假设有组合索引 index(a，b，c)，以下情况不考虑 select * 内容 (覆盖索引)
where a = 3 用到索引 a
where a = 3 and b = 3 用到索引 a，b
where a = 3 and b = 4 and c = 5 用到索引 a，b，c
where b = 3 或者 where b = 3 and c = 4 或者 where c = 4 索引失效
where a = 3 and c = 5 用到索引 a
where a = 3 and b > 4 and c = 5 用到索引a和b，c不能用在范围之后
where a = 3 and b like 'kk%' and c = 4 用到索引a，b，c。
where a = 3 and b like '%kk' and c = 4 使用到 a
where a = 3 and b like '%kk%' and c = 4 用到 a
where a = 3 and b like 'k%kk%' and c = 4 用到a，b，c
切记使用索引字段值的类型必须匹配，否则索引无效。
