为什么MySQL会为复合键创建一个自动的“只有一个”索引?

In this code:

在这段代码中:

CREATE TABLE institution (
    iid INT(6) AUTO_INCREMENT,
    name VARCHAR(40) NOT NULL,
    PRIMARY KEY (iid))
ENGINE = INNODB;

CREATE TABLE plan (
    pid INT(2) AUTO_INCREMENT,
    name VARCHAR(40) NOT NULL,
    PRIMARY KEY (pid))
ENGINE = INNODB;

CREATE TABLE subscription (
    iid INT(6),
    pid INT(2),
    PRIMARY KEY (iid, pid),
    CONSTRAINT iid_FOREIGN_KEY FOREIGN KEY (iid) REFERENCES institution (iid),
    CONSTRAINT pid_FOREIGN_KEY FOREIGN KEY (pid) REFERENCES plan (pid))
ENGINE = INNODB;
CREATE TABLE institut