Optimize tables MySQL - mysql

Question

I have a query that is executed in 35s, which is waaaaay too long.
Here are the 3 tables concerned by the query (each table is approx. 13000 lines long, and should be much longer in the future) :
Table 1 : Domains
CREATE TABLE IF NOT EXISTS `domain` (
`id_domain` int(11) NOT NULL AUTO_INCREMENT,
`domain_domain` varchar(255) NOT NULL,
`projet_domain` int(11) NOT NULL,
`date_crea_domain` int(11) NOT NULL,
`date_expi_domain` int(11) NOT NULL,
`active_domain` tinyint(1) NOT NULL,
`remarques_domain` text NOT NULL,
PRIMARY KEY (`id_domain`)
) ENGINE=InnoDB DEFAULT CHARSET=latin1;
Table 2 : Keywords
CREATE TABLE IF NOT EXISTS `kw` (
`id_kw` int(11) NOT NULL AUTO_INCREMENT,
`kw_kw` varchar(255) NOT NULL,
`clics_kw` int(11) NOT NULL,
`cpc_kw` float(11,3) NOT NULL,
`date_kw` int(11) NOT NULL,
PRIMARY KEY (`id_kw`)
) ENGINE=InnoDB DEFAULT CHARSET=latin1;
Table 3 : Linking between domain and keyword
CREATE TABLE IF NOT EXISTS `kw_domain` (
`id_kd` int(11) NOT NULL AUTO_INCREMENT,
`kw_kd` int(11) NOT NULL,
`domain_kd` int(11) NOT NULL,
`selected_kd` tinyint(1) NOT NULL,
PRIMARY KEY (`id_kd`),
KEY `kw_to_domain` (`kw_kd`,`domain_kd`)
) ENGINE=InnoDB DEFAULT CHARSET=latin1;
The query is as follows :
SELECT ng.*, kd.*, kg.*
FROM domain ng
LEFT JOIN kw_domain kd ON kd.domain_kd = ng.id_domain
LEFT JOIN kw kg ON kg.id_kw = kd.kw_kd
GROUP BY ng.id_domain
ORDER BY kd.selected_kd DESC, kd.id_kd DESC
Basically, it selects all domains, with, for each one of these domains, the last associated keyword.
Does anyone have an idea on how to optimize the tables or the query ?

Answer 1

The following will get the last keyword, according to your logic:
select ng.*,
(select kw_kd
from kw_domain kd
where kd.domain_kd = ng.id_domain and kd.selected_kd = 1
order by kd.id_kd desc
limit 1
) as kw_kd
from domain ng;
For performance, you want an index on kw_domain(domain_kd, selected_kd, kw_kd). In this case, the order of the fields matters.
You can use this as a subquery to get more information about the keyword:
select ng.*, kg.*
from (select ng.*,
(select kw_kd
from kw_domain kd
where kd.domain_kd = ng.id_domain and kd.selected_kd = 1
order by kd.id_kd desc
limit 1
) as kw_kd
from domain ng
) ng left join
kw kg
on kg.id_kw = ng.kw_kd;
In MySQL, group by can have poor performance, so this might work better, particularly with the right indexes.