Improve performance or redesign 'greatest-n-per-group' mysql query - mysql

Question

I'm using MySQL5 and I currently have a query that gets me the info I need but I feel like it could be improved in terms of performance.
Here's the query I built (roughly following this guide) :
SELECT d.*, dc.date_change, dc.cwd, h.name as hub
FROM livedata_dom AS d
LEFT JOIN ( SELECT dc1.*
FROM livedata_domcabling as dc1
LEFT JOIN livedata_domcabling AS dc2
ON dc1.dom_id = dc2.dom_id AND dc1.date_change < dc2.date_change
WHERE dc2.dom_id IS NULL
ORDER BY dc1.date_change desc) AS dc ON (d.id = dc.dom_id)
LEFT JOIN livedata_hub AS h ON (d.id = dc.dom_id AND dc.hub_id = h.id)
WHERE d.cluster = 'localhost'
GROUP BY d.id;
EDIT: Using ORDER BY + GROUP BY to avoid getting multiple dom entries in case 'domcabling' has an entry with null date_change and another one with a date for the same 'dom'.
I feel like I'm killing a mouse with a bazooka. This query takes more than 3 seconds with only about 5k entries in 'livedata_dom' and 'livedata_domcabling'. Also, EXPLAIN tells me that 2 filesorts are used:
+----+-------------+------------+--------+-----------------------------+-----------------------------+---------+-----------------+------+----------------------------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+------------+--------+-----------------------------+-----------------------------+---------+-----------------+------+----------------------------------------------+
| 1 | PRIMARY | d | ALL | NULL | NULL | NULL | NULL | 3 | Using where; Using temporary; Using filesort |
| 1 | PRIMARY | <derived2> | ALL | NULL | NULL | NULL | NULL | 3 | |
| 1 | PRIMARY | h | eq_ref | PRIMARY | PRIMARY | 4 | dc.hub_id | 1 | |
| 2 | DERIVED | dc1 | ALL | NULL | NULL | NULL | NULL | 4 | Using filesort |
| 2 | DERIVED | dc2 | ref | livedata_domcabling_dc592d9 | livedata_domcabling_dc592d9 | 4 | live.dc1.dom_id | 2 | Using where; Not exists |
+----+-------------+------------+--------+-----------------------------+-----------------------------+---------+-----------------+------+----------------------------------------------+
How could I change this query to make it more efficient?
Using the dummy data (provided below), this is the expected result:
+-----+-------+---------+--------+----------+------------+-----------+---------------------+------+-----------+
| id | mb_id | prod_id | string | position | name | cluster | date_change | cwd | hub |
+-----+-------+---------+--------+----------+------------+-----------+---------------------+------+-----------+
| 249 | 47 | 47 | 47 | 47 | SuperDOM47 | localhost | NULL | NULL | NULL |
| 250 | 48 | 48 | 48 | 48 | SuperDOM48 | localhost | 2014-04-16 05:23:00 | 32A | megahub01 |
| 251 | 49 | 49 | 49 | 49 | SuperDOM49 | localhost | NULL | 22B | megahub01 |
+-----+-------+---------+--------+----------+------------+-----------+---------------------+------+-----------+
Basically I need 1 row for every 'dom' entry, with
the 'domcabling' record with the highest date_change
if record does not exist, I need null fields
ONE entry may have a null date_change field per dom (null datetime field considered older than any other datetime)
the name of the 'hub', when a 'domcabling' entry is found, null otherwise
CREATE TABLE + dummy INSERT for the 3 tables:
livedata_dom (about 5000 entries)
CREATE TABLE `livedata_dom` (
`id` int(11) NOT NULL AUTO_INCREMENT,
`mb_id` varchar(12) NOT NULL,
`prod_id` varchar(8) NOT NULL,
`string` int(11) NOT NULL,
`position` int(11) NOT NULL,
`name` varchar(30) NOT NULL,
`cluster` varchar(9) NOT NULL,
PRIMARY KEY (`id`),
UNIQUE KEY `mb_id` (`mb_id`),
UNIQUE KEY `prod_id` (`prod_id`),
UNIQUE KEY `name` (`name`),
UNIQUE KEY `livedata_domgood_string_7bff074107b0e5a0_uniq` (`string`,`position`,`cluster`)
) ENGINE=InnoDB AUTO_INCREMENT=5485 DEFAULT CHARSET=latin1;
INSERT INTO `livedata_dom` VALUES (251,'49','49',49,49,'SuperDOM49','localhost'),(250,'48','48',48,48,'SuperDOM48','localhost'),(249,'47','47',47,47,'SuperDOM47','localhost');
livedata_domcabling (about 10000 entries and growing slowly)
CREATE TABLE `livedata_domcabling` (
`id` int(11) NOT NULL AUTO_INCREMENT,
`dom_id` int(11) NOT NULL,
`hub_id` int(11) NOT NULL,
`cwd` varchar(3) NOT NULL,
`date_change` datetime DEFAULT NULL,
PRIMARY KEY (`id`),
KEY `livedata_domcabling_dc592d9` (`dom_id`),
KEY `livedata_domcabling_4366aa6e` (`hub_id`),
CONSTRAINT `dom_id_refs_id_73e89ce0c50bf0a6` FOREIGN KEY (`dom_id`) REFERENCES `livedata_dom` (`id`),
CONSTRAINT `hub_id_refs_id_179c89d8bfd74cdf` FOREIGN KEY (`hub_id`) REFERENCES `livedata_hub` (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=5397 DEFAULT CHARSET=latin1;
INSERT INTO `livedata_domcabling` VALUES (1,251,1,'22B',NULL),(2,250,1,'33A',NULL),(6,250,1,'32A','2014-04-16 05:23:00'),(5,250,1,'22B','2013-05-22 00:00:00');
livedata_hub (about 100 entries)
CREATE TABLE `livedata_hub` (
`id` int(11) NOT NULL AUTO_INCREMENT,
`name` varchar(14) NOT NULL,
PRIMARY KEY (`id`),
UNIQUE KEY `name` (`name`)
) ENGINE=InnoDB AUTO_INCREMENT=98 DEFAULT CHARSET=latin;
INSERT INTO `livedata_hub` VALUES (1,'megahub01');

Answer 1

Try this rewriting (tested in SQL-Fiddle:
SELECT
d.*, dc.date_change, dc.cwd, h.name as hub
FROM
livedata_dom AS d
LEFT JOIN
livedata_domcabling as dc
ON dc.id =
( SELECT id
FROM livedata_domcabling AS dcc
WHERE dcc.dom_id = d.id
ORDER BY date_change DESC
LIMIT 1
)
LEFT JOIN
livedata_hub AS h
ON dc.hub_id = h.id
WHERE
d.cluster = 'localhost' ;
And index on (dom_id, date_change) would help efficiency.
I'm not sure about the selectivity of d.cluster = 'localhost' (how many rows of the livedata_dom table match this condiiton?) but adding an index on (cluster) might help as well.

Answer 2

set #rn := 0, #dom_id := 0;
select d.*, dc.date_change, dc.cwd, h.name as hub
from
livedata_dom d
left join (
select
hub_id, date_change, cwd, dom_id,
if(#dom_id = dom_id, #rn := #rn + 1, #rn := 1) as rn,
#dom_id := dom_id as dm_id
from
livedata_domcabling
order by dom_id, date_change desc
) dc on d.id = dc.dom_id
left join
livedata_hub h on h.id = dc.hub_id
where rn = 1 or rn is null
order by dom_id
The data you posted does not have the dom_id 249. And the #250 has one null date so it comes first. So your result does not reflect what I understand form your question.