$sample (aggregation)¶

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Definition
Behavior
Example

Definition¶

$sample¶

Randomly selects the specified number of documents from the input documents.

The $sample stage has the following syntax:

{ $sample: { size: <positive integer N> } }

N is the number of documents to randomly select.

Behavior¶

If all of the following conditions are true, $sample uses a pseudo-random cursor to select the N documents:

$sample is the first stage of the pipeline.
N is less than 5% of the total documents in the collection.
The collection contains more than 100 documents.

If any of the previous conditions are false, $sample:

Reads all documents that are output from a preceding aggregation stage or a collection scan.
Performs a random sort to select N documents.

Note

Random sorts are subject to the sort memory restrictions.

MMAPv1 May Return Duplicate Documents¶

If you are using the:

MMAPv1 storage engine, $sample may return the same document more than once in the result set.
WiredTiger or in-memory storage engine, $sample does not return duplicate documents. WiredTiger is the default storage engine as of MongoDB 3.2.

Example¶

Given a collection named users with the following documents:

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{ "_id" : 1, "name" : "dave123", "q1" : true, "q2" : true }
{ "_id" : 2, "name" : "dave2", "q1" : false, "q2" : false  }
{ "_id" : 3, "name" : "ahn", "q1" : true, "q2" : true  }
{ "_id" : 4, "name" : "li", "q1" : true, "q2" : false  }
{ "_id" : 5, "name" : "annT", "q1" : false, "q2" : true  }
{ "_id" : 6, "name" : "li", "q1" : true, "q2" : true  }
{ "_id" : 7, "name" : "ty", "q1" : false, "q2" : true  }

The following aggregation operation randomly selects 3 documents from the collection:

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db.users.aggregate(
   [ { $sample: { size: 3 } } ]
)

The operation returns three random documents.