EXPLAIN SELECT.., why TYPE = ALL?

EXPLAIN SELECT.., why TYPE = ALL?

Having these 3 tables:
users
CREATE TABLE `users` (
`user_id` MEDIUMINT(8) UNSIGNED NOT NULL AUTO_INCREMENT,
`first_name` VARCHAR(64) NOT NULL,
`last_name` VARCHAR(64) NOT NULL,
PRIMARY KEY (`user_id`)
)
COLLATE='utf8_general_ci'
ENGINE=InnoDB
AUTO_INCREMENT=1;
posts
CREATE TABLE `posts` (
`post_id` MEDIUMINT(8) UNSIGNED NOT NULL AUTO_INCREMENT,
`category_id` MEDIUMINT(8) UNSIGNED NOT NULL,
`author_id` MEDIUMINT(8) UNSIGNED NOT NULL,
`title` VARCHAR(128) NOT NULL,
`text` TEXT NOT NULL,
PRIMARY KEY (`post_id`),
INDEX `FK_posts__category_id` (`category_id`),
INDEX `FK_posts__author_id` (`author_id`),
CONSTRAINT `FK_posts__author_id` FOREIGN KEY (`author_id`) REFERENCES
`users` (`user_id`) ON UPDATE CASCADE,
CONSTRAINT `FK_posts__category_id` FOREIGN KEY (`category_id`)
REFERENCES `categories` (`category_id`) ON UPDATE CASCADE ON DELETE
CASCADE
)
COLLATE='utf8_general_ci'
ENGINE=InnoDB
AUTO_INCREMENT=1;
categories
CREATE TABLE `categories` (
`category_id` MEDIUMINT(8) UNSIGNED NOT NULL AUTO_INCREMENT,
`name` VARCHAR(64) NOT NULL,
PRIMARY KEY (`category_id`)
)
COLLATE='utf8_general_ci'
ENGINE=InnoDB
AUTO_INCREMENT=1;
And data in tables:
INSERT INTO `users` (`user_id`, `first_name`, `last_name`) VALUES
(1, 'John', 'Doe'),
(2, 'Pen', 'Poe'),
(3, 'Robert', 'Roe');
INSERT INTO `categories` (`category_id`, `name`) VALUES
(1, 'Category 1'),
(2, 'Category 2'),
(3, 'Category 3'),
(4, 'Category 4');
INSERT INTO `posts` (`post_id`, `category_id`, `author_id`, `title`,
`text`) VALUES
(1, 1, 1, 'title 1', 'text 1'),
(2, 1, 2, 'title 2', 'text 2');
I want to make a simple select (and let MySQL EXPLAIN it):
EXPLAIN SELECT p.post_id, p.title, p.text, c.category_id, c.name,
u.user_id, u.first_name, u.last_name
FROM posts AS p
JOIN categories AS c
ON c.category_id = p.category_id
JOIN users AS u
ON u.user_id = p.author_id
WHERE p.category_id = 1
I got this:

What I don't understand is, why has MySQL to do a full table scan at u
(users). I mean there will be only two users it has to retrieve data about
(with id 1 and 2), and these two can be found by primary key user_id. Can
somebody with more experience help me to understand this? Is there a
better way of creating indexes so MySQL don't has to make a full scan on
the users table to retrieve data about the post authors?
Thanks you!