CODEWITHSUNDEEP
mongodb
tspentzas
tspentzas

Reputation: 178

Mongo - Find a random document

I have a collection with users. I want to find a random collection, except mine.

db.mycoll.aggregate([{ $sample: { size: 1 } }])

This can return my document.

User.aggregate([
 { $match: { _id: { $nin: myID } } },
 { $sample: { size: 1 } }
])

Is this efficient for a really big collection?

Upvotes: 0

Views: 2185

Answers (1)

Atish
Atish

Reputation: 4435

Looking at the stages of your aggregation pipeline:

{ $match: { _id: { $nin: myID } } }

Pipeline Sequence Optimization

This will use the built-in index on _id. 

{ $sample: { size: 1 } }

This will select a sample record from the result of $match stage.

This is expected to be efficient provided not a not large size myID array.

Note:The inequality operator $nin is not very selective since it often matches a large portion of the index. As a result, in many cases, a $nin query with an index may perform no better than a $nin query that must scan all documents in a collection

Upvotes: 1

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