Mysql query optimisation, EXPLAIN and slow execution - mysql

Question

Having some real issues with a few queries, this one inparticular. Info below.
tgmp_games, about 20k rows
CREATE TABLE IF NOT EXISTS `tgmp_games` (
`g_id` int(8) NOT NULL AUTO_INCREMENT,
`site_id` int(6) NOT NULL,
`g_name` varchar(255) NOT NULL,
`g_link` varchar(255) NOT NULL,
`g_url` varchar(255) NOT NULL,
`g_platforms` varchar(128) NOT NULL,
`g_added` datetime NOT NULL,
`g_cover` varchar(255) NOT NULL,
`g_impressions` int(8) NOT NULL,
PRIMARY KEY (`g_id`),
KEY `g_platforms` (`g_platforms`),
KEY `site_id` (`site_id`),
KEY `g_link` (`g_link`),
KEY `g_release` (`g_release`),
KEY `g_genre` (`g_genre`),
KEY `g_name` (`g_name`),
KEY `g_impressions` (`g_impressions`)
) ENGINE=MyISAM DEFAULT CHARSET=latin1;
tgmp_reviews - about 200k rows
CREATE TABLE IF NOT EXISTS `tgmp_reviews` (
`r_id` int(8) NOT NULL AUTO_INCREMENT,
`site_id` int(6) NOT NULL,
`r_source` varchar(128) NOT NULL,
`r_date` date NOT NULL,
`r_score` int(3) NOT NULL,
`r_copy` text NOT NULL,
`r_link` text NOT NULL,
`r_int_link` text NOT NULL,
`r_parent` int(8) NOT NULL,
`r_platform` varchar(12) NOT NULL,
`r_impressions` int(8) NOT NULL,
PRIMARY KEY (`r_id`),
KEY `site_id` (`site_id`),
KEY `r_parent` (`r_parent`),
KEY `r_platform` (`r_platform`)
) ENGINE=InnoDB DEFAULT CHARSET=latin1 ;
Here is the query, takes 3 seconds ish
SELECT * FROM tgmp_games g
RIGHT JOIN tgmp_reviews r ON g_id = r.r_parent
WHERE g.site_id = '34'
GROUP BY g_name
ORDER BY g_impressions DESC LIMIT 15
EXPLAIN
id select_type table type possible_keys key key_len ref rows Extra
1 SIMPLE r ALL r_parent NULL NULL NULL 201133 Using temporary; Using filesort
1 SIMPLE g eq_ref PRIMARY,site_id PRIMARY 4 engine_comp.r.r_parent 1 Using where
I am just trying to grab the 15 most viewed games, then grab a single review (doesnt really matter which, I guess highest rated would be ideal, r_score) for each game.
Can someone help me figure out why this is so horribly inefficient?

Answer 1

I don't understand what is the purpose of having a GROUP BY g_name in your query, but this makes MySQL performing aggregates on the columns selected, or all columns from both table. So please try to exclude it and check if it helps.
Also, RIGHT JOIN makes database to query tgmp_reviews first, which is not what you want. I suppose LEFT JOIN is a better choice here. Please, try to change the join type.
If none of the first options helps, you need to redesign your query. As you need to obtain 15 most viewed games for the site, the query will be:
SELECT g_id
FROM tgmp_games g
WHERE site_id = 34
ORDER BY g_impressions DESC
LIMIT 15;
This is the very first part that should be executed by the database, as it provides the best selectivity. Then you can get the desired reviews for the games:
SELECT r_parent, max(r_score)
FROM tgmp_reviews r
WHERE r_parent IN (/*1st query*/)
GROUP BY r_parent;
Such construct will force database to execute the first query first (sorry for the tautology) and will give you the maximal score for each of the wanted games. I hope you will be able to use the obtained results for your purpose.

Answer 2

Your MyISAM table is small, you can try converting it to see if that resolves the issue. Do you have a reason for using MyISAM instead of InnoDB for that table?
You can also try running an analyze on each table to update the statistics to see if the optimizer chooses something different.