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$sample (aggregation)

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  • Definition
  • Behavior
  • Example
$sample

New in version 3.2.

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.

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. Random sorts are subject to the sort memory restrictions.

    Note

    Views are the result of aggregation pipelines. When you use $sample on a view, MongoDB appends the stage to the end of the view's aggregation pipeline syntax. Therefore, the $sample stage on a view is never the first stage and always results in a collection scan.

This section shows an aggregation pipeline example that uses the following users collection:

db.users.insertMany( [
{ _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.

Tip

See also:

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