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POMySQL: Why does an Order By ID runs much slower than Order By other Columns?
 

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  1. POMySQL: Why does an Order By ID runs much slower than Order By other Columns?
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    <p>I am using MySQL version 5.5.14 to run the following query, <strong>QUERY 1</strong>, from a table of 5 Million rows:</p> <pre><code>SELECT P.ID, P.Type, P.Name, P.cty , X(P.latlng) as 'lat', Y(P.latlng) as 'lng' , P.cur, P.ak, P.tn, P.St, P.Tm, P.flA, P.ldA, P.flN , P.lv, P.bd, P.bt, P.nb , P.ak * E.usD as 'usP' FROM PIG P INNER JOIN EEL E ON E.cur = P.cur WHERE act='1' AND flA &gt;= '1615' AND ldA &gt;= '0' AND yr &gt;= (YEAR(NOW()) - 100) AND lv &gt;= '0' AND bd &gt;= '3' AND bt &gt;= '2' AND nb &lt;= '5' AND cDate &gt;= NOW() AND MBRContains(LineString( Point(39.9097, -2.1973) , Point(65.5130, 41.7480) ), latlng) AND Type = 'g' AND tn = 'l' AND St + Tm - YEAR(NOW()) &gt;= '30' HAVING usP BETWEEN 300/2 AND 300 ORDER BY ak LIMIT 100; </code></pre> <p>Using an <strong>Index (Type, tn, act, flA)</strong>, I am able to obtain results within <strong>800ms</strong>. In <strong>QUERY 2</strong>, I changed the <code>ORDER BY</code> clause to <code>lv</code>, I am also able to obtain results within similar timings. In <strong>QUERY 3</strong>, I changed the <code>ORDER BY</code> clause to <code>ID</code> and the query time slowed dramatically to a full <strong>20s</strong> on an average of 10 trials.</p> <p>Running the <code>EXPLAIN SELECT</code> statement produces exactly the same query execution plan:</p> <pre><code>*************************** 1. row *************************** id: 1 select_type: SIMPLE table: P type: range possible_keys: Index key: Index key_len: 6 ref: NULL rows: 132478 Extra: Using where; Using filesort *************************** 2. row *************************** id: 1 select_type: SIMPLE table: E type: eq_ref possible_keys: PRIMARY key: PRIMARY key_len: 3 ref: BS.P.cur rows: 1 Extra: </code></pre> <p>My question is: <strong>why does ordering by ID in QUERY 3 runs so slow compared to the rest?</strong></p> <p>The partial table definition is as such:</p> <pre><code>CREATE TABLE `PIG` ( `ID` int(10) unsigned NOT NULL AUTO_INCREMENT, `lv` smallint(3) unsigned NOT NULL DEFAULT '0', `ak` int(10) unsigned NOT NULL DEFAULT '0', PRIMARY KEY (`ID`), KEY `id_ca` (`cty`,`ak`), KEY `Index` (`Type`, `tn`, `act`, `flA`), ) ENGINE=MyISAM AUTO_INCREMENT=5000001 DEFAULT CHARSET=latin1 CREATE TABLE `EEL` ( `cur` char(3) NOT NULL, `usD` decimal(11,10) NOT NULL, PRIMARY KEY (`cur`) ) ENGINE=MyISAM DEFAULT CHARSET=latin1 </code></pre> <p>UPDATE: After extensive testing of various <code>ORDER BY</code>s options, I have confirmed that the ID column which happens to be the Primary Key is the only one causing the slow query time.</p>
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    1. COnot an answer, but a comment on your queries/database. Use relevant names, for your and others sake. If you leave this application for a month or so, and then come back. You will have to figure out what every column contains. And for questions on forums like this it doesn't make much sense. It will require a little extra typing, but that should not be any problem.
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    2. CO@ Topener: I am sorry if using short-forms on column names makes it harder for you to understand my question. I have my concern in revealing the application. Hope you can understand ;p
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    3. COHow much time does the query need to run without `ORDER BY` but with `LIMIT 100` ?
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