MongoDB MapReduce-发射一键/一值不调用reduce [英] MongoDB MapReduce - Emit one key/one value doesnt call reduce

问题描述

因此，我一般对mongodb和mapreduce并不陌生，并且遇到了这个怪癖"问题.(或者至少在我看来是个怪癖)

So i'm new with mongodb and mapreduce in general and came across this "quirk" (or atleast in my mind a quirk)

说我的收藏夹中有这样的对象:

Say I have objects in my collection like so:

{'键':5，'值':5}

{'key':5, 'value':5}

{'键':5，'值':4}

{'key':5, 'value':4}

{'键':5，'值':1}

{'key':5, 'value':1}

{'键':4，'值':6}

{'key':4, 'value':6}

{'键':4，'值':4}

{'key':4, 'value':4}

{'key':3，'value':0}

{'key':3, 'value':0}

我的 ma​​p 函数只是发出键和值

My map function simply emits the key and the value

我的 reduce 函数仅将值添加 AND ，然后将它们加1(我这样做是为了检查reduce函数是否被调用)

My reduce function simply adds the values AND before returning them adds 1 (I did this to check to see if the reduce function is even called)

我的结果如下:

{'_ id':3，'value':0 }

{'_ id':4，'value':11.0}

{'_id':4, 'value': 11.0}

{'_ id':5，'value':11.0}

{'_id':5, 'value': 11.0}

如您所见，对于键4&5我对键3得到了预期的11 BUT答案(在该键集合中只有一个条目)，我得到了意外的0！

As you can see, for the keys 4 & 5 I get the expected answer of 11 BUT for the key 3 (with only one entry in the collection with that key) I get the unexpected 0!

这是mapreduce的自然行为吗?对于MongoDB?对于pymongo(我正在使用)?

Is this natural behavior of mapreduce in general? For MongoDB? For pymongo (which I am using)?

推荐答案

reduce函数将具有相同键的文档组合到一个文档中.如果map函数针对特定键发出单个文档(与键3一样)，则不会调用reduce函数.

The reduce function combines documents with the same key into one document. If the map function emits a single document for a particular key (as is the case with key 3), the reduce function will not be called.

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