加入后索引是否仍然存在？

Question

鉴于以下关系：

create table Students (
sid     integer,     -- 4 bytes
name    varchar(30), -- average length 20 bytes (incl padding)
bday    date,        -- 4 bytes
degree  char(4),     -- 4 bytes
primary key (id)
);

create table Courses (
cid     integer,     -- 4 bytes
code    char(8),     -- 8 bytes
term    char(4),     -- 4 bytes
title   varchar(40), -- average length 24 bytes (incl padding)
primary key (cid)
);

create table Enrolments (
course  integer,     -- 4 bytes
student integer,     -- 4 bytes
mark    integer,     -- 4 bytes
grade   char(2),     -- 4 bytes (with padding)
foreign key (course) references Courses(cid),
foreign key (student) references Students(sid)
);

主键cid和sid每个都有一个两级B树索引。

如果我最初加入Courses和Enrolments，然后将此结果加入Students，那么最终表结果是否仍有{{1}上的两个索引}和cid？

如果有人能够解释这个的机制，我会非常感激！