MySQL: How do I Optimize this JOIN Query? - mysql

Question

I have 3 tables; artist, album, song_cover, and song.
I would like to select an album and the total number of songs in that album.
Am currently using this query, but it is logged in the mysql-slow.log file. In the PHPMyAdmin, the query speed is inconsistent. Sometimes it will execute for 0.0005 seconds and other times, 2 seconds or more.
SELECT /*+ MAX_EXECUTION_TIME(1000) */ album.*,
artist_id, artist_aka, artist_slug, artist_profile_image, cover_filename,
(
SELECT COUNT(*)
FROM song
WHERE song.song_album_id = album.album_id
) AS TotalSongs
FROM album
LEFT JOIN artist ON album.album_artist = artist.artist_id
LEFT JOIN song_cover ON album.album_cover_id = song_cover.cover_id
ORDER BY album_id DESC LIMIT 0, 11
ROWS
artist: 15,978, album: 14,167, song: 67,559, song_cover: 12,668
EXPLAIN
Thank you in advance.

Answer 1

I would write it this way:
EXPLAIN SELECT b.*,
a.artist_id, a.artist_aka, a.artist_slug, a.artist_profile_image,
c.cover_filename,
COUNT(*) AS TotalSongs
FROM album AS b
INNER JOIN artist AS a ON b.album_artist = a.artist_id
LEFT OUTER JOIN song AS s ON s.song_album_id = b.album_id
LEFT OUTER JOIN song_cover AS c ON b.album_cover_id = c.cover_id
GROUP BY b.album_id
ORDER BY b.album_id DESC LIMIT 0, 11;
This eliminates the dependent subquery, in favor of another join and GROUP BY.
Here's the EXPLAIN report as near as I can guess at it:
+----+-------------+-------+------------+--------+-------------------------------------+---------------+---------+-----------------------+------+----------+---------------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+--------+-------------------------------------+---------------+---------+-----------------------+------+----------+---------------------+
| 1 | SIMPLE | b | NULL | index | PRIMARY,album_cover_id,album_artist | PRIMARY | 4 | NULL | 1 | 100.00 | Backward index scan |
| 1 | SIMPLE | a | NULL | eq_ref | PRIMARY | PRIMARY | 4 | test.b.album_artist | 1 | 100.00 | NULL |
| 1 | SIMPLE | s | NULL | ref | song_album_id | song_album_id | 4 | test.b.album_id | 1 | 100.00 | Using index |
| 1 | SIMPLE | c | NULL | eq_ref | PRIMARY | PRIMARY | 4 | test.b.album_cover_id | 1 | 100.00 | NULL |
+----+-------------+-------+------------+--------+-------------------------------------+---------------+---------+-----------------------+------+----------+---------------------+
I have no data in my tables, so the row counts are trivial.
There's still a problem that it's doing an index-scan of album, which in your case is 14,167 rows. That could be costly.
But the other joins are all using indexes. Two of them are type: eq_ref, indicating that it's joining to the primary key of those tables.
I changed the join to artist to an inner join. I can't see how an album could not reference an artist. But I suppose it's possible for an album to have no songs, hence the outer join.
I find it strange that you join album directly to song_cover. Wouldn't song_cover also need to reference the original song it's a cover of?