Cursor in MongoDB

When we run a query in MongoDB, it may look like all results are returned at once.

But behind the scenes, MongoDB uses something called a cursor to fetch documents efficiently.

What is a MongoDB Cursor?

A cursor is a pointer to the result set of a query.

When you run a find() query, MongoDB does not immediately return all matching documents.
Instead, it returns a cursor that:

• Points to the matching documents

• Fetches documents in batches

• Allows iteration over the result set

Why Does MongoDB Use Cursors?

Imagine a collection with millions of documents.

If MongoDB returned everything at once:

• High memory usage ❌

• Network overload ❌

• Slow performance ❌

Cursors solve this by:

• Stream large result sets efficiently

• Avoid loading all documents into memory

• Maintain consistent performance even for large collections

How Cursor Works Internally

1. Client sends a find() query

2. MongoDB:

   • Finds matching documents

   • Returns the first batch (default ~101 docs)

   • Creates a cursor ID

3. Once the first batch is fully consumed, the MongoDB driver automatically sends a getMore command to the server.

4. MongoDB sends the next batch

5. This cycle continues until:

   All documents are read, or

   The cursor is closed or times out

MongoDB Operations That Return Cursors

A few operations that send a response as a cursor are

• find()

• aggregate()

• listCollections()

• listIndexes()

Code:

Without Cursor

const data = await db.collection("users")
  .find({ age: { $gt: 20 } })
  .toArray();

data.forEach(user => {
  console.log(user);
});

→ Loads everything into memory first.

With Cursor

const cursor = db.collection("users")
  .find({ age: { $gt: 20 } });
for await (const doc of cursor) {
   console.log(doc)
}

→ Streams documents in batches, no memory blow-up.