Question

i have a very simple query that im trying to optimize, its taking 2~5 secs to execute.

This is my CREATE TABLE

CREATE TABLE `artist` (
  `id` INTEGER NOT NULL AUTO_INCREMENT,
  `name` VARCHAR(100) character set utf8 NOT NULL,
  `bio` MEDIUMTEXT character set utf8 DEFAULT NULL,
  `hits` INTEGER NOT NULL,
  PRIMARY KEY (`id`)
);

CREATE TABLE `album` (
  `id` INTEGER NOT NULL AUTO_INCREMENT,
  `artist_id` INTEGER NOT NULL,
  `title` VARCHAR(100) character set utf8 NOT NULL,
  `year` INTEGER,
  `hits` INTEGER NOT NULL,
  PRIMARY KEY (`id`),
KEY (`artist_id`)
);

CREATE TABLE `track` (
  `id` INTEGER NOT NULL AUTO_INCREMENT,
  `name` VARCHAR(100) character set utf8 NOT NULL,
  `lyric` MEDIUMTEXT character set utf8,
  `album_id` INTEGER NOT NULL,
  `hits` INTEGER NOT NULL,
  `date` datetime NOT NULL,
  PRIMARY KEY (`id`),
KEY (`album_id`)
);

ALTER TABLE `album` ADD FOREIGN KEY (artist_id) REFERENCES `artist` (`id`);
ALTER TABLE `track` ADD FOREIGN KEY (album_id) REFERENCES `album` (`id`);

and this is the query im running

SELECT DISTINCT artist.name, track.name
FROM track
LEFT JOIN album ON track.album_id = album.id
LEFT JOIN artist ON album.artist_id = artist.id
ORDER BY track.hits DESC
LIMIT 5 

Explain selects show this:

id  select_type     table   type    possible_keys   key     key_len     ref     rows    Extra
1   SIMPLE  track   ALL     NULL    NULL    NULL    NULL    103796  Using temporary; Using filesort
1   SIMPLE  album   eq_ref  PRIMARY     PRIMARY     4   lyrics.track.album_id   1    
1   SIMPLE  artist  eq_ref  PRIMARY     PRIMARY     4   lyrics.album.artist_id  1    

I’m new to MySQL but i guess using Using temporary; Using filesort is bad and thats why the query is very slow, can you guys hint me here? thanks!

update: The main problem here is that the very same song can be 5 times in the DB with different ID’s, because the same song can be in different albums. If i dont use distinct, this doesnt happen, bust i must for this reason

Answer 1

You can get it to use an index by adding

create index idx_tracks_on_album_id_name_hits on track(album_id, name, hits);

And since you are doing a DISTINCT across two tables, there will be no index to possibly find the unique rows so it puts it into a temp table to get rid of the duplicates.