MySQL联合索引VS单列索引

以一个一千万数据量的表格为例

1. 建表建索引

USE foo;
DROP TABLE IF EXISTS tmp;
CREATE TABLE tmp (
  id         INT UNSIGNED PRIMARY KEY AUTO_INCREMENT,
  school_id  INT UNSIGNED NOT NULL,
  student_id INT UNSIGNED NOT NULL,
  INDEX school_id(school_id),
  INDEX student_id(student_id),
  INDEX school_id_and_student_id(school_id, student_id)
);U