Question

how to create a partial index with $exists: false condition?

I'm working with MongoDB and want to create a partial index that only indexes documents where a specific field is missing ($exists: false). Can someone explain how to set up this type of partial index? What’s the correct syntax, and are there any performance considerations or best practices I should know about? Any help would be appreciated!

Answer 1

In MongoDB, you can use the partialFilterExpression option. 

Steps to use this :

1. Create a Partial Index with $exists: false:

Suppose you have a collection users and you want to create a partial index on the email field, but only for documents where the email field does not exist.

Use the following command:

db.users.createIndex(

  { email: 1 },  // Field to index

  {

    partialFilterExpression: { email: { $exists: false } }  // Condition for the index

  }

);

2. Explanation:

{ email: 1 }: This specifies the field you are indexing, and 1 indicates ascending order.

partialFilterExpression: { email: { $exists: false } }: This condition ensures that the index will only cover documents where the email field does not exist. The $exists: false condition excludes documents that contain the email field.

3. Verify the Index:

You can verify the index creation using the following command:

db.users.getIndexes();

This will show the indexes in the collection, and you should see the partial index with the specified condition.

4. Use the Index:

The partial index will be used for queries that filter on documents where the email field does not exist:

db.users.find({ email: { $exists: false } });