$sample (aggregation)¶

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Definition
Behavior
Example

Definition¶

$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> } }

Behavior¶

In order to get N random documents:

If N is greater than or equal to 5% of the total documents in the collection, $sample performs a collection scan, performs a sort, and then select the top N documents. As such, the $sample stage is subject to the sort memory restrictions.
If N is less than 5% of the total documents in the collection,
- If using WiredTiger Storage Engine, $sample uses a pseudo-random cursor over the collection to sample N documents.
- If using MMAPv1 Storage Engine, $sample uses the _id index to randomly select N documents.

Warning

$sample may output the same document more than once in its result set. For more information, see Cursor Isolation.

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.

← $group (aggregation) $sort (aggregation) →

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