Comments on: Using FORCE INDEX with Rails http://involution.com/2010/04/04/using-force-index-with-rails/ Tony Perrie's Weblog Tue, 15 May 2012 22:43:17 +0000 hourly 1 http://wordpress.org/?v=3.3.2 By: Tony Perrie http://involution.com/2010/04/04/using-force-index-with-rails/comment-page-1/#comment-387838 Tony Perrie Mon, 03 Oct 2011 23:08:13 +0000 http://involution.com/?p=1277#comment-387838 Thanks. I didn't know that. Thanks. I didn’t know that.

]]> By: Edward Rudd http://involution.com/2010/04/04/using-force-index-with-rails/comment-page-1/#comment-387835 Edward Rudd Mon, 03 Oct 2011 22:52:24 +0000 http://involution.com/?p=1277#comment-387835 The reason why MySQL won't use the index is because the index is sorted ASC, and you are requesting it be sorted DESC. For some reason rails lacks the ability to specify the sort direction when creating indexes. Creating an index thusly would solve your issue AND increase performance over what you are currently doing. CREATE INDEX posts_created_at_and_blog_id ON posts (create_at DESC, blog_id) though probably you want to do CREATE INDEX posts_blog_id_and_created_at ON posts (blog_id, created_at DESC) As I'm assuming your queries are like this. SELECT posts.* from posts where blog_id = 23 ORDER BY created_at DESC The reason why MySQL won’t use the index is because the index is sorted ASC, and you are requesting it be sorted DESC. For some reason rails lacks the ability to specify the sort direction when creating indexes.

Creating an index thusly would solve your issue AND increase performance over what you are currently doing.

CREATE INDEX posts_created_at_and_blog_id ON posts (create_at DESC, blog_id)

though probably you want to do

CREATE INDEX posts_blog_id_and_created_at ON posts (blog_id, created_at DESC)

As I’m assuming your queries are like this.

SELECT posts.* from posts where blog_id = 23 ORDER BY created_at DESC

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