# 复合索引 (C)

+ 覆盖索引：当通过索引可以完整获取数据，那么 TiDB 可以直接通过遍历索引取得数据，而无需回表，减少 I/O 操作。所以，在真正的实际应用中，覆盖索引是主要的提升性能的优化手段之一
```
select c1 from t2 where c1 = 1 and c2 = 10 and c3 between 1 and 200;
select c1, c2, c3 from t2 where c1 = 1 and c2 between 1 and 200;
```
-------------------------------------------------------------
[no notes]