Aggregation Commands Comparison¶

The following table provides a brief overview of the features of the MongoDB aggregation commands.

	`aggregate`	`mapReduce`
Description	Designed with specific goals of improving performance and usability for aggregation tasks. Uses a “pipeline” approach where objects are transformed as they pass through a series of pipeline operators such as `$group`, `$match`, and `$sort`. See Aggregation Pipeline Operators for more information on the pipeline operators.	Implements the Map-Reduce aggregation for processing large data sets.
Key Features	Pipeline operators can be repeated as needed. Pipeline operators need not produce one output document for every input document. Can also generate new documents or filter out documents.	In addition to grouping operations, can perform complex aggregation tasks as well as perform incremental aggregation on continuously growing datasets. See Map-Reduce Examples and Perform Incremental Map-Reduce.
Flexibility	Limited to the operators and expressions supported by the aggregation pipeline. However, can add computed fields, create new virtual sub-objects, and extract sub-fields into the top-level of results by using the `$project` pipeline operator. See `$project` for more information as well as Aggregation Pipeline Operators for more information on all the available pipeline operators.	Custom `map`, `reduce` and `finalize` JavaScript functions offer flexibility to aggregation logic. See `mapReduce` for details and restrictions on the functions.
Output Results	Returns results as a cursor. If the pipeline includes the `$out` stage, the cursor is empty. Changed in version 3.4: MongoDB 3.4 deprecates the use of `aggregate` command without the `cursor` option, unless the pipeline includes the `explain` option. When returning aggregation results inline using the `aggregate` command, specify the cursor option using the default batch size `cursor: {}` or specify the batch size in the cursor option `cursor: { batchSize: <num> }`.	Returns results in various options (inline, new collection, merge, replace, reduce). See `mapReduce` for details on the output options.
Sharding	Supports non-sharded and sharded input collections.	Supports non-sharded and sharded input collections.
Notes		Prior to 2.4, JavaScript code executed in a single thread.
More Information	See Aggregation Pipeline and `aggregate`.	See Map-Reduce and `mapReduce`.

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