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

See Aggregation Operators for more information on the pipeline operators.

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.

With the addition of $merge in version 4.2, 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.

Starting in version 4.4, can define custom aggregation expressions with $accumulator and $function.

In previous versions, can only use 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 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.

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 cllections, 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.

Supports non-sharded and sharded input collections.

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

• Aggregation Pipeline

• db.collection.aggregate()

• aggregate

• Map-Reduce

• db.collection.mapReduce()

• mapReduce.