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POBasic optimisation with an index for mysql
 

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    <p>i have a question about basic mysql database optimisation. I have 3 tables, Articles, Tags and Taggings (which is a join table).</p> <pre><code>Articles Taggings Tags id id id name article_id name tag_id </code></pre> <p>I am retrieving the articles that exactly match the tags specified, with the following query</p> <pre><code>SELECT *, COUNT(*) AS c FROM articles AS a JOIN taggings AS tng ON a.id = tng.article_id JOIN tags AS t ON t.id = tng.tag_id WHERE t.name IN ("Red","Green") GROUP BY a.id HAVING c = 2 </code></pre> <p>This query is slow, so I did an EXPLAIN, and got the following results:</p> <p><a href="http://dl.dropbox.com/u/2306276/EXPLAIN%20results.png" rel="nofollow noreferrer">alt text http://dl.dropbox.com/u/2306276/EXPLAIN%20results.png</a></p> <p>Now, I don't really understand what I am doing here, but i believe that "type: ALL" is not good, so thought i would add indexes(BTREE) to both article_id and tag_id in the taggings table, and run the query again. <a href="http://dl.dropbox.com/u/2306276/EXPLAIN%20results%202.png" rel="nofollow noreferrer">alt text http://dl.dropbox.com/u/2306276/EXPLAIN%20results%202.png</a> Well that didn't look any better to my uneducated eye, same number of rows as previous one, and the type is still ALL in two of the cases.</p> <p>So could someone tell me where I am going wrong please? will indexes not help me with this problem?</p> <p>My Tag table will remain relatively small, so I thought the query should scan the Tag table for the tags I have specified, and then (through the indexes) be able to instantly retrieve the associated properties, and it should all be very quick, obviously something wrong in my thinking.</p> <p>Thanks</p> <p>[EDIT] - for Jay's comments</p> <p>I added 10k articles, 30k taggings, and 6 tags, also added 2 indexs on tag.name and taggings.tag_id, the query still took a long time to run, 0.5-1 second, the EXPLAIN is below. <a href="http://dl.dropbox.com/u/2306276/EXPLAIN%20results%203.png" rel="nofollow noreferrer">alt text http://dl.dropbox.com/u/2306276/EXPLAIN%20results%203.png</a></p>
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    1. COis tags.name indexed?
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    2. COI have to [wonder if the GROUP BY is partly to blame for the poor performance, given MySQL's support of hidden columns](http://dev.mysql.com/doc/refman/5.0/en/group-by-hidden-columns.html)
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    3. CODo you have an index for tags.name? That would probably help if your database has a lot of records
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