Why when using a projection does it save data sent across the wire?

MongoDB University M220JS: MongoDB for Javascript Developers
#1

When doing the migration lesson I came across the statement that usinga projection saves data coming across the wire.

In my time spent I discovered that the statement is true when nothing is being found and returning 0. Otherwise if the search is bring back a truthy for each request the amount is realtively the same. If i search for dates and everything is a date it takes a long time. If I search for string and nothing is a string then the query comes back immediately.

I guess if the migration in this case has already been completed then it will save on an alert stating as such. but otherwise I am not seeing the benefit.

Also, could it be written differently or otherwise to save on the query in either condition?

here is the code sample.

// a projection is not required, but may help reduce the amount of data sent
    // over the wire!
    const predicate = { lastupdated: { $exists: true, $type: "string" } }
    const projection = { lastupdated: 1, _id: 0}
    const cursor = await mflix
      .collection("movies")
      .find(predicate, projection)
      .toArray()
#2

When you project only the projected fields are sent across the wire. In this case since we do not need the other fields because we are only interested in lastupdated. All other fields (13 or 14) like, title, cast, plot, … are not transferred from the mongod server to the client. In addition if the fields projected are an index the whole document does not have to be read from storage. In this schema the comments are another table but it could have been an array part of a movie document. Without projection and comments part of a movie you would have sent all comments of all selected movies just to update a single field.