Optimize LEFT JOIN on table with 30 000+ rows - mysql

Question

I have a website where visitors can leave comments. I want to add the ability to answer comments (i.e. nested comments).
At first this query was fast but after I populated the table with the existing comments (about 30000) a simple query like:
SELECT c.id, c2.id
FROM (SELECT id
FROM swb_comments
WHERE pageId = 1411
ORDER BY id DESC
LIMIT 10) AS c
LEFT JOIN swb_comments AS c2 ON c.id = c2.parentId
took over 2 seconds, with no childComments(!).
How do I optimize a query like this? On possible solution would be http://www.ferdychristant.com/blog//articles/DOMM-7QJPM7 (scroll to "The Flat Table Model done right") but this makes pagination rather difficult (how do I limit to 10 parent comments within 1 query?)
The table has 3 indexes, id, pageId and ParentId.
Thanks in advance!
EDIT:
Table definition added. This is the full definition with some differences to the above SELECT query, (i.e. pageId instead of numberId to avoid confussion)
CREATE TABLE `swb_comments` (
`id` mediumint(9) NOT NULL auto_increment,
`userId` mediumint(9) unsigned NOT NULL default '0',
`numberId` mediumint(9) unsigned default NULL,
`orgId` mediumint(9) unsigned default NULL,
`author` varchar(100) default NULL,
`email` varchar(255) NOT NULL,
`message` text NOT NULL,
`IP` varchar(40) NOT NULL,
`timestamp` varchar(25) NOT NULL,
`editedTimestamp` varchar(25) default NULL COMMENT 'last edited timestamp',
`status` varchar(20) NOT NULL default 'publish',
`parentId` mediumint(9) unsigned NOT NULL default '0',
`locale` varchar(10) NOT NULL,
PRIMARY KEY (`id`),
KEY `userId` (`userId`),
KEY `numberId` (`numberId`),
KEY `orgId` (`orgId`),
KEY `parentId` (`parentId`)
) ENGINE=MyISAM DEFAULT CHARSET=utf8 AUTO_INCREMENT=34748 ;

Answer 1

The issue is that MySQL cannot apply index if it need to deal with a result from a derived query (that's why you have NULL in the possible_keys column). So I suggest to filter out ten comments that you need:
SELECT * FROM swb_comments WHERE pageId = 1411 ORDER BY id DESC LIMIT 10
And after that send separate request to get answers for each comment id:
SELECT * FROM swb_comments WHERE parentId IN ($commentId1, $commentId2, ..., $commentId10)
In this case database engine will be able to apply pageId and parentId indexes efficiently.

Answer 2

If Mr Fedorenko is correct and the subquery is causing the optimiser difficulties, could you not try...
SELECT c.id, c2.id
FROM swb_comments c LEFT JOIN swb_comments c2 ON c.id = c2.parentID
WHERE c.pageId = 1411
ORDER BY c.id DESC
LIMIT 10;
and see if it's any improvement?
Later - I have created a table using your definition, filled it in with 30,000 skeletal rows, and tried both the queries. They both complete in too short a time to notice. The explain plans are here...
mysql> EXPLAIN SELECT c.id, c2.id
FROM swb_comments c LEFT JOIN swb_comments c2 ON c.id = c2.parentID
WHERE c.numberId = 1411 ORDER BY c.id DESC LIMIT 10;
+----+-------------+-------+------+---------------+----------+---------+------------+------+-----------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+------+---------------+----------+---------+------------+------+-----------------------------+
| 1 | SIMPLE | c | ref | numberId | numberId | 4 | const | 1 | Using where; Using filesort |
| 1 | SIMPLE | c2 | ref | parentId | parentId | 3 | books.c.id | 14 | |
+----+-------------+-------+------+---------------+----------+---------+------------+------+-----------------------------+
mysql> EXPLAIN SELECT c.id, c2.id
FROM swb_comments c LEFT JOIN swb_comments c2 ON c.id = c2.parentID
WHERE c.numberId = 1411 ORDER BY c.id DESC LIMIT 10;
+----+-------------+-------+------+---------------+----------+---------+------------+------+-----------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+------+---------------+----------+---------+------------+------+-----------------------------+
| 1 | SIMPLE | c | ref | numberId | numberId | 4 | const | 1 | Using where; Using filesort |
| 1 | SIMPLE | c2 | ref | parentId | parentId | 3 | books.c.id | 14 | |
+----+-------------+-------+------+---------------+----------+---------+------------+------+-----------------------------+
and are exactly what I'd expect.
This is very mysterious.
I'll think about it a bit more to see if there's anything else we can try.