Complete MongoDB Interview Guide
Questions
What is the difference between a sparse and a partial index in MongoDB?
The Scenario
You are a backend engineer at a social media company. You are designing a new service that will store user profiles. Some users have a location field, but others do not.
You want to create an index on the location field to speed up queries that search for users by location. However, you do not want the index to include the users who do not have a location field.
The Challenge
Explain the difference between a sparse and a partial index in MongoDB. What are the pros and cons of each approach, and which one would you choose for this use case?
A junior engineer might not be aware of sparse or partial indexes. They might just create a regular index on the `location` field, which would include all the documents in the collection, even the ones that do not have a `location` field.
A senior engineer would know that a sparse index is the perfect tool for this job. They would be able to explain the difference between a sparse and a partial index, and they would have a clear plan for how to use them to solve this problem.
Step 1: Understand the Key Differences
| Feature | Sparse Index | Partial Index |
|---|---|---|
| Inclusion | Only includes documents that have the indexed field. | Only includes documents that match a given filter expression. |
| Syntax | db.my_collection.createIndex({ my_field: 1 }, { sparse: true }) | db.my_collection.createIndex({ my_field: 1 }, { partialFilterExpression: { my_field: { $exists: true } } }) |
| Use Cases | When you want to index a field that only exists in some of the documents. | When you want to index a subset of the documents in a collection. |
Step 2: Choose the Right Tool for the Job
For our use case, we should use a sparse index. This is because we want to index the location field, but we only want the index to include the documents that have a location field.
A partial index would also work for this use case, but a sparse index is simpler to create and is more efficient for this specific scenario.
Practice Question
You want to create an index on a field that only contains a certain value. Which of the following would be the most appropriate?