Index usage in $lookup

[steevej (Steeve Juneau)]

The COLLSCAN we see at

inputStage: {
    stage: 'COLLSCAN',
    planNodeId: 1,
    filter: {},
    direction: 'forward'
  }

and

@Franck_Pachot1

inputStage: {
    stage: 'PROJECTION_DEFAULT',
    planNodeId: 2,
    transformBy: { _id: true, personId: '$attributes.personId' },
    inputStage: {
      stage: 'COLLSCAN',
      planNodeId: 1,
      filter: {},
      direction: 'forward'
    }
  }

are related to the survey collection. This is normal since no $match nor $sort is done on survey.

The values

@Franck_Pachot1

  strategy: 'IndexedLoopJoin',
  indexName: 'attributes.personId_1',
  indexKeyPattern: { 'attributes.personId': 1 },

in both plan make me feel like the index attributes.personId:1 of the collection person is used in both cases.