动态设置变量可能导致意外的副作用，例如从缓冲中刷新脏块。务必小心那些可以在线更改的设置，因为它们可能导致数据库做大量的工作。有时可以通过名称推断一个变
。）
总的来说，设置很大的排序缓存代价可能非常高，所以除非确定必须要这么大，否则不要增加排序缓存的大小。
如果查询必须使用一个更大的排序缓存才能比较好地执行，可以在查询执行前增加sort_buffer_size的值，执行完成后恢复DEFAULT。
eg：
SET @@session.sort_buffer_size :=可以将类似的代码封在函数中以方便使用。其它可以设置的单个连接级别的变量有read_buffer_size，read_rnd_buffer_size，tmp_table_size，以及myisam_sort_buffer_size。
特别说明：本文章是来自
个人感悟：理解可动态更改的变量产生的影响是蛮重要的，一不小心，可能导致负载飞一般的暴涨，CPU刷刷的飙升，香港虚拟主机，甚至宕机。
糗事：本人曾在线上数据库动态修改query_cache_szie的值，本来server的负载都40-50的样子啦，当时也不没考虑那么多，香港服务器租用，就直接改了，负载瞬间飙升到了200多，那个汗啊，，，
好的东西是拿出来分享的，香港空间，那样它就会更美！！！
本文出自 “Focus on the database” 博客，请务必保留此出处
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