mysql关系慢

<?php
$db = new mysqli(//editted out db credentials);

if($db->connect_errno > 0){
    die('Unable to connect to database [' . $db->connect_error . ']');
}
$sql = "SELECT m.ID, m.Title, GROUP_CONCAT(a.Title) AS Artist
FROM mp3s m
LEFT JOIN artist_relations ar ON ar.mp3ID = m.ID
LEFT JOIN artists a ON a.ID = ar.artistID
GROUP BY m.ID
ORDER BY ID
LIMIT 0,30;
";

if($result = $db->query($sql)){
echo "<table>";
while($row = $result->fetch_assoc()){
    echo "<tr>";  
    echo "<td>".$row['Title']."</td>";
    echo "<td>".$row['Artist']."</td>";
    echo "</tr>";  
}
echo "</table>";
}

?>

此查询工作正常，但速度非常慢 .

该数据库有3个字段：

`artists` : ID , Title
`mp3s` : ID , Title
`artist_relations` : mp3ID , artistID

我需要这个：

row1: titlemusic1 - artist1 , artist4 , artist5
row2: titlemusic2 - artist1
row3: titlemusic1 - artist3 , artist8
row4: titlemusic1 - artist9 , artist10
...

mp3s，artist＆artist_relations每个都有超过20000条记录

EXPLAIN SELECT m.ID, m.Title, .... :
    id  select_type     table   type    possible_keys   key     key_len     ref     rows    Extra   
    1   SIMPLE  m   ALL     NULL    NULL    NULL    NULL    23718   Using temporary; Using filesort
    1   SIMPLE  ar  ALL     NULL    NULL    NULL    NULL    24337   
    1   SIMPLE  a   eq_ref  PRIMARY     PRIMARY     4   ganools_rj.ar.artistID  1

- 表artist_relations的表结构

CREATE TABLE IF NOT NOT EXISTS artist_relations （ artistID int（11）NOT NULL， mp3ID int（11）NOT NULL）ENGINE = InnoDB DEFAULT CHARSET = latin1;

- 表格艺术家的表格结构

CREATE TABLE IF NOT NOT EXISTS artists （ ID int（11）NOT NULL AUTO_INCREMENT， Title varchar（155）NOT NULL PRIMARY KEY（ ID ））ENGINE = InnoDB DEFAULT CHARSET = latin1 AUTO_INCREMENT = 9005;

- 表mp3的表结构

CREATE TABLE IF NOT NOT EXISTS mp3s （ ID int（11）NOT NULL AUTO_INCREMENT， Title varchar（155）NOT NULL， imageURL varchar（155）NOT NULL， mp3URL varchar（155）NOT NULL， Description text， Lyric text， album varchar（155 ）DEFAULT NULL， plays int（11）DEFAULT NULL， pubDate date NOT NULL，PRIMARY KEY（ ID ））ENGINE = InnoDB DEFAULT CHARSET = latin1 AUTO_INCREMENT = 22936;