Map-Reduce
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Aggregation Pipeline as Alternative聚合管道作为备选方案
Starting in MongoDB 5.0, map-reduce is deprecated:从MongoDB 5.0开始，不赞成使用map-reduce：
Instead of map-reduce, you should use an aggregation pipeline.您应该使用聚合管道，而不是map-reduce。Aggregation pipelines provide better performance and usability than map-reduce.聚合管道提供了比映射减少更好的性能和可用性。You can rewrite map-reduce operations using aggregation pipeline stages, such as您可以使用聚合管道阶段（如$group,$merge, and others.$group、$merge和其他阶段）重写映射减少操作。For map-reduce operations that require custom functionality, you can use the对于需要自定义功能的map reduce操作，可以使用$accumulatorand$functionaggregation operators, available starting in version 4.4.$accumulator和$function聚合运算符，这些运算符从4.4版开始提供。You can use those operators to define custom aggregation expressions in JavaScript.您可以使用这些运算符在JavaScript中定义自定义聚合表达式。
For examples of aggregation pipeline alternatives to map-reduce, see:有关映射减少的聚合管道替代方案的示例，请参阅：
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，用于将大量数据浓缩为有用的聚合结果。To perform map-reduce operations, MongoDB provides the 为了执行mapReduce database command.map-reduce操作，MongoDB提供了mapReduce数据库命令。
Consider the following map-reduce operation:考虑以下映射缩减操作：
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). 在这个map-reduce操作中，MongoDB将map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. For those keys that have multiple values, MongoDB applies the reduce phase, which collects and condenses the aggregated data. map函数会发出键值对。对于那些具有多个值的键，MongoDB应用reduce阶段，该阶段集合并浓缩聚合的数据。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函数以进一步浓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，并且可以在开始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操作的结果作为文档返回，也可以将结果写入集合。
Map-Reduce JavaScript FunctionsMap Reduce JavaScript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.如果一个键映射了多个值，则该操作会将该键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函数的使用为映射reduce操作提供了灵活性。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运算末尾的结果进行最终修改，例如执行额外的计算。
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不再支持不推荐使用的BSON类型的JavaScript代码，其函数具有作用域（BSON type 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String（BSON类型2）或BSON类型JavaScript（BSON type 13)。To pass constant values which will be accessible in the 要传递map, reduce, and finalize functions, use the scope parameter.map、reduce和finalize函数中可访问的常数值，请使用scope参数。
The use of JavaScript code with scope for the 自4.2.1版本以来，一直不赞成使用具有mapReduce functions has been deprecated since version 4.2.1.mapReduce函数作用域的JavaScript代码。
Map-Reduce Results后果
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. 如果将映射减少输出写入集合，则可以对同一输入集合执行后续映射减少操作，以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 BSON Document Size limit, which is currently 16 megabytes. 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，该限制当前为16兆字节。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其他信息，请参阅mapReduce参考页。
Sharded Collections分片集合
MongoDB supports map-reduce operations on sharded collections.MongoDB支持对分片集合的map-reduce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 然而，从4.2版本开始，MongoDB反对使用map-reduce选项来创建新的分片集合，并反对使用sharded option for map-reduce. sharded选项来减少map。To output to a sharded collection, create the sharded collection first. 若要输出到分片集合，请首先创建分片集合。MongoDB 4.2 also deprecates the replacement of an existing sharded collection.MongoDB 4.2也反对替换现有的分片集合。
Views视图
Views do not support map-reduce operations.视图不支持map-reduce操作。