How to Efficiently Count Documents in a Firestore Collection?

Getting the Count of Documents in a Firestore Collection

Introduction

In Firebase Firestore, there is no direct method for obtaining the document count of a collection. However, with the recent update and the introduction of new features in Firestore, there are multiple approaches to achieve this.

New count() Method

As of October 20th, 2022, Firestore offers a new count() method that enables you to count documents without reading their actual content. It returns a query that counts the documents matching the query. For aggregation queries such as count(), you are billed one document read for each batch of up to 1000 index entries matched by the query.

Distributed Counter

On July 10th, 2021, Firebase introduced the Distributed Counter extension. It provides a highly scalable counter service tailored for applications tracking high-velocity actions like views, likes, or shares. Using this extension, you can overcome the maximum limit of one write operation per second.

Code Examples for Document Counting

1. Using count() Method

Query countQuery = db.collection("Posts")
                     .whereEqualTo("status", "published");

Task<QuerySnapshot> task = countQuery.get();
task.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());
        }
    }
});

2. Using Legacy Approach

db.collection("Posts")
   .get()
   .addOnCompleteListener(new OnCompleteListener<QuerySnapshot>() {
       @Override
       public void onComplete(@NonNull Task<QuerySnapshot> task) {
           if (task.isSuccessful()) {
               int count = 0;
               for (DocumentSnapshot document : task.getResult()) {
                   count++;
               }
               Log.d("TAG", count + "");
           } else {
               Log.d(TAG, "Error getting documents: ", task.getException());
           }
       }
   });

Other Considerations

Limitations of Legacy Approach:

Using the legacy approach works well for small datasets but can be inefficient for large datasets due to the read operation for every document.

Accuracy of Distributed Counter:

Please note that Cloud Functions are write-time best-effort, meaning there may be a slight delay in updating the counter, and it may not be immediately accurate, particularly if the dataset is large or if there is a high write load.

Conclusion

The new count() method provides a convenient and performant way to count documents in Firestore collections. If you require a counter with instant updates and scalability, consider using the Distributed Counter extension.

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