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POOptimizing MySQL Query
 

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    <p>We have a query that is currently killing our database and I know there has to be a way to optimize it. We have 3 tables:</p> <ol> <li>items - table of items where each items has an associated object_id, length, difficulty_rating, rating, avg_rating & status <li>lists - table of lists which are basically lists of items created by our users <li>list_items - table with 2 columns: list_id, item_id </ol> <p>We've been using the following query to display a simple HTML table that shows each list and a number of attributes related to the list including averages of attributes of the included list items:</p> <pre><code>select object_id, user_id, slug, title, description, items, city, state, country, created, updated, (select AVG(rating) from items where object_id IN (select object_id from list_items where list_id=lists.object_id) AND status="A" ) as 'avg_rating', (select AVG(avg_rating) from items where object_id IN (select object_id from list_items where list_id=lists.object_id) AND status="A" ) as 'avg_avg_rating', (select AVG(length) from items where object_id IN (select object_id from list_items where list_id=lists.object_id) AND status="A" ) as 'avg_length', (select AVG(difficulty_rating) from items where object_id IN (select object_id from list_items where list_id=lists.object_id) AND status="A" ) as 'avg_difficulty' from lists where user_id=$user_id AND status="A" order by $orderby LIMIT $start,$step </code></pre> <p>The reason why we haven't broken this up in 1 query to get all the lists and subsequent lookups to pull the averages for each list is because we want the user to be able to sort on the averages columns (i.e. 'order by avg_difficulty').</p> <p>Hopefully my explanation makes sense. There has to be a much more efficient way to do this and I'm hoping that a MySQL guru out there can point me in the right direction. Thanks!</p>
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    1. USJim Ferrans
    1. USRussell C.
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      1. USMaD70
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    1. COCan you please edit your question and include the DDL for all tables? Seeing indexes is key (no pun intended) to understanding if your bottleneck is simply bad/missing indexes.
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    2. COIt would be important to see what your `$orderby` actually is, as well as the result of an `EXPLAIN`. I have a feeling that it's probably using a temporary table and a filesort to order, due to improper indexing.
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      1. USjason
 

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