Introduction to MongoDB cursor.batchSize() Method

batchSize() is a method in MongoDB used to specify the number of documents returned per batch during batch processing. By default, MongoDB reads all documents that match the query or aggregation operation into memory and then returns them to the client. However, if there are a large number of documents, it can consume a lot of memory and cause performance issues. In this case, you can use the batchSize() method to return the documents in batches to reduce memory usage.

Syntax

The syntax of the batchSize() method is as follows:

db.collection.find().batchSize(size)

where size is a positive integer that represents the number of documents returned per batch.

Use Cases

Using the batchSize() method can improve the performance of MongoDB in the following scenarios:

• When the result set of a query or aggregation operation is very large, using batchSize() can reduce memory usage.
• When processing a large number of documents one by one, batchSize() can be used to return the documents in batches.

Example

Suppose we have a collection named users that contains a large number of documents. We want to get all the document data at once, but because the data is so large, getting all the data at once may cause performance issues. In this case, we can use the batchSize() method to limit the number of documents returned per query.

The following example code demonstrates how to use the batchSize() method to set the number of documents returned per batch to 1000:

const MongoClient = require("mongodb").MongoClient

const url = "mongodb://localhost:27017"
const dbName = "myproject"

MongoClient.connect(url, function (err, client) {
console.log("Connected successfully to server")

const db = client.db(dbName)
const collection = db.collection("users")

// Set batchSize
const cursor = collection.find({}).batchSize(1000)

cursor.forEach(
(doc) => {
console.log(doc)
},
(err) => {
console.log(err)
}
)

client.close()
})

In the code above, we first connect to the local MongoDB server using MongoClient and select the database myproject. Then we get the collection named users and use the batchSize() method to set the number of documents returned per batch to 1000. Finally, we use a cursor to iterate through the document data and print it to the console.

Conclusion

The batchSize() method can be used to limit the number of documents returned per query, thereby improving query performance and reducing memory usage. However, it should be noted that in some cases, using the batchSize() method may cause query performance to decrease, so it should be used carefully depending on the specific situation.