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mongodbmongodb-query
Itai
Itai

Reputation: 11

MongoDB - fast count of documents associated with indexed key

I need to query a mongoDB (version 4.0)collection with 100M documents based on indexed field. For example if I have a collection of users and each user has a field of city which is a string and this field is indexed I want to ask how many users have some exact value in the city field (db.users.count({address:"New York"})). I see that the response time to is changing according to the number of results if there are 20M it is around 5 sec. In the explain() I see that it using COUNT and there is index name

"stage" : "COUNT_SCAN"

and 

"indexName" : "address_1"

My question is doesn't mongodb save the number of documents related to each indexed field key for quick retrieval without scanning? Is there a quick way to retrieve this information?

Upvotes: 0

Views: 232

Answers (2)

TechFree
TechFree

Reputation: 2974

No it doesnt. However, another improvement possible:

db.users.ensureIndex({address:1});
db.users.find({{address: 'New York}).count();

Upvotes: 0

Nicolae Maties
Nicolae Maties

Reputation: 2675

You can do:

db.users.find({address: 'New York'}).count() which I think it's faster since you have indexed address key. I don't think that in mongoDB saves anywhere this information regarding how many items are in a collection based on an indexed key. Just retrieving the data using the indexed key and perform a .count() should be pretty quick.

Upvotes: 0

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