MySql范围查找时索引不生效问题的原因分析

CREATE TABLE `employees` (
 `emp_no` int(11) NOT NULL,
 `birth_date` date NOT NULL,
 `first_name` varchar(14) NOT NULL,
 `last_name` varchar(16) NOT NULL,
 `gender` enum('M','F') NOT NULL,
 `hire_date` date NOT NULL,
 `age` int(11) NOT NULL,
 PRIMARY KEY (`emp_no`),
 KEY `unique_birth_name` (`first_name`,`last_name`) USING BTREE
 ) ENGINE=InnoDB DEFAULT CHARSET=utf8;

EXPLAIN SELECT
 gender
FROM
 employees
ORDER BY
 first_name,
 last_name

EXPLAIN SELECT
 gender
FROM
 employees
WHERE first_name > 'Leah'
ORDER BY
 first_name,
 last_name

EXPLAIN SELECT
 gender
FROM
 employees
WHERE first_name > 'Tzvetan'
ORDER BY
 first_name,
 last_name

SELECT
 COUNT(first_name)
FROM
 employees
ORDER BY
 first_name,
 last_name

SELECT
 COUNT(first_name)
FROM
 employees
WHERE first_name > 'Leah'
ORDER BY
 first_name,
 last_name

SELECT
  COUNT(first_name)
FROM
  employees
WHERE first_name > 'Tzvetan'
ORDER BY
  first_name,
  last_name