firebase - Firestore query reads all the documents again (from database) when only one is modified even with persistence enabled

Question

Expected Behavior: I assumed that when some field of a document is modified, query will only return that document and so it would be considered only 1 read. Say I query a collection with n documents, I would hope that it would be n reads for the initial state, and if a document is modified later only that document would be read again from database, resulting in n+1 reads in total.

What I see is n reads initially, and n reads later whenever one document is modified regardless of Offline Persistence being enalbed or not; resulting in 2n reads with all "fromCache : false" metadata.

  myCollection: 
         doc1 : 
              value: "some string"
         ...
         docn: 
             value: "some text" 

later I change value for one document via console:

     myCollection: 
         doc1 : 
              value: "changed to other value"
         ...
         docn: 
             value: "some text" 

But I get all n documents again in the snapshot of query. Regardless of the fact that I can observe changes using "s.docChanges", it seems that snapshot has all the n documents.

  const someCollectionRef = firebase.firestore().collection("collection/document/subdocl??lection"); 
  someCollectionRef.onSnapshot(s => {

      var d = [];
      s.forEach(doc => {

           d.push(doc.data()));

           //fromCache is always false even when one document is 
           //modified it seems that all documents are read from db again 
           console.log(`metadata is ` , doc.metadata);  
      }
      //d seems to have all my documents even when only 1 is modified

  });

No difference with offline persistence enabled. I tested the same example with offline persistence enabled and no difference! Modifying a document still causes all the documents to be read with "fromCache : false" metadata, i.e, they are read from database. The only time data is being read from cache is when a query is refreshed but documents are all identical.

Answer 1

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