Partial indexes with filter in MongoDB - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a partial index in MongoDB?

A partial index is an index built only on the documents in a collection that meet a specified filter condition. It helps improve performance and reduce index size by indexing only relevant documents.

beginner

How do you create a partial index with a filter in MongoDB?

Use the createIndex() method with the option { partialFilterExpression: <filter> }, where <filter> defines the condition documents must meet to be included in the index.

intermediate

Why use a partial index instead of a full index?

Partial indexes save space and improve write performance by indexing only documents that match the filter, making queries on those documents faster without indexing the entire collection.

intermediate

Example: What does this partial index do? { age: 1 }, { partialFilterExpression: { age: { $gte: 18 } } }

This index only includes documents where the age field is 18 or older. Queries filtering on age >= 18 will be faster, but documents with age less than 18 are not indexed.

intermediate

Can a partial index speed up queries that do not match the filter condition?

No. Partial indexes only help queries that match the filter condition. Queries outside the filter will not use the partial index and may scan the collection.

What does a partial index in MongoDB do?

A Indexes only documents matching a filter condition

B Indexes all documents in a collection

C Indexes documents randomly

D Indexes documents based on size

A partial index only includes documents that match the specified filter condition.

Which option is used to specify the filter condition when creating a partial index?

A filterCondition

B indexFilter

C partialFilterExpression

D filterExpression

The option partialFilterExpression defines the filter condition for a partial index.

If a query filters documents outside the partial index filter, what happens?

A The query uses a random index

B The query fails

C The partial index is used anyway

D The query scans the whole collection

Queries outside the partial index filter do not use the partial index and scan the collection.

What is a benefit of using partial indexes?

A They reduce index size and improve write speed

B They make queries slower

C They index all fields automatically

D They delete documents automatically

Partial indexes reduce index size and improve write performance by indexing fewer documents.

Which of these is a valid partial index filter expression?

A { name: 'John' }

B All of the above

C { status: { $exists: true } }

D { age: { $lt: 30 } }

Any valid MongoDB query filter can be used as a partialFilterExpression.

Explain what a partial index is and why you might use one in MongoDB.

Think about indexing only some documents instead of all.

Describe how to create a partial index with a filter and give an example.

Remember the option name partialFilterExpression.