Firestore listener mechanism efficiency

Question

If I understand correctly, in the initialization phase of addSnapshotListener you get a list of all the documents (even if it is 500 trillion documents) from the QuerySnapshot if you call the getDocuments function. Then, every time you modify or add a document to a collection, you get from QuerySnapshot all the documents that have been modified by calling the getDocumentChanges function or all the existing documents by calling getDocuments.

That means both at the initialization stage and after every change, I always get a list of all the documents. That's logical? Assuming I have 500 trillion documents under the same collection (just for the sake of exaggeration), at every change and initialization of the app will I get them all?

Is that really the case? Or is it some kind of lazy instantiation or something? Because if so, when I would like to question the whole collection, no matter what I get at first the whole list?

Answer 1

The QuerySnapshot always contains all documents that match the query (or collection reference). Even when there's an update and only a subset of the documents matched by the query is changed, the QuerySnapshot still contains all of the documents, even though in its communication between the SDK and the backend serves, Firestore only synchronizes the modified documents. If you only want to process the changes, you can process just the changes between the snapshots.