> db.c.drop();
> db.c.insert({a: [1, 2, 3]});
> db.c.find({$expr: {$gt: ["$a", 4]}});
// No results. This is incorrect because the array [1, 2, 3] is considered greater than
// the number 4 in the aggregation language. In fact, all arrays are considered greater
// than all numbers in aggregation, since the first part of the comparison is the
// canonical type.

// The explain shows that the match expression {a: {$gt: 4}} has been generated by
// the $expr rewrite optimization.
> db.c.find({$expr: {$gt: ["$a", 4]}}).explain()
{
	"queryPlanner" : {
		"plannerVersion" : 1,
		"namespace" : "test.c",
		"indexFilterSet" : false,
		"parsedQuery" : {
			"$and" : [
				{
					"a" : {
						"$gt" : 4
					}
				},
				{
					"$expr" : {
						"$gt" : [
							"$a",
							{
								"$const" : 4
							}
						]
					}
				}
			]
		},
		"winningPlan" : {
			"stage" : "COLLSCAN",
			"filter" : {
				"$and" : [
					{
						"a" : {
							"$gt" : 4
						}
					},
					{
						"$expr" : {
							"$gt" : [
								"$a",
								{
									"$const" : 4
								}
							]
						}
					}
				]
			},
			"direction" : "forward"
		},
		"rejectedPlans" : [ ]
	},
	"serverInfo" : {
		"host" : "storchbox",
		"port" : 27017,
		"version" : "0.0.0",
		"gitVersion" : "unknown"
	},
	"ok" : 1
}