redundant $sort in aggregation prevents best $limit $sort consolidation

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    • Minor Change
    • v4.7, v4.4
    • Query 2020-06-29, Query 2020-07-13, Query 2020-07-27
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      If there is an extra $sort that's identical to one before it, and there's a $limit after it, it causes an unfortunate full $sort of every document before the second sort which absorbed $limit is executed.

      db.c.explain().aggregate([ {$sort: {a: 1}},{$sort:{a:1}},{$limit:5}]);
{
	"stages" : [
		{
			"$cursor" : {
				"query" : {

				},
				"sort" : {
					"a" : 1
				},
				"queryPlanner" : {
					"plannerVersion" : 1,
					"namespace" : "test.c",
					"indexFilterSet" : false,
					"parsedQuery" : {

					},
					"winningPlan" : {
						"stage" : "EOF"
					},
					"rejectedPlans" : [ ]
				}
			}
		},
		{
			"$sort" : {
				"sortKey" : {
					"a" : 1
				},
				"limit" : NumberLong(5)
			}
		}
	],
	"ok" : 1
}

            Assignee:
            Ribhav Jain (Inactive)
            Reporter:
            Asya Kamsky
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              Created:
              Updated:
              Resolved: