Introduction
MongoDB is a leading NoSQL database, favored for its flexibility, scalability, and performance. A common requirement when working with large collections is to get a quick estimation of the number of documents they contain. This is exactly what the estimateDocumentCount() method is designed for.
In this tutorial, we dive deep into how and when to use estimateDocumentCount() in MongoDB, especially with large collections. We’ll start from the basics and progressively introduce more advanced use cases. Let’s buckle up and boost your MongoDB skills.
Understanding estimateDocumentCount()
The estimateDocumentCount() method in MongoDB provides an estimate of the count of documents in a collection using metadata rather than scanning the entire collection. This approach is faster, especially with large datasets, but may not always provide an exact count, particularly if documents are frequently added or removed.
Here is the basic syntax used to get an estimated count of documents:
db.collection.estimateDocumentCount()This will return an estimated count based on collection statistics.
Basic Usage
Let’s begin with an example of a simple usage on a sample collection called ‘products’.
const estimatedCount = db.products.estimateDocumentCount();
console.log(`Estimated number of documents: ${estimatedCount}`);
Output:
Estimated number of documents: 12000
Note that the count returned is fast, but again, it’s only an estimate.
When to Use estimateDocumentCount()
This method is best used when:
- You need a fast count for large collections.
- Exactness is not critical to your application logic.
- You want to avoid performance overheads.
Keep in mind that regular document insertions, deletions, or collection modifications could result in inaccurate estimates. For precisely accurate counts, use the countDocuments() method.
Comparing estimateDocumentCount() with countDocuments()
To understand the difference, let’s also count the documents using the countDocuments() method and compare the outputs.
const exactCount = db.products.countDocuments();
console.log(`Exact number of documents: ${exactCount}`);
Output:
Exact number of documents: 11998
Here you can see the actual count might be slightly different from the estimate, depending on the operations that might have happened on the collection.
Advanced Usage of estimateDocumentCount()
While estimateDocumentCount() has a basic use case, it can be involved in more complex operational scripts. Sometimes, you may need to perform conditional logic based on whether your collection has reached a certain size. Here’s how to do it:
const threshold = 10000;
const estimatedCount = db.products.estimateDocumentCount();
if (estimatedCount > threshold) {
console.log('The collection has exceeded the threshold.');
// Additional logic here
} else {
console.log('The collection is below the threshold.');
}
Output:
The collection has exceeded the threshold.
Handling Sharded Collections
When dealing with sharded collections, the estimateDocumentCount() method may not be accurate due to the possibility of chunks being moved around. In this situation, you might want to fall back to countDocuments() for more accuracy, while remaining aware of the higher performance cost.
Conclusion
In summary, the estimateDocumentCount() method is a powerful tool for quickly evaluating the size of large collections. It’s perfect when speed is a priority, and an exact count is not essential. However, for precise document counts in critical operations, consider using countDocuments() despite its higher resource usage.
