Avoid filesort with INNER JOIN + ORDER BY

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.