$sample (aggregation)

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.

Example

Given a collection named users with the following documents:

{ "_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:

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

The operation returns three random documents.