> For the complete MongoDB documentation index, see www.mongodb.com/docs/llms.txt

# Review Drop Index Recommendations

The Performance Advisor suggests dropping unused, redundant, and hidden indexes to improve write performance and increase storage space.

**See also:**

To learn more about the impact of unnecessary indexes, see [Remove Unnecessary Indexes.](https://www.mongodb.com/docs/manual/data-modeling/design-antipatterns/unnecessary-indexes/)

## Unused Indexes

An index is unused if it doesn't support any queries.

The Performance Advisor flags an index as unused if it hasn't supported a query in 7 or more days after it was created or the server was restarted.

**Note:**

The Performance Advisor inspects only the 20 most active collections for unused indexes.

## Redundant Indexes

An index is redundant if another index supports any query that it could.

The Performance Advisor flags an index as redundant if it matches the prefix of another index in the same collection.

**Example:**

If a collection contains the indexes:

- `{ a: 1 }`

- `{ b: -1 }`

- `{ a: 1, b: -1 }`

`{ a: 1 }` is redundant because it matches the prefix `{ a: 1, b: -1 }`.

`{ b: -1 }` is not redundant because it does not match any prefix.

In the Performance Advisor, redundant indexes are marked with a red `Redundant` badge. Below each redundant index, the Performance Advisor displays the related indexes that cover it.

**Note:**

Related indexes are displayed for you to verify that the redundant index can be dropped safely. Related indexes are not recommended for removal.

## Hidden Indexes

Atlas doesn't use hidden indexes to support queries. They still impact write performance and consume storage space. To learn more, see [Hidden Indexes.](https://www.mongodb.com/docs/manual/core/index-hidden/)

You can hide an index to evaluate the impact of dropping an index before you drop it. Unhiding an index also takes less time than rebuilding a dropped index. To hide and unhide an index by using the Atlas UI, see [Manage Atlas Indexes.](https://www.mongodb.com/docs/atlas/atlas-ui/indexes.md#std-label-atlas-ui-indexes)

The Performance Advisor always recommends dropping hidden indexes. If you determine that a hidden index is unnecessary, drop it.

**Note:**

The Atlas Performance Advisor page displays hidden indexes from your top 20 most active namespaces at the time you load the page. If you have more than 20 namespaces, the Performance Advisor page might not display all of your namespaces with hidden indexes.

## Drop or Hide an Index

**Note:**

Consider [hiding indexes](https://www.mongodb.com/docs/atlas/performance-advisor/drop-indexes.md#std-label-hidden-indexes) before you drop them.

To drop or hide an index by using the Performance Advisor:

1. In Atlas, go to the Clusters page for your project.

   If it's not already displayed, select the organization that contains your desired project from the  Organizations menu in the navigation bar.

   If it's not already displayed, select your desired project from the Projects menu in the navigation bar.

   In the sidebar, click Clusters under the Database heading.

   The [Clusters](https://cloud.mongodb.com/go?l=https%3A%2F%2Fcloud.mongodb.com%2Fv2%2F%3Cproject%3E%23%2Fclusters) page displays.

2. Click the replica set where the collection resides.

   If the replica set is part of a sharded cluster, first click the sharded cluster containing the replica set.

3. Click Performance Advisor.

4. View recommendations.

   In the Performance Advisor tab, click Explore Recommendations on the Drop Indexes card.

5. On the index that you want to drop or hide, click Drop Index.

   The Performance Advisor displays a dialog box with a link to the [Atlas UI](https://www.mongodb.com/docs/atlas/atlas-ui/indexes.md#std-label-atlas-ui-indexes) and a copyable [MongoDB Shell](https://www.mongodb.com/docs/mongodb-shell/) command to drop that index.

   The dialog box also provides a copyable MongoDB Shell command to hide that index.

6. Drop or hide the index by using the Atlas UI or MongoDB Shell.

   To drop or hide an index by using the Atlas UI, click the Indexes tab, then click the Drop Index or Hide Index icon next to the index. Atlas displays a dialog box to confirm your selection. For more information, see [Manage Atlas Indexes.](https://www.mongodb.com/docs/atlas/atlas-ui/indexes.md#std-label-atlas-ui-indexes)

   To drop or hide an index by using the MongoDB Shell, paste and run the command provided by the Performance Advisor.