Map-Reduce¶
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Aggregation Pipeline as Alternative聚合管道作为替代方案
Aggregation pipeline provides better performance and a more coherent interface than map-reduce, and various map-reduce operations can be rewritten using aggregation pipeline operators, such as 聚合管道提供了比map-reduce更好的性能和更一致的接口，并且可以使用聚合管道运算符重写各种map-reduce操作，例如$group, $merge, $accumulator, etc.$group、$merge、$accumulator等。
For examples of aggregation alternatives to map-reduce operations, see Map-Reduce Examples. 有关映射减少操作的聚合替代方案的示例，请参阅映射减少示例。See also Map-Reduce to Aggregation Pipeline.另请参见Map-Reduce 到聚合管道。
Map-reduce is a data processing paradigm for condensing large volumes of data into useful aggregated results. Map reduce是一种数据处理范式，用于将大量数据压缩成有用的聚合结果。To perform map-reduce operations, MongoDB provides the 为了执行map-reduce操作，MongoDB提供了mapReduce database command.mapReduce数据库命令。
Consider the following map-reduce operation:考虑下面的映射缩小操作：
In this map-reduce operation, MongoDB applies the map phase to each input document (i.e. the documents in the collection that match the query condition). 在此map reduce操作中，MongoDB将映射阶段应用于每个输入文档（即集合中与查询条件匹配的文档）。The map function emits key-value pairs. map函数发出键值对。For those keys that have multiple values, MongoDB applies the reduce phase, which collects and condenses the aggregated data. 对于具有多个值的键，MongoDB应用reduce阶段，该阶段收集并压缩聚合的数据。MongoDB then stores the results in a collection. 然后，MongoDB将结果存储在一个集合中。Optionally, the output of the reduce function may pass through a finalize function to further condense or process the results of the aggregation.可选地，reduce函数的输出可以通过finalize函数来进一步压缩或处理聚合的结果。
All map-reduce functions in MongoDB are JavaScript and run within the MongoDB中的所有map reduce函数都是JavaScript，并在mongod process. mongod进程中运行。Map-reduce operations take the documents of a single collection as the input and can perform any arbitrary sorting and limiting before beginning the map stage. Map reduce操作将单个集合的文档作为输入，可以在开始Map阶段之前执行任意排序和限制。mapReduce can return the results of a map-reduce operation as a document, or may write the results to collections.mapReduce可以将map-reduce操作的结果作为文档返回，也可以将结果写入集合。
Map-Reduce JavaScript Functions函数¶
In MongoDB, map-reduce operations use custom JavaScript functions to map, or associate, values to a key. 在MongoDB中，map reduce操作使用自定义JavaScript函数将值映射或关联到键。If a key has multiple values mapped to it, the operation reduces the values for the key to a single object.如果一个关键点映射了多个值，则该操作会将关键点的值减少为单个对象。
The use of custom JavaScript functions provide flexibility to map-reduce operations. 自定义JavaScript函数的使用提供了映射reduce操作的灵活性。For instance, when processing a document, the map function can create more than one key and value mapping or no mapping. 例如，在处理文档时，map函数可以创建多个键和值映射，也可以不创建映射。Map-reduce operations can also use a custom JavaScript function to make final modifications to the results at the end of the map and reduce operation, such as perform additional calculations.Map reduce操作还可以使用自定义JavaScript函数对Map和reduce操作末尾的结果进行最终修改，例如执行其他计算。
Note
Starting in MongoDB 4.4, 从MongoDB 4.4开始，mapReduce no longer supports the deprecated BSON type JavaScript code with scope (BSON type 15) for its functions. mapReduce不再为其函数支持不推荐使用的BSON类型——带范围的JavaScript代码（BSON类型15）。The map, reduce, and finalize functions must be either BSON type String (BSON type 2) or BSON type JavaScript (BSON type 13). map、reduce和finalize函数必须是BSON类型字符串（BSON类型2）或BSON类型JavaScript(BSON类型13）。To pass constant values which will be accessible in the 要传递可在map, reduce, and finalize functions, use the scope parameter.map、reduce和finalize函数中访问的常量值，请使用scope参数。
The use of JavaScript code with scope for the 自版本4.2.1以来，已不推荐使用具有mapReduce functions has been deprecated since version 4.2.1.mapReduce函数作用域的JavaScript代码。
Map-Reduce Results映射减少结果¶
In MongoDB, the map-reduce operation can write results to a collection or return the results inline. 在MongoDB中，map reduce操作可以将结果写入集合或内联返回结果。If you write map-reduce output to a collection, you can perform subsequent map-reduce operations on the same input collection that merge replace, merge, or reduce new results with previous results. 如果将map reduce输出写入集合，则可以对同一输入集合执行后续的map reduce操作，将新结果与以前的结果合并、替换、合并或减少。See 有关详细信息和示例，请参阅mapReduce and Perform Incremental Map-Reduce for details and examples.mapReduce和执行增量Map-Reduce。
When returning the results of a map-reduce operation inline, the result documents must be within the 内联返回map reduce操作的结果时，结果文档必须在BSON文档大小限制内，当前为16 MB。BSON Document Size limit, which is currently 16 megabytes. For additional information on limits and restrictions on map-reduce operations, see the mapReduce reference page.有关map reduce操作的限制和限制的更多信息，请参阅mapReduce参考页。
Sharded Collections分片集合¶
MongoDB supports map-reduce operations on sharded collections.MongoDB支持分片集合上的映射缩减操作。
However, starting in version 4.2, MongoDB deprecates the map-reduce option to create a new sharded collection and the use of the 但是，从版本4.2开始，MongoDB不推荐使用map-reduce选项来创建新的分片集合，也不推荐使用sharded option for map-reduce. sharded选项来创建map-reduce。To output to a sharded collection, create the sharded collection first. 要输出到分片集合，请首先创建分片集合。MongoDB 4.2 also deprecates the replacement of an existing sharded collection.MongoDB 4.2还反对替换现有的分片集合。