Mysql: Perform of NOT EXISTS. Is it possible to improve perfomance?

Question

I have two tables posts and comments. Table comments have post_id attribute. I need to get all p

Answer 1

No idea whether it is faster, you could check it out:

SELECT * FROM posts
LEFT JOIN comments 
ON comment.postid = post.id
AND comment.comment_type='good'
WHERE comment.postid IS NULL

Assuming postid is never NULL / a non NULLable column.

Answer 2

You are right - you can do better. See this article by Quassnoi for the details but the conclusion is:

That’s why the best way to search for missing values in MySQL is using a LEFT JOIN / IS NULL or NOT IN rather than NOT EXISTS.

Your query rewritten using NOT IN could look like this:

SELECT *
FROM posts  
WHERE posts.id NOT IN (SELECT post_id
                       FROM comments
                       WHERE comments.comment_type = 'good'
                       AND comments.created_at BETWEEN '2010-05-01 00:00:00'
                                                   AND '2010-05-01 23:59:59')