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POMySQL Query Optimization - Random Record
 

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    <p>I'm having a terrible time with a MySQL query. I've spent most of my weekend and most of my day today attempting to make this query run a bit faster. I've made it considerably faster, but I know I can make it better.</p> <pre><code>SELECT m.id,other_fields,C.contacts_count FROM marketingDatabase AS m LEFT OUTER JOIN (SELECT COUNT(*) as contacts_count, rid FROM contacts WHERE status = 'Active' AND install_id = 'XXXX' GROUP BY rid) as C ON C.rid = m.id WHERE (RAND()*2612&lt;50) AND do_not_call != 'true' AND `ACTUAL SALES VOLUME` &gt;= '800000' AND `ACTUAL SALES VOLUME` &lt;= '1200000' AND status = 'Pending' AND install_id = 'XXXXX' ORDER BY RAND() </code></pre> <p>I have an index on 'install_id', 'category' and 'status' but the EXPLAIN shows it was sorting based on 9100 rows.</p> <p>My Explain is here: <a href="https://s3.amazonaws.com/jas-so-question/Screen+Shot+2012-03-13+at+12.34.04+AM.png" rel="nofollow">https://s3.amazonaws.com/jas-so-question/Screen+Shot+2012-03-13+at+12.34.04+AM.png</a></p> <p>Anybody have any suggestions on what I can do to make this a bit faster? The entire point of the query is to select a random record from an account's records (install_id) that matches certain criteria like sales volume, status and do_not_call. I'm currently gathering 25 records and caching it (using PHP) so I only have to run this query once every 25 requests, but I'm already dealing with thousands of requests per day. It currently takes 0.2 seconds to run. I realize that by using ORDER BY RAND() I'm already taking a major performance hit, but it's just sorting 25 rows.</p> <p>Thanks in advance for the help.</p> <p>**EDIT: I forgot to mention that the 'contact_sort' index is on the 'contacts' table, and indexes install_id, status, and rid. (rid references Record ID in marketingDatabase so it knows which record a contact belongs to.</p> <p>**EDIT 2: The 2612 number in the query represents the number of rows in marketingDatabase that match the criteria (install_id, status, actual sales volume, etc.)</p>
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    1. COTake a look at http://dev.mysql.com/doc/refman/5.0/en/group-by-optimization.html There are restrictions and complexities that arise when using GROUP BY and indexes. To quote the manual "The most important preconditions for using indexes for GROUP BY are that all GROUP BY columns reference attributes from the same index, and that the index stores its keys in order (for example, this is a BTREE index and not a HASH index)." You are grouping by a non-indexed field, and also selecting and ordering the entire table in the sub-query. I think (not sure) that ordering by RAND() is also a bad idea.
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    2. COORDER BY RAND() is a very, very [bad idea](http://www.webtrenches.com/post.cfm/avoid-rand-in-mysql).
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    3. COYeah, I know it's a bad idea, although I've read because the ORDER BY RAND() is only ordering 25 rows, it's not that big of a deal. Am I wrong? How should I look to fix it and still get a random record? I do not have any column that is auto-increment without gaps...my id column is auto-increment but has gaps where rows have been deleted.
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