New in version 3.2.
Randomly selects the specified number of documents from its input.
$samplestage has the following syntax:
$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:
$sampleis 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.