$sample (aggregation)
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Definition
-
$sample
-
New in version 3.2.
Randomly selects the specified number of documents from its input.
The
$sample
stage has the following syntax:
Behavior
$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.
Warning
$sample
may output the same document more than once in its result set. For more information, see Cursor Isolation.