MongoDB Enterprise > db.group_conversion_to_distinct_scan.explain().aggregate([{ $sort: { str: 1, d: 1 } }, { $group: { _id: "$str", accum: { $first: "$d" } } }]);
{
        "explainVersion" : "2",
        "queryPlanner" : {
                "namespace" : "test.group_conversion_to_distinct_scan",
                "indexFilterSet" : false,
                "parsedQuery" : {

                },
                "queryHash" : "FB9A89A9",
                "planCacheKey" : "0C288E96",
                "optimizedPipeline" : true,
                "maxIndexedOrSolutionsReached" : false,
                "maxIndexedAndSolutionsReached" : false,
                "maxScansToExplodeReached" : false,
                "winningPlan" : {
                        "queryPlan" : {
                                "stage" : "GROUP",
                                "planNodeId" : 4,
                                "inputStage" : {
                                        "stage" : "SORT",
                                        "planNodeId" : 3,
                                        "sortPattern" : {
                                                "str" : 1,
                                                "d" : 1
                                        },
                                        "memLimit" : 104857600,
                                        "type" : "simple",
                                        "inputStage" : {
                                                "stage" : "PROJECTION_SIMPLE",
                                                "planNodeId" : 2,
                                                "transformBy" : {
                                                        "d" : true,
                                                        "str" : true,
                                                        "_id" : false
                                                },
                                                "inputStage" : {
                                                        "stage" : "COLLSCAN",
                                                        "planNodeId" : 1,
                                                        "filter" : {

                                                        },
                                                        "direction" : "forward"
                                                }
                                        }
                                }
                        },
                        "slotBasedPlan" : {
                                "slots" : "$$RESULT=s15 env: { s3 = 1685019940032 (NOW), s2 = Nothing (SEARCH_META), s1 = TimeZoneDatabase(Asia/Makassar...Pacific/Noumea) (timeZoneDB) }",
                                "stages" : "[4] mkbson s15 [_id = s12, accum = s13] true false \n[4] group [s12] [s13 = first((s4 ?: null))] spillSlots[s14] mergingExprs[first((s14 ?: null))] \n[4] project [s12 = (s5 ?: null)] \n[3] sort [s10, s11] [asc, asc] [s4, s5] \n[3] project [s10 = \n    if (isArray(s5) ?: false) \n    then (_internalLeast(s5) ?: undefined) \n    else (s5 ?: null) \n, s11 = \n    if (isArray(s4) ?: false) \n    then (_internalLeast(s4) ?: undefined) \n    else (s4 ?: null) \n] \n[3] project [s9 = (!((isArray(s5) ?: false)) || (!((isArray(s4) ?: false)) || fail(2, \"cannot sort with keys that are parallel arrays\")))] \n[2] mkbson s8 s6 [d, str] keep [] true false \n[1] scan s6 s7 none none none none none none lowPriority [s4 = d, s5 = str] @\"47ba07a1-1d34-490b-886f-42ec54c3e327\" true false "
                        }
                },
                "rejectedPlans" : [ ]
        },
        "command" : {
                "aggregate" : "group_conversion_to_distinct_scan",
                "pipeline" : [
                        {
                                "$sort" : {
                                        "str" : 1,
                                        "d" : 1
                                }
                        },
                        {
                                "$group" : {
                                        "_id" : "$str",
                                        "accum" : {
                                                "$first" : "$d"
                                        }
                                }
                        }
                ],
                "explain" : true,
                "cursor" : {

                },
                "lsid" : {
                        "id" : UUID("34460f2a-08bd-4247-a5d1-5a4210607ef9")
                },
                "$db" : "test"
        },
        "serverInfo" : {
                "host" : "ip-10-0-1-70",
                "port" : 20000,
                "version" : "7.1.0-alpha-1019-gd5dff6d",
                "gitVersion" : "d5dff6d99d6b569864e7dd71a8bea88f9017aa3c"
        },
        "serverParameters" : {
                "internalQueryFacetBufferSizeBytes" : 104857600,
                "internalQueryFacetMaxOutputDocSizeBytes" : 104857600,
                "internalLookupStageIntermediateDocumentMaxSizeBytes" : 104857600,
                "internalDocumentSourceGroupMaxMemoryBytes" : 104857600,
                "internalQueryMaxBlockingSortMemoryUsageBytes" : 104857600,
                "internalQueryProhibitBlockingMergeOnMongoS" : 0,
                "internalQueryMaxAddToSetBytes" : 104857600,
                "internalDocumentSourceSetWindowFieldsMaxMemoryBytes" : 104857600,
                "internalQueryFrameworkControl" : "trySbeEngine"
        },
        "ok" : 1
}