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Aggregation Commands Comparison¶
The following table provides a brief overview of the features of the MongoDB aggregation commands.
aggregate | mapReduce | group | |
---|---|---|---|
Description | 2.2 新版功能. 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. | Provides grouping functionality. Is slower than the aggregate command and has less functionality than the mapReduce command. |
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. |
Can either group by existing fields or with a custom keyf JavaScript function, can group by calculated fields. See group for information and example using the keyf function. |
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. |
Custom reduce and finalize JavaScript functions offer flexibility to grouping logic. See group for details and restrictions on these functions. |
Output Results | Returns results in various options (inline as a document that contains the result set, a cursor to the result set) or stores the results in a collection. The result is subject to the BSON Document size limit if returned inline as a document that contains the result set. 在 2.6 版更改: Can return results as a cursor or store the results to a collection. |
Returns results in various options (inline, new collection, merge, replace, reduce). See mapReduce for details on the output options. 在 2.2 版更改: Provides much better support for sharded map-reduce output than previous versions. |
Returns results inline as an array of grouped items. The result set must fit within the maximum BSON document size limit. 在 2.2 版更改: The returned array can contain at most 20,000 elements; i.e. at most 20,000 unique groupings. Previous versions had a limit of 10,000 elements. |
Sharding | Supports non-sharded and sharded input collections. | Supports non-sharded and sharded input collections. | Does not support sharded collection. |
Notes | Prior to 2.4, JavaScript code executed in a single thread. | Prior to 2.4, JavaScript code executed in a single thread. | |
More Information | See 聚合管道 and aggregate. | See 映射化简 and mapReduce. | See group. |