mongodb count vs.count with count [英] mongodb count vs find with count

问题描述

我正在对mongo(版本2.4和3.2)集合中的文档进行计数.馆藏非常大，共有3,820,085个文档.我需要计算所有引用为_id的文档.我尝试了两个不同的查询:

I am performing a count of documents in a mongo (versions 2.4 and 3.2) collection. The collection is very big, 3821085 documents. I need to count all documents with a reference _id. I have tried two different queries:

db.SampleCollection.find({"field._id" : ObjectId("UUID")}).count() db.SampleCollection.count({"field._id" : ObjectId("UUID")})

此查询需要很长时间.太多的时间让我没有完成，超过5分钟，我被吓到要杀死它.

This query takes a very long time. So much time that I have not let it complete, more than 5 minutes and I get scared and kill it.

对于此集合field._id不是索引.我没有与此查询一起使用索引的相关信息.

For this collection field._id is not an index. I do not have relevant info to use an index with this query.

是否有更好的方法来计算mongo中的文档.

Is there a better approach to count document in mongo.

更新:

我了解我需要在字段field._id上建立索引.如果我确实有该字段的索引，那么在大集合db.SampleCollection.find(...).count()或db.SampleCollection.count(...)上哪种方法会更好?还是两者没有区别?

I understand that I need an index on the field field._id. If I did have an index for the field which approach would perform better on a large collection db.SampleCollection.find(...).count() or db.SampleCollection.count(...)? Or is there no difference between the two?

推荐答案

在您的方案中，您应该有一个索引.

In your scenario, you should have an index.

索引支持在MongoDB中高效执行查询.没有索引，MongoDB必须执行集合扫描，即扫描集合中的每个文档，以选择与查询语句匹配的那些文档.

https://docs.mongodb.com/manual/indexes/

更新:

现在提出的问题有所不同. "collection.find({}).count()"比"collection.count()"更快吗?

the question asked now is different. Is "collection.find({}).count()" more fast then "collection.count()"?

根据MongoDB文档:

According to the MongoDB documentation:

count()等效于db.collection.find(query).count()构造. https://docs.mongodb.com/manual/reference/method/db.collection.count/

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