db.index.explain().aggregate({$sort:{a:1}},{$group:{_id:0, a:{$first:"$a"}}})
{
  explainVersion: '2',
  queryPlanner: {
    namespace: 'test.index',
    indexFilterSet: false,
    parsedQuery: {},
    queryHash: '5416DB9E',
    planCacheKey: '2C4F0CBD',
    optimizedPipeline: true,
    maxIndexedOrSolutionsReached: false,
    maxIndexedAndSolutionsReached: false,
    maxScansToExplodeReached: false,
    winningPlan: {
      queryPlan: {
        stage: 'GROUP',
        planNodeId: 3,
        inputStage: {
          stage: 'PROJECTION_COVERED',
          planNodeId: 2,
          transformBy: { a: true, _id: false },
          inputStage: {
            stage: 'IXSCAN',
            planNodeId: 1,
            keyPattern: { a: 1, b: 1 },
            indexName: 'a_1_b_1',
            isMultiKey: false,
            multiKeyPaths: { a: [], b: [] },
            isUnique: false,
            isSparse: false,
            isPartial: false,
            indexVersion: 2,
            direction: 'forward',
            indexBounds: { a: [ '[MinKey, MaxKey]' ], b: [ '[MinKey, MaxKey]' ] }
          }
        }
      },
      slotBasedPlan: {
        slots: '$$RESULT=s10 env: { s2 = Nothing (SEARCH_META), s4 = 1710513544145 (NOW), s1 = TimeZoneDatabase(Etc/GMT-14...Africa/Bujumbura) (timeZoneDB), s3 = Timestamp(1710513542, 1) (CLUSTER_TIME) }',
        stages: '[3] mkbson s10 [_id = s9, a = s7] true false \n' +
          '[3] project [s9 = 0] \n' +
          '[3] group [] [s7 = first((s5 ?: null))] spillSlots[s8] mergingExprs[first((s8 ?: null))] \n' +
          '[1] ixseek KS(0A0A0104) KS(F0F0FE04) none s6 none none lowPriority [s5 = 0] @"ba1ce78f-0524-475b-8c44-f2488f71f8aa" @"a_1_b_1" true '
      }
    },
    rejectedPlans: []
  },