What is a Covering Index?
The concept of "covering indexes" explained in under 100 words. #Under100
![What is a Covering Index?](/content/images/size/w2000/2021/08/baby-1399332_1920.jpg)
In a typical index, the database uses the index to look up the primary key, which it then uses to look up the row in the original table. Two lookups.
![](https://nolongerset.com/content/images/2021/08/image-58.png)
If you only need a small subset of fields, you can save a lookup by creating a compound index that includes those fields.
![](https://nolongerset.com/content/images/2021/08/image-57.png)
An index is considered a covering index in the context of a SELECT statement that requires only the "covered" fields. Thus, the CityName, Country index covers the following SQL statement:
SELECT CityName, Country
FROM City
WHERE CityName = 'Berlin'
Bottom Line
A covering index saves a lookup.