performance - MySQL ORDER BY DESC is fast but ASC is very slow

Question

For some reason when I sort this query by DESC it's super fast, but if sorted by ASC it's extremely slow.

This takes about 150 milliseconds:

SELECT posts.id
FROM posts USE INDEX (published)
WHERE posts.feed_id IN ( 4953,622,1,1852,4952,76,623,624,10 )
ORDER BY posts.published DESC
LIMIT 0, 50;

This takes about 32 seconds:

SELECT posts.id
FROM posts USE INDEX (published)
WHERE posts.feed_id IN ( 4953,622,1,1852,4952,76,623,624,10 )
ORDER BY posts.published ASC
LIMIT 0, 50;

The EXPLAIN is the same for both queries.

id  select_type table   type    possible_keys   key key_len ref rows    Extra
1   SIMPLE  posts   index   NULL    published   5   NULL    50  Using where

I've tracked it down to "USE INDEX (published)". If I take that out it's the same performance both ways. But the EXPLAIN shows the query is less efficient overall.

id  select_type table   type    possible_keys   key key_len ref rows    Extra
1   SIMPLE  posts   range   feed_id feed_id 4   N  759 Using where; Using filesort

And here's the table.

CREATE TABLE `posts` (
  `id` int(20) NOT NULL AUTO_INCREMENT,
  `feed_id` int(11) NOT NULL,
  `post_url` varchar(255) NOT NULL,
  `title` varchar(255) NOT NULL,
  `content` blob,
  `author` varchar(255) DEFAULT NULL,
  `published` int(12) DEFAULT NULL,
  `updated` datetime NOT NULL,
  `created` datetime NOT NULL,
  PRIMARY KEY (`id`),
  UNIQUE KEY `post_url` (`post_url`,`feed_id`),
  KEY `feed_id` (`feed_id`),
  KEY `published` (`published`)
) ENGINE=InnoDB AUTO_INCREMENT=196530 DEFAULT CHARSET=latin1;

Is there a fix for this?

Answer 1

Your index is sorted desc so when you ask for ascending it needs to do a lot more work to bring it back in that order