Map-Reduce
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Starting in MongoDB 5.0, map-reduce is deprecated:从MongoDB 5.0开始，不推荐使用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map-reduce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（例如$group,$merge, and others.$group、$merge等）重写map-reduce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，您可以使用$accumulatorand$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reduce的聚合管道备选方案的示例，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，用于将大量数据压缩为有用的聚合结果。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:考虑以下map-reduce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（即集合中匹配查询条件的文档）。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，该阶段集合并压缩聚合的数据。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.或者，reduce函数的输出可以通过finalize函数进一步压缩或处理聚合结果。
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.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 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函数在Map和reduce操作结束时对结果进行最终修改，例如执行其他计算。
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不再支持其函数的范围为（BSON类型15）的不推荐使用的BSON类型JavaScript代码。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类别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，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-reducte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和实施增量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. 当以内联方式返回map-reduce操作的结果时，结果文档必须在BSON文档大小限制（当前为16 MB）内。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参考页面。
Sharded Collections分片集合
MongoDB supports map-reduce operations on sharded collections.MongoDB支持对分片集合执行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版开始，MongoDB不推荐使用map-reduce选项创建新的分片集合，也不推荐使用sharded option for map-reduce. shared选项进行map-reducte。To output to a sharded collection, create the sharded collection first. 要输出到分片集合，请先创建分片集合。MongoDB 4.2 also deprecates the replacement of an existing sharded collection.MongoDB 4.2也不赞成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。