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

                                        },
                                        "collation" : {
                                                "locale" : "en_US",
                                                "caseLevel" : false,
                                                "caseFirst" : "off",
                                                "strength" : 2,
                                                "numericOrdering" : false,
                                                "alternate" : "non-ignorable",
                                                "maxVariable" : "punct",
                                                "normalization" : false,
                                                "backwards" : false,
                                                "version" : "57.1"
                                        },
                                        "queryHash" : "2F8A5CEE",
                                        "planCacheKey" : "2F8A5CEE",
                                        "maxIndexedOrSolutionsReached" : false,
                                        "maxIndexedAndSolutionsReached" : false,
                                        "maxScansToExplodeReached" : false,
                                        "winningPlan" : {
                                                "stage" : "FETCH",
                                                "inputStage" : {
                                                        "stage" : "DISTINCT_SCAN",
                                                        "keyPattern" : {
                                                                "str" : 1,
                                                                "d" : 1
                                                        },
                                                        "indexName" : "str_1_d_1",
                                                        "collation" : {
                                                                "locale" : "en_US",
                                                                "caseLevel" : false,
                                                                "caseFirst" : "off",
                                                                "strength" : 2,
                                                                "numericOrdering" : false,
                                                                "alternate" : "non-ignorable",
                                                                "maxVariable" : "punct",
                                                                "normalization" : false,
                                                                "backwards" : false,
                                                                "version" : "57.1"
                                                        },
                                                        "isMultiKey" : false,
                                                        "multiKeyPaths" : {
                                                                "str" : [ ],
                                                                "d" : [ ]
                                                        },
                                                        "isUnique" : false,
                                                        "isSparse" : false,
                                                        "isPartial" : false,
                                                        "indexVersion" : 2,
                                                        "direction" : "forward",
                                                        "indexBounds" : {
                                                                "str" : [
                                                                        "[MinKey, MaxKey]"
                                                                ],
                                                                "d" : [
                                                                        "[MinKey, MaxKey]"
                                                                ]
                                                        }
                                                }
                                        },
                                        "rejectedPlans" : [ ]
                                }
                        }
                },
                {
                        "$groupByDistinctScan" : {
                                "newRoot" : {
                                        "_id" : "$str",
                                        "accum" : "$d"
                                }
                        }
                }
        ],
        "serverInfo" : {
                "host" : "ip-10-0-1-70",
                "port" : 20000,
                "version" : "7.1.0-alpha-1019-gd5dff6d",
                "gitVersion" : "d5dff6d99d6b569864e7dd71a8bea88f9017aa3c"
        },
        "serverParameters" : {
                "internalQueryFacetBufferSizeBytes" : 104857600,
                "internalQueryFacetMaxOutputDocSizeBytes" : 104857600,
                "internalLookupStageIntermediateDocumentMaxSizeBytes" : 104857600,
                "internalDocumentSourceGroupMaxMemoryBytes" : 104857600,
                "internalQueryMaxBlockingSortMemoryUsageBytes" : 104857600,
                "internalQueryProhibitBlockingMergeOnMongoS" : 0,
                "internalQueryMaxAddToSetBytes" : 104857600,
                "internalDocumentSourceSetWindowFieldsMaxMemoryBytes" : 104857600,
                "internalQueryFrameworkControl" : "trySbeEngine"
        },
        "command" : {
                "aggregate" : "group_conversion_to_distinct_scan",
                "pipeline" : [
                        {
                                "$sort" : {
                                        "str" : 1,
                                        "d" : 1
                                }
                        },
                        {
                                "$group" : {
                                        "_id" : "$str",
                                        "accum" : {
                                                "$first" : "$d"
                                        }
                                }
                        }
                ],
                "collation" : {
                        "locale" : "en_US",
                        "strength" : 2
                },
                "explain" : true,
                "cursor" : {

                },
                "lsid" : {
                        "id" : UUID("18b834ac-f35c-4ec3-aa60-70ec9aac98a6")
                },
                "$db" : "test"
        },
        "ok" : 1
}