Why compound wildcard index still do fetch?

[iamabs2001 (Abhijeet Sharma)]

@Anderson_Lin

db.my_coll.createIndex(
  {
    year: 1,
    month: 1,
    day: 1,
    hour: 1,
    appId: 1,
    os: 1,
    "attributes.$**": 1
  },
  {
    name: "compound_wildcard_test"
  }
)

It might be because mongodb is trying to group certain fields but it’s missing some context, you can try adding a full projection of the usefull fields before grouping in the aggregate pipeline.