How SELECT statement can cause disk write IO - create tables

create_tables.sql

CREATE TABLE author (
    id bigserial primary key, 
    name varchar(64) not null, country varchar(64)
);

CREATE TABLE book (
    id bigserial primary key, 
    title varchar(128) not null, 
    author_id bigint NOT NULL, 
    CONSTRAINT fk_books_author_id FOREIGN KEY (author_id) REFERENCES author(id)
);

CREATE INDEX idx_book_author_id ON book(author_id);