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    <p>The EXPLAIN report may say "Using filesort" but that's misleading. It doesn't mean it's writing to a file, it only means it's sorting without the benefit of an index.</p> <p>The EXPLAIN report may say "Using temporary" but this doesn't mean it's using a temporary table on disk. It can make a small temp table in memory. The table must fit within the <em>lesser</em> of <a href="http://dev.mysql.com/doc/refman/5.1/en/server-system-variables.html#sysvar_max_heap_table_size" rel="noreferrer">max_heap_table_size</a> and <a href="http://dev.mysql.com/doc/refman/5.1/en/server-system-variables.html#sysvar_tmp_table_size" rel="noreferrer">tmp_table_size</a>. If you increase tmp_table_size, you should also increase max_heap_table_size to match. </p> <p>See also <a href="http://dev.mysql.com/doc/refman/5.1/en/internal-temporary-tables.html" rel="noreferrer">http://dev.mysql.com/doc/refman/5.1/en/internal-temporary-tables.html</a> for more info on temporary table management.</p> <p>But 4 gigs for that value is very high! Consider that this memory can potentially be used <em>per connection</em>. The default is 16 megs, so you've increased it by a factor of 256.</p> <p>So we want to find which queries caused the temp disk tables.</p> <p>If you run MySQL 5.1 or later, you can <a href="http://dev.mysql.com/doc/refman/5.1/en/server-system-variables.html#sysvar_long_query_time" rel="noreferrer">SET GLOBAL long_query_time=0</a> to make <em>all</em> queries output to the slow query log. Be sure to do this only temporarily and set it back to a nonzero value when you're done! :-)</p> <p>If you run <a href="http://www.percona.com/software/percona-server/" rel="noreferrer">Percona Server</a>, the <a href="http://www.percona.com/docs/wiki/percona-server%3afeatures%3aslow_extended_51" rel="noreferrer">slow query log is extended</a> with additional information and configurability, including whether the query caused a temp table or a temp disk table. You can even filter the slow-query log to include only queries that cause a temp table or temp disk table (the the docs I link to).</p> <p>You can also process Percona Server's slow-query log with <a href="http://www.maatkit.org/doc/mk-query-digest.html" rel="noreferrer">mk-query-digest</a> and filter for queries that cause a temp disk table.</p> <pre><code>mk-query-digest /path/to/slow.log --no-report --print \ --filter '($event-&gt;{Disk_tmp_table }||"") eq "Yes"' </code></pre>
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    1. POMysql - Finding cause of temp disk tables
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