Aggregation Commands Comparison

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 stages such as $group, $match, and $sort.

For more information on pipeline stages, see Aggregation Stages.

Implements the Map-Reduce aggregation for processing large data sets.

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.

Using the $merge stage, you can create on-demand materialized views, where the content of the output collection can be updated incrementally the pipeline is run. $merge can incorporate results (insert new documents, merge documents, replace documents, keep existing documents, fail the operation, process documents with a custom update pipeline) into an existing collection.

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.

You can define custom accumulators with $accumulator and custom expressions with $function.

You can also use the $project pipeline stage to add computed fields and reshape existing document fields.

See $project for more information as well as Aggregation Stages for more information on all the available pipeline stages.

Custom map, reduce and finalize JavaScript functions offer flexibility to aggregation logic.

See mapReduce for details and restrictions on the functions.

Returns results as a cursor. If the pipeline includes the $out stage or $merge stage, the cursor is empty.

With $out, you can replace an existing output collection completely or output to a new collection. See $out for details.

With $merge, you can output to a new or existing collection. For existing collections, you can specify how to incorporate the results into the output collection (insert new documents, merge documents, replace documents, keep existing documents, fail the operation, process documents with a custom update pipeline). See $merge for details.

Returns results in various options (inline, new collection, merge, replace, reduce). See mapReduce for details on the output options.

Supports non-sharded and sharded input collections.

$merge can output to a non-sharded or sharded collection.

Supports non-sharded and sharded input collections.

• Aggregation Pipeline

• db.collection.aggregate()

• aggregate

• Map-Reduce

• db.collection.mapReduce()

• mapReduce.