estimatedDocumentCount for speed in MongoDB - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

What does estimatedDocumentCount() return?

Consider a MongoDB collection with 1000 documents. You run db.collection.estimatedDocumentCount(). What does this method return?

MongoDB

db.collection.estimatedDocumentCount()

A The exact count of documents matching a query filter.

B The size of the collection in megabytes.

C A list of all documents in the collection.

D An approximate count of documents in the collection, usually fast and without filters.

💡 Hint

Think about whether this method uses filters or scans all documents.

estimatedDocumentCount() returns a fast, approximate count of all documents in the collection without applying any filters. It is faster than countDocuments() which applies filters and counts exactly.

🧠 Conceptual

intermediate

Why use estimatedDocumentCount() over countDocuments()?

Which reason best explains why you might choose estimatedDocumentCount() instead of countDocuments()?

A Because it provides an exact count with filters applied.

B Because it is faster and uses collection metadata without scanning documents.

C Because it returns documents instead of counts.

D Because it updates the collection size automatically.

💡 Hint

Think about speed and how the method counts documents.

estimatedDocumentCount() is faster because it uses collection metadata to estimate the number of documents, avoiding scanning all documents. It does not apply filters and is approximate.

📝 Syntax

advanced

Which code correctly uses estimatedDocumentCount() in Node.js?

Choose the correct way to get the estimated document count from a MongoDB collection using Node.js MongoDB driver.

MongoDB

const count = await collection.???();

A estimatedCount()

B countDocuments()

C estimatedDocumentCount()

D count()

💡 Hint

Check the exact method name for estimated count.

The correct method name is estimatedDocumentCount(). Other options are either deprecated or incorrect.

❓ optimization

advanced

When is estimatedDocumentCount() NOT suitable?

You want to count documents matching a filter in a large collection. Why might estimatedDocumentCount() be a bad choice?

A Because it only returns an approximate count of all documents without filters.

B Because it is slower than scanning all documents.

C Because it modifies the documents during counting.

D Because it returns the size of the collection in bytes.

💡 Hint

Think about whether filters are applied.

estimatedDocumentCount() does not accept filters and returns an approximate count of all documents. For filtered counts, use countDocuments().

🔧 Debug

expert

Why does estimatedDocumentCount() return a non-zero number after deleting all documents?

You run estimatedDocumentCount() on a collection after deleting all documents, but it still returns a non-zero number. Why?

A Because estimatedDocumentCount() uses collection metadata that may not update immediately after deletions.

B Because the method counts documents with filters by default.

C Because the collection is locked and cannot be counted.

D Because estimatedDocumentCount() counts only documents in the cache.

💡 Hint

Think about how metadata updates in MongoDB.

estimatedDocumentCount() relies on collection metadata which may lag behind actual document deletions, causing it to return outdated counts temporarily.