MySQL optimize count query - mysql

Question

I've got a question about MySQL performance.
These are my tables:
(about 140.000 records)
CREATE TABLE IF NOT EXISTS `article` (
`id` int(11) NOT NULL AUTO_INCREMENT,
`label` varchar(256) COLLATE utf8_unicode_ci NOT NULL,
`title` varchar(256) COLLATE utf8_unicode_ci NOT NULL,
`intro` text COLLATE utf8_unicode_ci NOT NULL,
`content` text COLLATE utf8_unicode_ci NOT NULL,
`date` int(11) NOT NULL,
`active` int(1) NOT NULL,
`language_id` int(11) NOT NULL,
`category_id` int(11) NOT NULL,
`indexed` int(1) NOT NULL,
PRIMARY KEY (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci AUTO_INCREMENT=132911 ;
(about 400.000 records)
CREATE TABLE IF NOT EXISTS `article_category` (
`article_id` int(11) NOT NULL,
`category_id` int(11) NOT NULL
) ENGINE=InnoDB DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci;
RUNNING THIS COUNT QUERY:
SELECT SQL_NO_CACHE COUNT(id) as total
FROM (`article`)
LEFT JOIN `article_category` ON `article_category`.`article_id` = `article`.`id`
WHERE `article`.`language_id` = 1
AND `article_category`.`category_id` = '<catid>'
This query takes a lot of resources, so I am wondering how to optimize this query.
After executing it's beeing cached, so after the first run I am fine.
RUNNING THE EXPLAIN FUNCTION:
AFTER CREATING AN INDEX:
ALTER TABLE `article_category` ADD INDEX ( `article_id` , `category_id` ) ;
After adding indexes and changing LEFT JOIN to JOIN the query runs alot faster!
Thanks for these fast replys :)
QUERY I USE NOW (I removed the language_id because it was not that neccesary):
SELECT COUNT(id) as total
FROM (`article`)
JOIN `article_category` ON `article_category`.`article_id` = `article`.`id`
AND `article_category`.`category_id` = '<catid>'
I've read something about forcing an index, but I think thats not neccesary anymore because the tables are already indexed, right?
Thanks alot!
Martijn

Answer 1

You haven't created necessary index on the table
Table article_category - Create a compound index on (article_id, category_id)
Table article -Create a compound index on (id, language_id)
If this doesn't help post the explain statement.

Answer 2

The columns used in a JOIN condition should have an index, so you need to index article_id.