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POReturning random rows from mysql database without using rand()
 

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  1. POReturning random rows from mysql database without using rand()
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    <p>I would like to be able to pull back 15 or so records from a database. I've seen that using <code>WHERE id = rand()</code> can cause performance issues as my database gets larger. All solutions I've seen are geared towards selecting a single random record. I would like to get multiples.</p> <p>Does anyone know of an efficient way to do this for large databases?</p> <p>edit:</p> <p><strong>Further Edit and Testing:</strong></p> <p>I made a fairly simple table, on a new database using MyISAM. I gave this 3 fields: <code>autokey</code> (unsigned auto number key) <code>bigdata</code> (a large blob) and <code>somemore</code> (a medium int). <strong>I then applied random data to the table and ran a series of queries using Navicat. Here are the results:</strong></p> <p><code>Query 1: select * from test order by rand() limit 15</code></p> <pre><code> Query 2: select * from test join (select round(rand()*(select max(autokey) from test)) as val from test limit 15) as rnd on rnd.val=test.autokey;` </code></pre> <p>(I tried both select and select distinct and it made no discernible difference)</p> <p>and:</p> <pre><code>Query 3 (I only ran this on the second test): SELECT * FROM ( SELECT @cnt := COUNT(*) + 1, @lim := 10 FROM test ) vars STRAIGHT_JOIN ( SELECT r.*, @lim := @lim - 1 FROM test r WHERE (@cnt := @cnt - 1) AND RAND(20090301) &lt; @lim / @cnt ) i </code></pre> <pre> ROWS: QUERY 1: QUERY 2: QUERY 3: 2,060,922 2.977s 0.002s N/A 3,043,406 5.334s 0.001s 1.260 </pre> <p>I would like to do more rows so I can see how query 3 scales, but at the moment, <strong>it seems as though the clear winner is query 2</strong>.</p> <p>Before I wrap up this testing and declare an answer, and while I have all this data and the test environment set up, <strong>can anyone recommend any further testing?</strong></p>
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    1. USChris Sobolewski
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    1. COThese might be helpful: http://stackoverflow.com/questions/142242/what-is-the-best-way-to-pick-a-random-row-from-a-table-in-mysql, http://stackoverflow.com/questions/1244555/how-can-i-optimize-mysqls-order-by-rand-function, http://stackoverflow.com/questions/1868102/order-by-rand-alternative
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