1. 跳过列,where a = 1 and c = 3,最多用到索引列a;where b = 2 and c = 3,一个也用不到,必须从最左列开始 2. 前面是范围查询,where a = 1 and b > 2 and c = 3,最多用到 a, b两个索引列; 3. 顺序颠倒,where c = 3 and b = 2 and a = 1,一个也用不到; 4. 索引列上...