Avoid filesort with INNER JOIN + ORDER BY - mysql

Question

I've been reading other posts but I didn't managed to fix my query.
Using DESC order the query is x20 times slower, I must improve that.
This is the query:
SELECT posts.post_id, posts.post_b_id, posts.post_title, posts.post_cont, posts.thumb, posts.post_user, boards.board_title_l, boards.board_title
FROM posts
INNER JOIN follow ON posts.post_b_id = follow.board_id
INNER JOIN boards ON posts.post_b_id = boards.board_id
WHERE follow.user_id =1
ORDER BY posts.post_id DESC
LIMIT 10
And these are the tables (Updated):
CREATE TABLE IF NOT EXISTS `posts` (
`post_id` int(11) NOT NULL AUTO_INCREMENT,
`post_b_id` int(11) unsigned NOT NULL,
`post_title` varchar(50) COLLATE utf8_bin NOT NULL,
`post_cont` text COLLATE utf8_bin NOT NULL,
`post_mintxt` varchar(255) COLLATE utf8_bin NOT NULL,
`post_type` char(3) COLLATE utf8_bin NOT NULL,
`thumb` varchar(200) COLLATE utf8_bin NOT NULL,
`post_user` varchar(16) COLLATE utf8_bin NOT NULL,
`published` enum('0','1') COLLATE utf8_bin NOT NULL,
`post_ip` varchar(94) COLLATE utf8_bin NOT NULL,
`post_ip_dat` int(11) unsigned NOT NULL,
`post_up` int(10) unsigned NOT NULL DEFAULT '0',
`post_down` int(10) unsigned NOT NULL DEFAULT '0',
PRIMARY KEY (`post_id`),
KEY `post_b_id` (`post_b_id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8 COLLATE=utf8_bin AUTO_INCREMENT=405 ;
CREATE TABLE IF NOT EXISTS `boards` (
`board_id` int(10) unsigned NOT NULL AUTO_INCREMENT,
`board_title_l` varchar(19) COLLATE utf8_bin NOT NULL,
`board_user_id` int(10) unsigned NOT NULL,
`board_title` varchar(19) COLLATE utf8_bin NOT NULL,
`board_user` varchar(16) COLLATE utf8_bin NOT NULL,
`board_txt` tinyint(1) unsigned NOT NULL,
`board_img` tinyint(1) unsigned NOT NULL,
`board_vid` tinyint(1) unsigned NOT NULL,
`board_desc` varchar(100) COLLATE utf8_bin NOT NULL,
`board_mod_p` tinyint(3) unsigned NOT NULL DEFAULT '0',
`board_ip` varchar(94) COLLATE utf8_bin NOT NULL,
`board_dat_ip` int(11) unsigned NOT NULL,
PRIMARY KEY (`board_id`),
UNIQUE KEY `board_title_l` (`board_title_l`),
KEY `board_user_id` (`board_user_id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8 COLLATE=utf8_bin AUTO_INCREMENT=89 ;
CREATE TABLE IF NOT EXISTS `follow` (
`user_id` int(10) unsigned NOT NULL,
`board_id` int(10) unsigned NOT NULL,
PRIMARY KEY (`user_id`,`board_id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8 COLLATE=utf8_bin;
Using default ASC order it only uses index and where, with DESC uses index, where, temporary and filesort.
id select_type table type possible_keys key key_len ref rows filtered Extra
1 SIMPLE follow ref user_id user_id 4 const 2 100.00 Using index; Using temporary; Using filesort
1 SIMPLE boards eq_ref PRIMARY PRIMARY 4 xxxx.follow.board_id 1 100.00
1 SIMPLE posts ref post_b_id post_b_id 4 xxxx.boards.board_id 3 100.00 Using where
How I can make the query receiving the results in DESC order without filesort and temporary.
UPDATE: I made a new query, no temporary or filesort, but type: index, filtered: 7340.00. Almost as fast as ASC order if the posts are at the end of the table, but slow if the posts that is searching are at the beginning. So seems better but it's not enough.
SELECT posts.post_id, posts.post_b_id, posts.post_title, posts.post_cont, posts.thumb, posts.post_user, boards.board_title_l, boards.board_title
FROM posts INNER JOIN boards ON posts.post_b_id = boards.board_id
WHERE posts.post_b_id
IN (
SELECT follow.board_id
FROM follow
WHERE follow.user_id = 1
)
ORDER BY posts.post_id DESC
LIMIT 10
Explain:
id select_type table type possible_keys key key_len ref rows filtered Extra
1 PRIMARY posts index post_b_id PRIMARY 8 NULL 10 7340.00 Using where
1 PRIMARY boards eq_ref PRIMARY PRIMARY 4 xxxx.posts.post_b_id 1 100.00
2 DEPENDENT SUBQUERY follow eq_ref user_id user_id 8 const,func 1 100.00 Using index
UPDATE: Explain for the query from dened's answer:
id select_type table type possible_keys key key_len ref rows filtered Extra
1 PRIMARY <derived2>ALL NULL NULL NULL NULL 10 100.00
1 PRIMARY posts eq_ref PRIMARY,post_b_id PRIMARY 4 sq.post_id 1 100.00
1 PRIMARY boards eq_ref PRIMARY PRIMARY 4 xxxx.posts.post_b_id 1 100.00
2 DERIVED follow ref PRIMARY PRIMARY 4 1 100.00 Using index; Using temporary; Using filesort
2 DERIVED posts ref post_b_id post_b_id 4 xxxx.follow.board_id 6 100.00 Using index
Times:
Original query no order (ASC): 0.187500 seconds
Original query DESC: 2.812500 seconds
Second query posts at the end (DESC): 0.218750 seconds
Second query posts at the beginning (DESC): 3.293750 seconds
dened's query DESC: 0.421875 seconds
dened's query no order (ASC): 0.323750 seconds
Interesting note, if I add ORDER BY ASC is as slow as DESC.
Alter the table order will be a god way, but as I said in the comments I wasn't able to do that.

Answer 1

You can help MySQL optimizer by moving all the filtering work to a subquery that accesses only indices (manipulating indices is usually much faster than manipulating other data), and fetching rest of the data in the outermost query:
SELECT posts.post_id,
posts.post_b_id,
posts.post_title,
posts.post_cont,
posts.thumb,
posts.post_user,
boards.board_title_l,
boards.board_title
FROM (SELECT post_id
FROM posts
JOIN follow
ON posts.post_b_id = follow.board_id
WHERE follow.user_id = 1
ORDER BY post_id DESC
LIMIT 10) sq
JOIN posts
ON posts.post_id = sq.post_id
JOIN boards
ON boards.board_id = posts.post_b_id
Note that I omit ORDER BY posts.post_id DESC from the outer query, because it is usually faster to sort the final result in your code rather than sorting using a MySQL query (MySQL often uses filesort for that).
P.S. You can replace the unique key in the follow table with a primary key.

Answer 2

Increasing the sort_buffer_size parameter will increase the amount of memory MySQL uses before resorting to a temporary disk file and should help considerably.