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POMySQL: Searching same table multiple times with joins
 

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  1. POMySQL: Searching same table multiple times with joins
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    <p>I am writing a script to do a partial-word search on messages for a user. Each conversation has a mail_id, and each individual message has a msg_id.</p> <p>I have a table, mail_word_index, which contains a row for each word in a message. </p> <pre><code>CREATE TABLE IF NOT EXISTS `mail_word_index` ( `id` int(10) unsigned NOT NULL AUTO_INCREMENT, `sender_id` int(10) unsigned NOT NULL DEFAULT '0', `dest_id` int(10) unsigned NOT NULL DEFAULT '0', `mail_id` int(10) unsigned NOT NULL DEFAULT '0', `msg_id` int(10) unsigned NOT NULL DEFAULT '0', `word` varchar(15) NOT NULL DEFAULT '', PRIMARY KEY (`id`), KEY `dest_id` (`dest_id`,`word`), KEY `sender_id` (`sender_id`,`word`), KEY `multiple_words` (`mail_id`,`msg_id`,`word`) ) ENGINE=MyISAM ; </code></pre> <p>I have a query which takes 0.01 seconds to finish</p> <pre><code>SELECT DISTINCT w1.mail_id FROM mail_word_index AS w1, mail_word_index AS w2 WHERE w1.sender_id=1 AND w1.word LIKE 'str%' AND w1.mail_id=w2.mail_id AND w1.msg_id=w2.msg_id AND w2.word LIKE 'con%' LIMIT 20 </code></pre> <p>However, searching one word at a time only takes 0.002 seconds to finish each one, and 0.004 seconds total:</p> <pre><code>SELECT DISTINCT w1.mail_id FROM mail_word_index AS w1 WHERE w1.sender_id=1 AND w1.word LIKE 'str%' LIMIT 20 SELECT DISTINCT w1.mail_id FROM mail_word_index AS w1 WHERE w1.sender_id=1 AND w1.word LIKE 'con%' LIMIT 20 </code></pre> <p>The inner join seems to slow down the first query. How would I change the first query to make it faster?</p> <p>The EXPLAIN tells me this:</p> <pre><code>id select_type table type possible_keys key key_len ref rows Extra 1 SIMPLE w1 range sender_id,multiple_words sender_id 21 NULL 1 Using where; Using temporary 1 SIMPLE w2 ref multiple_words multiple_words 8 game-node4.w1.mail_id,game-node4.w1.msg_id 8 Using where; Using index; Distinct </code></pre>
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      1. VTUpMod
    1. COYou're *seriously* worrying about **6ms**??
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      1. USeggyal
    2. COAfaik, your `multiple_words` is already the best possible index for that query. It'll be slower because MySQL has to join.
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    3. COAs the table grows larger, and more words are searched at a time, the time difference between the query with the multiple joins, and the total time of each individual query grows much larger
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