MongoDB Enterprise > db.c.drop()
false
MongoDB Enterprise > for (let i = 0; i < 1000; i++) { db.c.insert({_id: i}); }
WriteResult({ "nInserted" : 1 })
MongoDB Enterprise > db.c.explain(true).aggregate([{$sample: {size: 10}}])
{
	"stages" : [
		{
			"$cursor" : {
				"query" : {

				},
				"queryPlanner" : {
					"plannerVersion" : 1,
					"namespace" : "test.c",
					"indexFilterSet" : false,
					"winningPlan" : {
						"stage" : "MULTI_ITERATOR"
					},
					"rejectedPlans" : [ ]
				},
				"executionStats" : {
					"executionSuccess" : true,
					"nReturned" : 0,
					"executionTimeMillis" : 43,
					"totalKeysExamined" : 0,
					"totalDocsExamined" : 0,
					"executionStages" : {
						"stage" : "MULTI_ITERATOR",
						"nReturned" : 0,
						"executionTimeMillisEstimate" : 0,
						"works" : 0,
						"advanced" : 0,
						"needTime" : 0,
						"needYield" : 0,
						"saveState" : 0,
						"restoreState" : 0,
						"isEOF" : 0,
						"invalidates" : 0
					},
					"allPlansExecution" : [ ]
				}
			}
		},
		{
			"$sampleFromRandomCursor" : {
				"size" : NumberLong(10)
			}
		}
	],
	"ok" : 1
}

MongoDB Enterprise > db.c.aggregate([{$sample: {size: 10}}])
{ "_id" : 266 }
{ "_id" : 789 }
{ "_id" : 999 }
{ "_id" : 987 }
{ "_id" : 685 }
{ "_id" : 979 }
{ "_id" : 633 }
{ "_id" : 484 }
{ "_id" : 847 }
{ "_id" : 22 }