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POGROUP BY Optimization
 

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  1. POGROUP BY Optimization
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    <p>Hey. I´ve got these two tables in a 1:n relation.</p> <pre><code>CREATE TABLE IF NOT EXISTS `de_locations` ( `id` int(11) NOT NULL auto_increment, `user_id` int(11) default NULL, `author_id` int(11) NOT NULL, `city_id` int(11) NOT NULL, `district_id` int(11) NOT NULL, `title` varchar(150) collate utf8_unicode_ci NOT NULL, `description` tinytext collate utf8_unicode_ci, `lat` double NOT NULL, `lng` double NOT NULL, `stars` double default '0', `comments` mediumint(9) default '0', `flag` tinyint(4) default '0', PRIMARY KEY (`id`), KEY `user_id` (`user_id`), KEY `flag` (`flag`), KEY `rating_district` (`district_id`,`stars`,`comments`), KEY `rating_city` (`city_id`,`stars`,`comments`) ) ENGINE=MyISAM DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci AUTO_INCREMENT=15 ; </code></pre> <p>and</p> <pre><code>CREATE TABLE IF NOT EXISTS `de_location2category` ( `id` int(11) NOT NULL auto_increment, `location_id` int(11) NOT NULL, `cat_id` mediumint(9) NOT NULL, PRIMARY KEY (`id`), UNIQUE KEY `rel` (`location_id`,`cat_id`) ) ENGINE=MyISAM DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci AUTO_INCREMENT=14 ; </code></pre> <p>A location could be placed in more than one category.</p> <p>For example:</p> <p>Location: "Pizza Hut" Categories: "Italian Food", "Fast Food"</p> <p>These categories are child categories of the parent category food.</p> <p>Now I want to select all the locations within the category food.</p> <pre><code>SELECT a.id, a.title, a.description, a.street, a.hnr, ROUND(a.stars) as stars, a.comments, a.lat, a.lng FROM de_locations as a INNER JOIN de_location2category as b ON b.location_id = a.id WHERE b.cat_id BETWEEN 0 AND 100 AND a.city_id = 1000 GROUP BY a.id ORDER BY a.stars DESC, a.comments DESC </code></pre> <p>I need the GROUP BY because I don´t want duplicate locations if they where related to more than one category. But this query builds an temporary table and uses filesort. If I leave the GROUP BY everything is fine, but I need it …</p> <p>Have I got to add another index? Or is anything wrong with my scheme? How would you solve this problem? Thanks a lot.</p>
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    1. USBrian Rasmussen
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    1. CO"But this query builds an temporary table and uses filesort." So what? That's the way relational databases work. What's wrong with that? How else do you propose it removes duplicates?
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      1. POGROUP BY Optimization
      1. USS.Lott
    2. COhttp://dev.mysql.com/doc/refman/5.0/en/order-by-optimization.html "In some cases, MySQL cannot use indexes to resolve the ORDER BY, although it still uses indexes to find the rows that match the WHERE clause. These cases include the following: *You use ORDER BY on different keys: SELECT * FROM t1 ORDER BY key1, key2;*"
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      1. USAmber
    3. CO"What´s wrong with that?" The performance. In a simple SELECT a GROUP BY not always force temporary tables. Isn´t there a way to achieve that in this case? Also the index "rating_city" is not used with GROUP BY.
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