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# What is a Database Index?
- URL: https://nolongerset.com/under-100-database-index/
- Published: 2021-08-26T00:51:32.000Z
- Updated: 2026-05-08T13:02:45.000Z
- Description: The concept of database indexes explained in under 100 words. #Under100
- Author: Mike Wolfe
- Tags: Under 100, #Import 2026-05-20 02:59

![](https://storage.ghost.io/c/eb/d7/ebd732c1-5f03-4f07-b386-5d08557e15c9/content/images/2021/08/image-49.png)

Original table

An index is an internal copy of a table sorted by the index field(s). The only columns are the index field(s) and primary key.

![](https://storage.ghost.io/c/eb/d7/ebd732c1-5f03-4f07-b386-5d08557e15c9/content/images/2021/08/image-50.png)

The internal table maintained for the CityName index.

The database uses the index to look up the primary key. It uses the primary key to look up the row in the original table.

### Advantages

- Faster lookups

### Disadvantages

Each index is an extra table for the database to maintain, which means:

- Extra storage space
- Slower INSERT/UPDATE/DELETE

### Bottom Line

It's important to understand the tradeoffs when adding (or omitting) indexes.

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