FULL(j) */

  e1.first_name, e1.last_name, j.job_id, sum(e2.salary) total_sal

  FROM employees e1, employees e2, job_history j

  WHERE e1.employee_id = e2.manager_id

  AND e1.employee_id = j.employee_id

  AND e1.hire_date = j.start_date

  GROUP BY e1.first_name, e1.last_name, j.job_id

  ORDER BY total_sal;

LEADING - about join order

USE_NL - use nested loops algorithm for ....

INDEX - use index ......

USE_MERGE - merge join algorithm

FULL(j) - full table scan of j