MySQL组合索引与最左匹配原则详解

CREATE TABLE `student` (
 `id` int(11) NOT NULL,
 `name` varchar(10) NOT NULL,
 `age` int(11) NOT NULL,
 PRIMARY KEY (`id`),
 KEY `idx_id_name_age` (`id`,`name`,`age`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8

DROP PROCEDURE pro10;
CREATE PROCEDURE pro10()
BEGIN
	DECLARE i INT;
	DECLARE char_str varchar(100) DEFAULT 'abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789';
	DECLARE return_str varchar(255) DEFAULT '';
	DECLARE age INT;
	SET i = 1;
	WHILE i < 5000000 do
		SET return_str = substring(char_str, FLOOR(1 + RAND()*62), 8);
		SET i = i+1;
		SET age = FLOOR(RAND() * 100);
		INSERT INTO student(id, name, age) values(i, return_str, age);
	END WHILE;
END;

CALL pro10();

EXPLAIN SELECT * FROM student WHERE id = 2;

EXPLAIN SELECT * FROM student WHERE id = 2 AND name = 'defghijk';

EXPLAIN SELECT * FROM student WHERE id = 2 AND name = 'defghijk' and age = 8;

EXPLAIN SELECT * FROM student WHERE id = 2 AND age = 8;

EXPLAIN SELECT * FROM student WHERE name = 'defghijk' AND age = 8;

EXPLAIN SELECT * FROM student WHERE name = 'defghijk' AND id = 2;

EXPLAIN SELECT * FROM student WHERE age = 8 AND id = 2;

EXPLAIN SELECT * FROM student WHERE name = 'defghijk' and age = 8 AND id = 2;