Why compound wildcard index still do fetch?

Working with Data
, , ,
3

Hi,

I tried $project but no luck, it still uses $fetch.
Then I add attribute in $match, and it won’t do $fetch, like below:

[
  {
    $match: {
      year: 2024,
      month: 8,
      day: 1,
      "attributes.country": 'JP'
    }
  },
  {
    $group: {
      _id: {
        appId: "$appId",
        os: "$os",
        country: "$attributes.country"
      },
      value: {
        $sum: 1
      }
    }
  }
]

image
image1898×988 82.3 KB

BUT this is not what I want,
I don’t need to match country but group by country.

Can’t compound wildcard index be used in $group only?

Any comments are welcome!
Thank you very much.