A subquery that should be independent is not. Why?

Question

I have a table files with files and a table reades with read accesses to these files. In the table reades there is a column file_id where refers to the respective column in files.

Now I would like to list all files which have not been accessed and tried this:

SELECT * FROM files WHERE file_id NOT IN (SELECT file_id FROM reades)

This is terribly slow. The reason is that mySQL thinks that the subquery is dependent on the query:

+----+--------------------+--------+------+---------------+------+---------+------+------+----------+-------------+
| id | select_type        | table  | type | possible_keys | key  | key_len | ref  | rows | filtered | Extra       |
+----+--------------------+--------+------+---------------+------+---------+------+------+----------+-------------+
|  1 | PRIMARY            | files  | ALL  | NULL          | NULL | NULL    | NULL | 1053 |   100.00 | Using where |
|  2 | DEPENDENT SUBQUERY | reades | ALL  | NULL          | NULL | NULL    | NULL | 3242 |   100.00 | Using where |
+----+--------------------+--------+------+---------------+------+---------+------+------+----------+-------------+

But why? The subquery is completely independent and more or less just meant to return a list of ids.

(To be precise: Each file_id can appear multiple times in reades, of course, as there can be arbitrarily many read operations for each file.)

Answer 1

Try replacing the subquery with a join:

SELECT * 
FROM files f
LEFT OUTER JOIN reades r on r.file_id = f.file_id
WHERE r.file_id IS NULL

Here's a link to an article about this problem. The writer of that article wrote a stored procedure to force MySQL to evaluate subqueries as independant. I doubt that's necessary in this case though.