OPTIONS

Create Indexes to Support Your Queries

An index supports a query when the index contains all the fields scanned by the query. The query scans the index and not the collection. Creating indexes that support queries results in greatly increased query performance.

This document describes strategies for creating indexes that support queries.

Create a Single-Key Index if All Queries Use the Same, Single Key

If you only ever query on a single key in a given collection, then you need to create just one single-key index for that collection. For example, you might create an index on category in the product collection:

db.products.ensureIndex( { "category": 1 } )

Create Compound Indexes to Support Several Different Queries

If you sometimes query on only one key and at other times query on that key combined with a second key, then creating a compound index is more efficient than creating a single-key index. MongoDB will use the compound index for both queries. For example, you might create an index on both category and item.

db.products.ensureIndex( { "category": 1, "item": 1 } )

This allows you both options. You can query on just category, and you also can query on category combined with item. A single compound index on multiple fields can support all the queries that search a “prefix” subset of those fields.

Example

The following index on a collection:

{ x: 1, y: 1, z: 1 }

Can support queries that the following indexes support:

{ x: 1 }
{ x: 1, y: 1 }

There are some situations where the prefix indexes may offer better query performance: for example if z is a large array.

The { x: 1, y: 1, z: 1 } index can also support many of the same queries as the following index:

{ x: 1, z: 1 }

Also, { x: 1, z: 1 } has an additional use. Given the following query:

db.collection.find( { x: 5 } ).sort( { z: 1} )

The { x: 1, z: 1 } index supports both the query and the sort operation, while the { x: 1, y: 1, z: 1 } index only supports the query. For more information on sorting, see Use Indexes to Sort Query Results.

Starting in version 2.6, MongoDB can use index intersection to fulfill queries. The choice between creating compound indexes that support your queries or relying on index intersection depends on the specifics of your system. See Index Intersection and Compound Indexes for more details.