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POWhy is mySQL query, left join 'considerably' faster than my inner join
 

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  1. POWhy is mySQL query, left join 'considerably' faster than my inner join
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    <p>I've researched this, but I still cannot explain why:</p> <pre><code>SELECT cl.`cl_boolean`, l.`l_name` FROM `card_legality` cl INNER JOIN `legality` l ON l.`legality_id` = cl.`legality_id` WHERE cl.`card_id` = 23155 </code></pre> <p>Is significantly slower than:</p> <pre><code>SELECT cl.`cl_boolean`, l.`l_name` FROM `card_legality` cl LEFT JOIN `legality` l ON l.`legality_id` = cl.`legality_id` WHERE cl.`card_id` = 23155 </code></pre> <p>115ms Vs 478ms. They are both using InnoDB and there are relationships defined. The 'card_legality' contains approx 200k rows, while the 'legality' table contains 11 rows. Here is the structure for each:</p> <pre><code>CREATE TABLE `card_legality` ( `card_id` varchar(8) NOT NULL DEFAULT '', `legality_id` int(3) NOT NULL, `cl_boolean` tinyint(1) NOT NULL, PRIMARY KEY (`card_id`,`legality_id`), KEY `legality_id` (`legality_id`), CONSTRAINT `card_legality_ibfk_2` FOREIGN KEY (`legality_id`) REFERENCES `legality` (`legality_id`), CONSTRAINT `card_legality_ibfk_1` FOREIGN KEY (`card_id`) REFERENCES `card` (`card_id`) ) ENGINE=InnoDB DEFAULT CHARSET=latin1; </code></pre> <p>And:</p> <pre><code>CREATE TABLE `legality` ( `legality_id` int(3) NOT NULL AUTO_INCREMENT, `l_name` varchar(16) NOT NULL DEFAULT '', PRIMARY KEY (`legality_id`) ) ENGINE=InnoDB AUTO_INCREMENT=12 DEFAULT CHARSET=latin1; </code></pre> <p>I could simply use LEFT-JOIN, but it doesn't seem quite right... any thoughts, please?</p> <p><strong>UPDATE:</strong> As requested, I've included the results of explain for each. I had run it previously, but I dont pretend to have a thorough understanding of it..</p> <pre><code>id select_type table type possible_keys key key_len ref rows Extra 1 SIMPLE cl ALL PRIMARY NULL NULL NULL 199747 Using where 1 SIMPLE l eq_ref PRIMARY PRIMARY 4 hexproof.co.uk.cl.legality_id 1 </code></pre> <p>AND, inner join:</p> <pre><code>id select_type table type possible_keys key key_len ref rows Extra 1 SIMPLE l ALL PRIMARY NULL NULL NULL 11 1 SIMPLE cl ref PRIMARY,legality_id legality_id 4 hexproof.co.uk.l.legality_id 33799 Using where </code></pre>
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    1. COBy the way `card_id` is a VARCHAR as I have no choice, I wouldn't normally accept that.
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