USE employees;

/* Creating an index for an index-covered query */
    CREATE INDEX dept_no_from_date_idx ON dept_emp (dept_no, from_date);

/* Show `dept_emp` table structure, indexes and generic data */
    SHOW TABLE STATUS LIKE "dept_emp";
    DESCRIBE dept_emp;
    SHOW KEYS IN dept_emp;

/* The EXPLAIN shows that the subquery doesn't use a covering-index */
EXPLAIN SELECT SQL_NO_CACHE * FROM dept_emp INNER JOIN (
        /* The subquery should use a covering index, but isn't */
        SELECT SQL_NO_CACHE emp_no, dept_no FROM dept_emp WHERE dept_no="d001" ORDER BY from_date DESC LIMIT 20000,50
    ) AS `der` USING (`emp_no`, `dept_no`);

/* The EXPLAIN shows that the subquery DOES use a covering-index,
        thanks to the FORCE INDEX clause */
EXPLAIN SELECT SQL_NO_CACHE * FROM dept_emp INNER JOIN (
        /* The subquery use a covering index */
        SELECT SQL_NO_CACHE emp_no, dept_no FROM dept_emp FORCE INDEX(dept_no_from_date_idx) WHERE dept_no="d001" ORDER BY from_date DESC LIMIT 20000,50
    ) AS `der` USING (`emp_no`, `dept_no`);