Work with Indexes
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Overview
In this guide, you can learn how to use indexes with the C++ driver. Indexes can improve the efficiency of queries and add additional functionality to querying and storing documents.
Without indexes, MongoDB must scan every document in a collection to find the documents that match each query. If an appropriate index exists for a query, MongoDB can use the index to limit the documents it must inspect and improve potential query performance.
Operational Considerations
To improve query performance, build indexes on fields that appear often in your application's queries and operations that return sorted results. Each index that you add consumes disk space and memory when active, so we recommend that you track index memory and disk usage for capacity planning. In addition, when a write operation updates an indexed field, MongoDB also updates any related index.
Because MongoDB supports dynamic schemas, applications can query against fields whose names are not known in advance or are arbitrary. MongoDB 4.2 introduced wildcard indexes to help support these queries. Wildcard indexes are not designed to replace workload-based index planning.
For more information about designing your data model and choosing indexes appropriate for your application, see the Data Modeling and Indexes guide in the MongoDB Server manual.
Sample Data
The examples in this guide use the movies collection in the sample_mflix
database from the Atlas sample datasets. To access this collection
from your C++ application, instantiate a mongocxx::client that connects to an Atlas cluster
and assign the following values to your db and collection variables:
auto db = client["sample_mflix"]; auto collection = db["movies"];
To learn how to create a free MongoDB Atlas cluster and load the sample datasets, see the Get Started with Atlas guide.
Create an Index
MongoDB supports several index types to help query your data. The following pages describe different index types and provide sample code to create each type of index:
Remove an Index
You can remove any unused index except the default unique index on the
_id field.
The following sections provide examples that show how to remove one or more indexes from a collection.
Remove a Single Index
To remove a single index from a collection, call the drop_one() method and pass in
the index name or an instance of the index you want to remove.
The following example removes an index with the name "title_1" from the movies
collection:
collection.indexes().drop_one("title_1");
Note
You cannot remove a single field from a compound text index. You must drop the entire index and create a new one to update the indexed fields.
Remove All Indexes
Starting with MongoDB 4.2, you can drop all indexes by calling the
drop_all() method on the index view in your collection:
collection.indexes().drop_all();
For earlier versions of MongoDB, pass "*" as a parameter to your call to
drop_one() on the index view in your collection:
collection.indexes().drop_one("*");
Additional Information
To learn more about indexes in MongoDB, see the Indexes guide in the MongoDB Server manual.
API Documentation
To learn more about the methods or types discussed in this guide, see the following API documentation: