How to calculate read/writes on Firestore query/where operation?

Question

The Firebase Firestore documentation says:

Get multiple documents from a collection

You can also retrieve multiple documents with one request by querying documents in a collection. For example, you can use where() to query for all of the documents that meet a certain condition, then use get() to retrieve the results:

var citiesRef = db.collection('cities');
var query = citiesRef.where('capital', '==', true).get()
    .then(snapshot => {
      snapshot.forEach(doc => {
        console.log(doc.id, '=>', doc.data());
      });
    })
    .catch(err => {
      console.log('Error getting documents', err);
    });

If I have a collection with N documents, would I incur a N-read fee or a single read, for the above query?

Is there any way to retrieve the read/write count on a per call basis using the SDK?

As some background for my rational for asking, I have a single collection with a large number of documents (around 20,000). I want to export the entire collection's documents in the most cost efficient manner (least reads).

Answer 1

Every document yielded by a query counts as a document read. If your query matches and returns N documents, it will cost N document reads.