How Can I Efficiently Count Documents in a Firestore Collection?

Counting Documents in a Firestore Collection

Edit: October 20th, 2022

Firestore now provides a count() method that counts the documents in a collection or the documents returned by a query without reading the documents. This eliminates the need for maintaining a counter.

Query query = db.collection("Posts").orderBy("createdAt");
Task<Long> task = query.count();
task.addOnCompleteListener(new OnCompleteListener<Long>() {
    @Override
    public void onComplete(@NonNull Task<Long> task) {
        long count = task.getResult();
    }
});

Edit: July 10th, 2021

Firebase introduces the Distributed Counter Extension, which provides a highly scalable counter service for high-velocity actions like views, likes, or shares. This extension allows counting operations to exceed the limitation of one write operation per second.

Original Answer:

Retrieving Count of Documents Without getDocumentCount() Method

Since Firestore lacks a dedicated getDocumentCount() method, alternative approaches are necessary to count the number of documents in a collection.

db.collection("Posts").get()
        .addOnCompleteListener(new OnCompleteListener<QuerySnapshot>() {
            @Override
            public void onComplete(@NonNull Task<QuerySnapshot> task) {
                if (task.isSuccessful()) {
                    int count = task.getResult().size();
                    Log.d(" TAG", count + "" );
                } else {
                    Log.d(TAG, "Error getting documents: ", task.getException());
                }
            }
        });

Limitations and Alternatives

This approach is suitable for small datasets. For larger datasets, consider alternative methods:

• Cloud Functions Counter: Update a counter in the Cloud Functions whenever a document is added or deleted. This is efficient but limited to less than one operation per second.
• Distributed Counters: Implement distributed counters as explained in the Firebase documentation. This approach accommodates high-rate operations but incurs additional cost.
• Firebase Realtime Database: Store the count in the Firebase Realtime Database for near real-time updates and cost-effectiveness.

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