Question

I’m not great with MySQL, so I often find myself preparing sub-optimal queries that work, but I know must be horribly inefficient. I’m hoping you guys could give me some pointers on why the following query doesn’t work well, and what methods I should use to accomplish similar queries.

I have the following table structure:

TABLE Files
files_id => INT(12), PRIMARY, AUTO INCREMENT, NOT NULL
files_name => VARCHAR(255), NOT NULL
(some other fields such as file type etc)

TABLE File_Permissions
perm_id => INT(12), PRIMARY, AUTO INCREMENT, NOT NULL
perm_files_id => INT(12), NOT NULL
perm_users_id => INT(12), NOT NULL

I pull a list of the files a user is allowed to view with the following SQL:

SELECT files_name FROM Files WHERE files_id IN 
    (SELECT perm_files_id FROM File_Permissions WHERE perm_users_id = 'xxxxxx');

This, as far as I can tell, will go through each of the thousands of records in the Files table, and for each one execute a subquery that selects from the File_Permissions table to check against the user’s ID.

This takes almost 2 seconds per query. I’m sure something is fundamentally wrong with this, I just don’t know what it is.

Thanks so much for the help!

Answer 1

Most queries that involve an IN clause for a subquery can be refactored to use a join. In your case:

SELECT files_name 
FROM Files 
JOIN File_Permissions ON files_id = perm_files_id
WHERE perm_users_id = 'xxxxxx';

The above query will create a result set of the join between the two tables, then filter by the conditions. This requires two passes instead of N+1.