mongodb.js

db.createCollection('a');
db.a.insertMany([
  { day: 'Monday', packages: 1000 },
  { day: 'Tuesday', packages: 100 },
  { day: 'Monday', packages: 1000 },
]);

db.a.createIndex({ day: 1 });

db.createCollection('b', { viewOn: 'a', pipeline: [
  {
    $group: {
      _id: { day: '$day' },
      packages: { $sum: '$packages' },
    }
  }
] });

db.b.find({ day: 'Monday' }).explain();
/*
The result is
{
  "$group" : {
    "_id" : {
      "day" : "$day"
    },
    "packages" : {
      "$sum" : "$packages"
    }
  }
},
{
  "$match" : {
    "day" : {
      "$eq" : "Monday"
    }
  }
}
*/