How to improve Flowable<Object> data reading from Firebase db using RxJava 2? - firebase

Question

I have Recycler Viewer that displays data from Fire Base db however initial List contains around 4k elements. I am trying to show only first 15 elements instead of waiting for full list to be loaded however not sure how to do it.
I am trying to take(x) elements via Subscriber however it does not improve reading performance (it still waits for 4k elements from Firebase DB). How to speed up this?
Subscriber - Presenter
#Override
public void onBindViewHolder(final ListContentFragment.ViewHolder holder, int position) {
modelInterface.getDataFromFireBase("FinalSymbols")
.take(15)
.subscribeOn(Schedulers.newThread())
.observeOn(AndroidSchedulers.mainThread())
.subscribe(new Consumer<DataSnapshot>() {
#Override
public void accept(DataSnapshot dataFromDb) throws Exception {
//update TextView inside Recycler Viewer
holder.name.setText(dataFromDb.child(String.valueOf(holder.getAdapterPosition())).child("description").getValue().toString());
holder.description.setText(dataFromDb.child(String.valueOf(holder.getAdapterPosition())).child("categoryName").getValue().toString());
}
}
);
}
Publisher - source of Data (FireBase db)
#Override
public Flowable<DataSnapshot> getDataFromFireBase(final String childName) {
return Flowable.create(new FlowableOnSubscribe<DataSnapshot>() {
#Override
public void subscribe(final FlowableEmitter<DataSnapshot> e) throws Exception {
databaseReference.child(childName).addValueEventListener(new ValueEventListener() {
#Override
public void onDataChange(DataSnapshot dataSnapshot) {
e.onNext(dataSnapshot);
e.onComplete();
}
#Override
public void onCancelled(DatabaseError databaseError) {
}
});
}
}, BackpressureStrategy.BUFFER);

Answer 1

I believe you need to use the method limitToFirst().
Something like this:
#Override
public Flowable<DataSnapshot> getDataFromFireBase(final String childName) {
return Flowable.create(new FlowableOnSubscribe<DataSnapshot>() {
#Override
public void subscribe(final FlowableEmitter<DataSnapshot> e) throws Exception {
databaseReference.child(childName).limitToFirst(15).addValueEventListener(new ValueEventListener() {
#Override
public void onDataChange(DataSnapshot dataSnapshot) {
e.onNext(dataSnapshot);
e.onComplete();
}
#Override
public void onCancelled(DatabaseError databaseError) {
}
});
}
}, BackpressureStrategy.BUFFER);