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POSlow SQL query because of ORDER BY not using index
 

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  1. POSlow SQL query because of ORDER BY not using index
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    <p>I've got this query:</p> <pre><code>SELECT cl.title, cl.URL, cl.ID AS ad_id, cl.cat_id, cl.price, cs.name AS cat_name, pix.file_name, area.area_name FROM classifieds cl FORCE INDEX (advertiser_id) INNER JOIN classifieds_pix pix ON cl.ID = pix.classified_id INNER JOIN cat_names_sub cs ON cl.cat_id = cs.ID INNER JOIN zip_codes zip ON cl.zip_id = zip.zip_id INNER JOIN area_names area ON zip.area_id = area.id WHERE cl.confirmed = 1 AND cl.price != '' AND cl.country = 'de' GROUP BY cl.advertiser_id ORDER BY cl.timestamp DESC LIMIT 5 </code></pre> <p>It takes > 1 sec when <code>classifieds</code> contains 168k rows, which is too long. <code>FORCE INDEX (advertiser_id)</code> allowed me to bring it down to 0.00x secs without the <code>ORDER BY</code> clause. The <code>timestamp</code> column is indexed, too, and I tried adding <code>FORCE INDEX (timestamp)</code>, but it didn't help. </p> <p><code>EXPLAIN</code> says <code>Using where; Using temporary; Using filesort</code> on the first <code>SELECT</code> from the <code>classifieds</code> table - which obviously causes the performance issue.</p> <p>Can you help me out on this one?</p> <p>Thanks in advance!</p> <p>PS: The purpose of this query is to get the 5 latest classifieds (including some additional information such as picture, category, zip code and area name). Furthermore, only one classified should be shown per advertiser. Can this be so hard?</p> <p>PPS: I tried to pin the problem down as much as possible and ended up with this query:</p> <pre><code>SELECT cl.title FROM classifieds cl GROUP BY cl.advertiser_id ORDER BY cl.timestamp DESC LIMIT 5 </code></pre> <p>It takes incredible 23 secs! With <code>FORCE INDEX (advertiser_id)</code> I can take it to 1 sec. If I remove either the GROUP BY or the ORDER BY, it goes down to 0.0003 secs.</p> <p>Something's gotta be wrong with my tables/indexes? I should not need <code>FORCE INDEX</code> (btw: <code>USE INDEX</code> doesn't work - I need to force it!) and it shouldn't take that long!</p>
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    1. COI'm confused by what it means to do your SELECT in the context of the GROUP BY cl.advertiser_id.
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      1. USNarveson
    2. COThe `GROUP BY` makes sure only one classified per advertiser is selected.
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    3. COcan you post your query explain plan, show indexes from <all tables in your query>, show create table <all tables in your query> - just throw it all into http://pastie.org/ to save space then we might be able to help - tia
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      1. USJon Black
 

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