How to improve this MySQL query? - mysql

Question

This is what I saw in slow-query.log
# User#Host: appUser[appUser] # [192.168.1.10]
# Query_time: 65.118800 Lock_time: 0.000257 Rows_sent: 7 Rows_examined: 78056
SET timestamp=1312883519;
select
this_.id as id2_0_,
this_.appearance_count as appearance2_2_0_,
this_.author as author2_0_,
this_.date_added as date4_2_0_,
this_.date_last_ranked as date5_2_0_,
this_.date_updated as date6_2_0_,
this_.description as descript7_2_0_,
this_.frequency_updated as frequency8_2_0_,
this_.hidden as hidden2_0_,
this_.link as link2_0_,
this_.lock_id as lock11_2_0_,
this_.plain_text_description as plain12_2_0_,
this_.processor_done as processor13_2_0_,
this_.processor_lock_id as processor14_2_0_,
this_.published_date as published15_2_0_,
this_.raw_link as raw16_2_0_,
this_.retweet_count as retweet17_2_0_,
this_.source_url as source18_2_0_,
this_.subtype as subtype2_0_,
this_.thumbnail_img_url as thumbnail20_2_0_,
this_.title as title2_0_,
this_.total_hit_count as total22_2_0_,
this_.total_lift_count as total23_2_0_,
this_.total_share_count as total24_2_0_,
this_.trend_value as trend25_2_0_,
this_.type as type2_0_
from
news this_
where
exists (select 1 from news_sub_topic where this_.id=news_sub_topics_id)
order by
this_.trend_value desc,
this_.retweet_count desc,
this_.total_lift_count desc,
this_.published_date desc
limit 7;
this is what EXPLAIN gives:
+----+--------------------+----------------+------+--------------------------------------+-------------------+---------+------------------------+-------+-----------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+--------------------+----------------+------+--------------------------------------+-------------------+---------+------------------------+-------+-----------------------------+
| 1 | PRIMARY | this_ | ALL | NULL | NULL | NULL | NULL | 50910 | Using where; Using filesort |
| 2 | DEPENDENT SUBQUERY | news_sub_topic | ref | FK420C980470B0479,FK420C98041F791FA1 | FK420C980470B0479 | 9 | causelift_web.this_.id | 1 | Using where; Using index |
+----+--------------------+----------------+------+--------------------------------------+-------------------+---------+------------------------+-------+-----------------------------+
2 rows in set (3.28 sec)
this is the CREATE sytax for the news and news_sub_topic table:
CREATE TABLE `news` (
`id` bigint(20) NOT NULL AUTO_INCREMENT,
`appearance_count` int(11) NOT NULL,
`author` longtext COLLATE utf8_unicode_ci,
`date_added` datetime NOT NULL,
`date_updated` datetime DEFAULT NULL,
`description` longtext COLLATE utf8_unicode_ci,
`link` longtext COLLATE utf8_unicode_ci,
`published_date` datetime NOT NULL,
`raw_link` longtext COLLATE utf8_unicode_ci,
`subtype` varchar(5) COLLATE utf8_unicode_ci NOT NULL,
`thumbnail_img_url` varchar(255) COLLATE utf8_unicode_ci DEFAULT NULL,
`title` longtext COLLATE utf8_unicode_ci,
`type` varchar(5) COLLATE utf8_unicode_ci NOT NULL,
`date_last_ranked` datetime DEFAULT NULL,
`lock_id` int(11) DEFAULT NULL,
`plain_text_description` longtext COLLATE utf8_unicode_ci,
`source_url` longtext COLLATE utf8_unicode_ci,
`retweet_count` int(11) DEFAULT NULL,
`frequency_updated` bit(1) DEFAULT NULL,
`hidden` bit(1) DEFAULT NULL,
`processor_done` bit(1) DEFAULT NULL,
`processor_lock_id` int(11) DEFAULT NULL,
`total_hit_count` int(11) DEFAULT NULL,
`total_lift_count` int(11) DEFAULT NULL,
`total_share_count` int(11) DEFAULT NULL,
`trend_value` float DEFAULT NULL,
PRIMARY KEY (`id`),
KEY `link_idx` (`link`(255)),
KEY `type_idx` (`type`),
KEY `processor_lock_id_idx` (`processor_lock_id`),
KEY `processor_done_idx` (`processor_done`),
KEY `hidden_idx` (`hidden`),
KEY `title_idx` (`title`(255)),
KEY `published_date_idx` (`published_date`)
) ENGINE=InnoDB AUTO_INCREMENT=321136 DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci;
CREATE TABLE `news_sub_topic` (
`news_sub_topics_id` bigint(20) DEFAULT NULL,
`sub_topic_id` bigint(20) DEFAULT NULL,
KEY `FK420C98045D8019D4` (`sub_topic_id`),
KEY `FK420C980470B0479` (`news_sub_topics_id`),
CONSTRAINT `FK420C98041F791FA1` FOREIGN KEY (`news_sub_topics_id`) REFERENCES `news` (`id`),
CONSTRAINT `FK420C98045D8019D4` FOREIGN KEY (`sub_topic_id`) REFERENCES `sub_topic` (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci;
anyone can explain why sometimes it took 65 seconds for this query to finish? but sometimes it is fast? what other factors should i look into (DB config, OS, etc.).
thanks for any leads on this.

Answer 1

my feeling is the 'WHERE' clause is your problem.
Try this code instead:
from
news this_
inner join news_sub_topic topic on this.id=topic.news_sub_topics_id
The explanation is the because the lookup against news is using a filesort and not an index, likely because the where clause isn't giving a good hint to the optimize.