We’ve just learned how to create and manage indexes in MongoDB. Let’s recap some key takeaways from this lesson:

• An index is a data structure that captures a subset of a collection’s data in an easy-to-traverse form. We can use the .createIndex() method to create an index.
• A single field index is an index that references one field from a document.
• We can use the .explain() method with the "executionStats" argument to gain insight into the performance implications of our index on our query.
• A compound index is an index that contains references to multiple fields within a document.
• Multikey indexes are automatically created whenever we create an index on a field that contains an array value. Multikey indexes create an index key for each element in the array.
• A compound index cannot support two multikey indexes.
• The .dropIndex() method deletes an index without modifying the original collection.

In addition to the syntax we’ve learned throughout this lesson, MongoDB offers us other syntax and commands that can be useful when indexing collections:

• Partial Indexes only index documents in a collection that meet specific filter criteria. By indexing a subset of a collection’s documents, partial indexes consume less storage and have improved performance.
• Sparse Indexes only index documents that include the specified index field. Any documents that do not have the field will be excluded from the index. Much like partial indexes, these indexes can use significantly less storage and have relatively improved performance compared to non-sparse indexes.
• TTL Indexes are special single-field indexes that MongoDB can use to automatically remove documents from a collection after a certain amount of time or at a specific clock time.
• Unique Indexes enforce unique values for the indexed fields. Creating a unique index on a collection will restrict the insertion or update of documents where the indexed field’s value matches an existing value in the index.

Before moving on, spend some time creating and deleting indexes using what you’ve learned in this lesson!