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$sample (aggregation)

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$sample

New in version 3.2.

Randomly selects the specified number of documents from its input.

The $sample stage has the following syntax:

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

$sample uses one of two methods to obtain N random documents, depending on the size of the collection, the size of N, and $sample's position in the pipeline.

If all the following conditions are met, $sample uses a pseudo-random cursor to select 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 above conditions are NOT met, $sample performs a collection scan followed by a random sort to select N documents. In this case, the $sample stage is subject to the sort memory restrictions.

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.

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.

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