Aggregation
Overview
In this guide, you can learn how to use the Java driver to perform aggregation operations.
Aggregation operations process data in your MongoDB collections and return computed results. The MongoDB Aggregation framework, which is part of the Query API, is modeled on the concept of data processing pipelines. Documents enter a pipeline comprised of one or more stages, and this pipeline transforms the documents into an aggregated result.
An aggregation operation is similar to a car factory. A car factory has an assembly line, which contains assembly stations with specialized tools to do specific jobs, like drills and welders. Raw parts enter the factory, and then the assembly line transforms and assembles them into a finished product.
The aggregation pipeline is the assembly line, aggregation stages are the assembly stations, and operator expressions are the specialized tools.
For more information about aggregation in the Java driver, see the following pages:
Compare Aggregation and Find Operations
You can use find operations to perform the following actions:
Select what documents to return
Select what fields to return
Sort the results
You can use aggregation operations to perform the following actions:
Perform find operations
Rename fields
Calculate fields
Summarize data
Group values
Aggregation operations have some limitations you must keep in mind:
Returned documents must not violate the BSON document size limit of 16 megabytes.
Pipeline stages have a memory limit of 100 megabytes by default. If required, you can exceed this limit by using the allowDiskUse method.
Important
$graphLookup exception
The $graphLookup stage has a strict memory limit of 100 megabytes and will ignore
allowDiskUse.