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POMySQL Explain: what's causing 'Using temporary; Using filesort'
 

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  1. POMySQL Explain: what's causing 'Using temporary; Using filesort'
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    <p>I'm planning on creating a view using this SQL SELECT, but the explain for it shows it's using temporary and using filesort. I can't figure out what indices I need in order to fix this problem. Mostly, I'm wondering why it's using filesort intead of using an index to sort.</p> <p>Here are my tables:</p> <pre><code>CREATE TABLE `learning_signatures` ( `signature_id` int(11) NOT NULL AUTO_INCREMENT, `signature_file` varchar(100) NOT NULL, `signature_md5` varchar(32) NOT NULL, `image_file` varchar(100) NOT NULL, PRIMARY KEY (`signature_id`), UNIQUE KEY `unique_signature_md5` (`signature_md5`) ) ENGINE=InnoDB AUTO_INCREMENT=640 DEFAULT CHARSET=latin1 CREATE TABLE `learning_user_suggestions` ( `user_suggestion_id` int(11) NOT NULL AUTO_INCREMENT, `signature_id` int(11) NOT NULL, `ch` char(1) NOT NULL, `time_suggested` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP, `user_id` int(11) NOT NULL, PRIMARY KEY (`user_suggestion_id`), KEY `char_index` (`ch`), KEY `ls_sig_id_indx` (`signature_id`), KEY `user_id_indx` (`user_id`), KEY `sig_char_indx` (`signature_id`,`ch`) ) ENGINE=InnoDB AUTO_INCREMENT=1173 DEFAULT CHARSET=latin1 </code></pre> <p>And here is the problematic SQL statement I'm planning on using in my view:</p> <pre><code>select ls.signature_id, ls.signature_file, ls.signature_md5, ls.image_file, sug.ch , count(sug.ch) AS suggestion_count from (`learning_signatures` `ls` left join `learning_user_suggestions` `sug` on(ls.signature_id = sug.signature_id)) group by ls.signature_id, sug.ch; </code></pre> <p>Output from explain:</p> <pre><code>id select_type table type possible_keys key key_len ref rows Extra 1 SIMPLE ls ALL NULL NULL NULL NULL 514 "Using temporary; Using filesort" 1 SIMPLE sug ref ls_sig_id_indx,sig_char_indx ls_sig_id_indx 4 wwf.ls.signature_id 1 </code></pre> <p>Another example, this time using a where clause:</p> <pre><code>explain select ls.signature_id, ls.signature_file, ls.signature_md5, ls.image_file, sug.ch , count(sug.ch) AS suggestion_count from (`learning_signatures` `ls` left join `learning_user_suggestions` `sug` on(ls.signature_id = sug.signature_id)) WHERE signature_md5 = '75f8a5b1176ecc2487b90bacad9bc4c' group by ls.signature_id, sug.ch; </code></pre> <p>Explain output:</p> <pre><code>id select_type table type possible_keys key key_len ref rows Extra 1 SIMPLE ls const unique_signature_md5 unique_signature_md5 34 const 1 "Using temporary; Using filesort" 1 SIMPLE sug ref ls_sig_id_indx,sig_char_indx ls_sig_id_indx 4 const 1 </code></pre>
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    1. This table or related slice is empty.
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      1. POMySQL Explain: what's causing 'Using temporary; Using filesort'
      1. USTodd Chaffee
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      1. POMySQL Explain: what's causing 'Using temporary; Using filesort'
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      1. POMySQL Explain: what's causing 'Using temporary; Using filesort'
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    1. COPost EXPLAIN output, post your CREATE VIEW statement..
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      1. POMySQL Explain: what's causing 'Using temporary; Using filesort'
      1. USMichael J.V.
    2. COGood point, I just added the explain output. I haven't created the view yet, I want to optimize the SELECT first
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      1. POMySQL Explain: what's causing 'Using temporary; Using filesort'
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    3. COSo what is exactly wrong here? You don't have a factor that limits the dataset, entire learning_signatures table will be pulled. Using temporary is probably because of your innodb_buffer_pool size.
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      1. USMichael J.V.
 

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