MySQL如何优化GROUP BY

SELECT c1, c2 FROM t1 GROUP BY c1, c2;SELECT DISTINCT c1, c2 FROM t1;SELECT c1, MIN(c2) FROM t1 GROUP BY c1;SELECT c1, c2 FROM t1 WHERE c1  const GROUP BY c1, c2;SELECT c2 FROM t1 WHERE c1 <p>不满足条件示例：</p><pre class="brush:php;toolbar:false">1. 除了MIN()或MAX()，还有其它累积函数，例如： SELECT c1, SUM(c2) FROM t1 GROUP BY c1;<br>2. GROUP BY子句中的域不引用索引开头，例如：

 SELECT c1,c2 FROM t1 GROUP BY c2, c3;<br>3. 查询引用了GROUP BY 部分后面的关键字的一部分，并且没有等于常量的等式，例如： SELECT c1,c3 FROM t1 GROUP BY c1, c2；

1. GROUP BY 中有一个漏洞，但已经由条件c2 = 'a'覆盖。 SELECT c1，c2，c3 FROM t1 WHERE c2 = 'a' GROUP BY c1，c3;<br>2. GROUP BY 不是满足最左前缀，但是有一个条件提供该元素的常量： SELECT c1，c2，c3 FROM t1 WHERE c1 = 'a' GROUP BY c2，c3;