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    <p>For your query to run fast, you need:</p> <ol> <li><p>Create an index: <code>CREATE INDEX ix_timestamp_id ON items (timestamp, id)</code></p> <ul> <li>Ensure that <code>id</code>'s on <code>sources</code>, <code>authors</code> and <code>keywords</code> are primary keys.</li> <li><p>Force MySQL to use this index for items, and perform <code>NESTED LOOP</code> joins for other items:</p> <p>EXPLAIN EXTENDED SELECT <code>Item</code>.*, <code>Source</code> . * , <code>Keyword</code> . * , <code>Author</code> . * FROM <code>items</code> AS <code>Item</code> FORCE INDEX FOR ORDER BY (ix_timestamp_id) JOIN <code>items_keywords</code> AS <code>ItemsKeyword</code> FORCE INDEX (ix_item_keyword) ON ( <code>Item</code>.<code>id</code> = <code>ItemsKeyword</code>.<code>item_id</code> AND <code>ItemsKeyword</code>.<code>keyword_id</code> IN ( SELECT <code>keyword_id</code> FROM <code>keywords_profiles</code> AS <code>KeywordsProfile</code> FORCE INDEX (ix_keyword_profile) WHERE <code>KeywordsProfile</code>.<code>profile_id</code> = 17 ) ) JOIN <code>sources</code> AS <code>Source</code> FORCE INDEX (primary) ON ( <code>Item</code>.<code>source_id</code> = <code>Source</code>.<code>id</code> ) JOIN <code>authors</code> AS <code>Author</code> FORCE INDEX (primary) ON ( <code>Item</code>.<code>author_id</code> = <code>Author</code>.<code>id</code> ) JOIN <code>keywords</code> AS <code>Keyword</code> FORCE INDEX (primary) ON ( <code>Keyword</code>.<code>id</code> = <code>ItemsKeyword</code>.<code>keyword_id</code> ) ORDER BY <code>Item</code>.timestamp DESC, <code>Item</code>.id DESC LIMIT 0, 20</p></li> </ul></li> </ol> <p>As you can see, we get rid of <code>GROUP BY</code>, push the subquery into the <code>JOIN</code> condition and force <code>PRIMARY KEY</code>s to be used for joins.</p> <p>That's how we ensure that <code>NESTED LOOPS</code> with <code>items</code> as a leading tables will be used for all joins.</p> <p>As a result:</p> <pre> 1, 'PRIMARY', 'Item', 'index', '', 'ix_timestamp_id', '12', '', 20, 2622845.00, '' 1, 'PRIMARY', 'Author', 'eq_ref', 'PRIMARY', 'PRIMARY', '4', 'test.Item.author_id', 1, 100.00, '' 1, 'PRIMARY', 'Source', 'eq_ref', 'PRIMARY', 'PRIMARY', '4', 'test.Item.source_id', 1, 100.00, '' 1, 'PRIMARY', 'ItemsKeyword', 'ref', 'PRIMARY', 'PRIMARY', '4', 'test.Item.id', 1, 100.00, 'Using where; Using index' 1, 'PRIMARY', 'Keyword', 'eq_ref', 'PRIMARY', 'PRIMARY', '4', 'test.ItemsKeyword.keyword_id', 1, 100.00, '' 2, 'DEPENDENT SUBQUERY', 'KeywordsProfile', 'unique_subquery', 'PRIMARY', 'PRIMARY', '8', 'func,const', 1, 100.00, 'Using index; Using where' </pre> <p>, and when we run this, we get</p> <pre><code>20 rows fetched in 0,0038s (0,0019s) </code></pre> <p>There are <code>500k</code> in <code>items</code>, <code>600k</code> in <code>items_keywords</code>, <code>512</code> values in <code>keywords</code> and <code>512</code> values in <code>keywords_profiles</code> (all with profile <code>17</code>).</p>
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    1. POWhat's wrong with this query? EXPLAIN looks fine to me
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    1. USQuassnoi
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    1. COreally really appreciate your help here, but this didn't work. CPU spiked above 100% and stayed there for about five minutes before I finally killed it. explain revealed that there was still a temp table / filesort issue. once I removed the FORCE INDEX on everything but items, it took ~60 secs :(
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    2. CODid you create an index on (timestamp, id)? Did you put correct index name into FORCE INDEX?
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    3. COAh, and make sure ItemsKeyword has a UNIQUE INDEX or PRIMARY KEY defined on (item_id, keyword_id) (in that order)
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