Why MYSQL doesn't use the index for the same query if I query more columns?

I have the following table:

create table stuff (
       id mediumint unsigned not null auto_increment primary key,
       title varchar(150) not null,
       link varchar(250) not null,
       time timestamp default current_timestamp not null,
       content varchar(1500)
);   

If I EXPLAIN the query

select id from stuff order by id;

then it says it uses they primary key as an index for ordering the results. But with this query:

select id,title from stuff order by id;

EXPLAIN says no possible ke开发者_如何学JAVAys and it resorts to filesort.

Why is that? Isn't the data of a certain row stored together in the database? If it can order the results using the index when I'm querying only the id then why adding an other column to the query makes a difference? The primary key identifies the row already, so I think it should use the primary key for ordering in the second case too.

Can you explain why this is not the case?

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Sure, because it is more performant in this query: you need to read full index and after that iteratively read row by row from data. This is extremely unefficient. Instead of this mysql just prefers to read the data right from the data file.

Also, what kind of storage engine do you use? Seems like mysam.

For this case innodb would be more efficient, since it uses clustered indexes over primary key (which is monotonously growing in your case).