MySQL EXPLAIN output explanation - mysql

Question

I have a slow MySQl query that takes about 15 seconds to run. So I did some investigation and discovered I can use the EXPLAIN statement to see where the bottleneck is. So I did that, but really can't decipher these results.
If I had to take a stab, I would say the first line is a problem as there are null values for the keys. However, if that is so, I can't understand why as the classType1 table IS indexed on the appropriate columns.
Could someone please offer some explanation as to where the problems might lay?
Thanks so much.
EDIT: Ok, I have added the query as well hoping that it might offer some more light to the issues. Unfortunately I just won't be able to explain to you what it's doing, so if any help could be offered based on what's provided, that would be great.
id, select_type, table, type, possible_keys, key, key_len, ref, rows, Extra
1, 'PRIMARY', 'classType1', 'system', 'PRIMARY', '', '', '', 1, 'Using temporary; Using filesort'
1, 'PRIMARY', 'user', 'const', 'PRIMARY', 'PRIMARY', '4', 'const', 1, 'Using index'
1, 'PRIMARY', 'class1', 'ref', 'IX_classificationType,IX_classificationValue,IX_classificationObjectType,IX_classificationObjectId', 'IX_classificationObjectId', '8', 'const', 3, 'Using where'
1, 'PRIMARY', 'classVal1', 'eq_ref', 'PRIMARY', 'PRIMARY', '4', 'ccms.class1.classificationValue', 1, 'Using where; Using index'
1, 'PRIMARY', 'class2', 'ref', 'IX_classificationType,IX_classificationValue,IX_classificationObjectType,IX_classificationObjectId', 'IX_classificationValue', '4', 'ccms.class1.classificationValue', 368, 'Using where'
1, 'PRIMARY', 'album', 'eq_ref', 'PRIMARY,IX_albumType,IX_albumIsDisabled,IX_albumIsActive,IX_albumCSI,IX_albumOwner,IX_albumPublishDate', 'PRIMARY', '4', 'ccms.class2.classificationObjectId', 1, 'Using where'
1, 'PRIMARY', 'profile', 'eq_ref', 'PRIMARY,IX_profileUserId', 'PRIMARY', '4', 'ccms.album.albumOwnerId', 1, 'Using where'
1, 'PRIMARY', 'albumUser', 'eq_ref', 'PRIMARY,IX_userIsAccountPublic', 'PRIMARY', '4', 'ccms.profile.profileUserId', 1, 'Using where'
1, 'PRIMARY', 'photo', 'eq_ref', 'PRIMARY,FK_photoAlbumId', 'PRIMARY', '8', 'ccms.album.albumCoverPhotoId', 1, 'Using where'
2, 'DEPENDENT SUBQUERY', 'class3', 'ref', 'IX_classificationObjectType,IX_classificationObjectId', 'IX_classificationObjectId', '8', 'ccms.class2.classificationObjectId', 1, 'Using where'
3, 'DEPENDENT SUBQUERY', 'class4', 'ref', 'IX_classificationType,IX_classificationValue,IX_classificationObjectType,IX_classificationObjectId', 'IX_classificationObjectId', '8', 'const', 3, 'Using where'
Query is...
SELECT profileDisplayName,albumPublishDate,profileId,albumId,albumPath,albumName,albumCoverPhotoId,photoFilename,fnAlbumGetNudityClassification(albumId) AS albumNudityClassification,fnAlbumGetNumberOfPhotos(albumId,1,0) AS albumNumberOfPhotos,albumDescription,albumCSD,albumUSD,photoId,fnGetAlbumPhotoViewCount(albumId) AS albumNumberOfPhotoViews
FROM user
-- Join User Classifications
INNER JOIN classification class1
ON class1.classificationObjectId = user.userId AND class1.classificationObjectType = 1
INNER JOIN classificationType classType1
ON class1.classificationType = classType1.classificationTypeId
INNER JOIN classificationTypeValue classVal1
ON class1.classificationValue = classVal1.classificationTypeValueId
-- Join Album Classifications
INNER JOIN classification class2
ON class2.classificationObjectType = 3
AND class1.classificationType = class2.classificationType AND class1.classificationValue = class2.classificationValue
INNER JOIN album
ON album.albumId = class2.classificationObjectId
AND albumIsActive = 1
AND albumIsDisabled = 0
LEFT JOIN profile
ON albumOwnerId = profileId AND albumOwnerType = 0
LEFT JOIN user albumUser
ON albumUser.userId = profileUserId
AND albumUser.userIsAccountPublic = 1
LEFT JOIN photo
ON album.albumId = photo.photoAlbumId AND photo.photoId = album.albumCoverPhotoId
WHERE 0 =
(
SELECT COUNT(*)
FROM classification class3
WHERE class3.classificationObjectType = 3
AND class3.classificationObjectId = class2.classificationObjectId
AND NOT EXISTS
(
SELECT 1
FROM classification class4
WHERE class4.classificationObjectType = 1
AND class4.classificationObjectId = user.userId
AND class4.classificationType = class3.classificationType AND class4.classificationValue = class3.classificationValue
)
)
AND class1.classificationObjectId = 8
AND (albumPublishDate <= {ts '2011-01-28 20:48:39'} || albumCSI =
8)
AND album.albumType NOT IN (1)
AND fnAlbumGetNumberOfPhotos(albumId,1,0) > 0
AND albumUser.userIsAccountPublic IS NOT NULL
ORDER BY albumPublishDate DESC
LIMIT 0, 15

Answer 1

without seeing the actual structure or query, I would look for 2 things...
I know you said they are... but... make sure all the appropriate fields are indexed
example: you have an index on field "active" (to filter out only active records) and another one (let's say primary key index) on id_classType1... unless you do a unique index on "id_classType1, active", a query similar to this:
SELECT * FROM classType1 WHERE id_classType1 IN (1,2,3) AND active = 1
... would need to either combine those indexes or look them up separately. However, if you have an index on both, id_classType1 AND active (and that index is a type UNIQUE), SQL will use it and find the combinations much quicker.
secondly, you seem to have dependent subqueries in your EXPLAIN statement, which can alone slow your query down quite a lot... have a look here for a possible workaround: http://forums.mysql.com/read.php?115,128477,128477
my first try would be to replace those subqueries by JOINs and then perhaps try to optimize it further by removing them altogether (if possible) or making a separate queries for those subqueries
EDIT
this query is more complex that any other I've ever seen, so take these as somehow limited tips:
try removing subqueries (just put anything you know will work and give results there temporarily)
I see a lot of INNER JOINS in the query, which can be quite slow, as they need to join all rows from both tables (source: http://dev.mysql.com/doc/refman/5.0/en/join.html) - maybe there's a way to replace them somehow?
also - and this is something I remember from the past (might not be true or relevant) - shouldn't WHERE-like statements be in the WHERE clause, not the JOIN clause? for example, I would put the following from the 1st JOIN into a WHERE section: class1.classificationObjectType = 1
that's just about all - and one question: how many rows do those table have? no need for exact numbers, just trying to see on approx. how many records the query runs, as it takes so long

Answer 2

Ok ,so through process of elimination I managed to find the issue.
In my column list, I had a call: fnGetAlbumPhotoViewCount(albumId) AS albumNumberOfPhotoViews
One of the tables joined in that call had a column that was not indexed. Simple enough.
Now my question is, EXPLAIN could not show me that. If you look, there is in fact no reference to the pageview table or columns anywhere in the EXPLAIN output.
So what tool could I have used to weed out this issue??
Thanks