db.testcoll1.createIndex({testField1:1})
db.testcoll2.explain().aggregate( [ { $unionWith : { coll: "testcoll1", pipeline : [ { $match : { testField1 : { $gt : 2 } } } ] } } ] )
// shows correct plan
db.testcoll2.explain("executionStats").aggregate( [ { $unionWith : { coll: "testcoll1", pipeline : [ { $match : { testField1 : { $gt : 2 } } } ] } } ] )
// shows incorrect plan!
{
	"stages" : [
		{
			"$cursor" : {
				"queryPlanner" : {
					"plannerVersion" : 1,
					"namespace" : "test.testcoll2",
					"indexFilterSet" : false,
					"parsedQuery" : {

					},
					"winningPlan" : {
						"stage" : "EOF"
					},
					"rejectedPlans" : [ ]
				},
				"executionStats" : {
					"executionSuccess" : true,
					"nReturned" : 0,
					"executionTimeMillis" : 0,
					"totalKeysExamined" : 0,
					"totalDocsExamined" : 0,
					"executionStages" : {
						"stage" : "EOF",
						"nReturned" : 0,
						"executionTimeMillisEstimate" : 0,
						"works" : 1,
						"advanced" : 0,
						"needTime" : 0,
						"needYield" : 0,
						"saveState" : 1,
						"restoreState" : 1,
						"isEOF" : 1
					},
					"allPlansExecution" : [ ]
				}
			},
			"nReturned" : NumberLong(0),
			"executionTimeMillisEstimate" : NumberLong(0)
		},
		{
			"$unionWith" : {
				"coll" : "testcoll1",
				"pipeline" : [
					{
						"$cursor" : {
							"queryPlanner" : {
								"plannerVersion" : 1,
								"namespace" : "test.testcoll1",
								"indexFilterSet" : false,
								"parsedQuery" : {

								},
								"queryHash" : "8B3D4AB8",
								"planCacheKey" : "8B3D4AB8",
								"winningPlan" : {
									"stage" : "COLLSCAN",
									"direction" : "forward"
								},
								"rejectedPlans" : [ ]
							},
							"executionStats" : {
								"executionSuccess" : true,
								"nReturned" : 0,
								"executionTimeMillis" : 0,
								"totalKeysExamined" : 0,
								"totalDocsExamined" : 0,
								"executionStages" : {
									"stage" : "COLLSCAN",
									"nReturned" : 0,
									"executionTimeMillisEstimate" : 0,
									"works" : 0,
									"advanced" : 0,
									"needTime" : 0,
									"needYield" : 0,
									"saveState" : 1,
									"restoreState" : 0,
									"isEOF" : 0,
									"direction" : "forward",
									"docsExamined" : 0
								},
								"allPlansExecution" : [ ]
							}
						},
						"nReturned" : NumberLong(0),
						"executionTimeMillisEstimate" : NumberLong(0)
					}
				]
			},
			"nReturned" : NumberLong(0),
			"executionTimeMillisEstimate" : NumberLong(1)
		}
	],
	"serverInfo" : {
		"host" : "asyas-mbp-4.lan",
		"port" : 27017,
		"version" : "4.4.0-rc7",
		"gitVersion" : "b79b53f55a5c148fd297b81a45c08d08e2cf8f94"
	},
	"ok" : 1
}