Preventing Out-of-Memory Crashes in MongoDB Sorting

Sorting large datasets in MongoDB can sometimes lead to frustrating out-of-memory crashes. But don’t worry — I've got some simple tips to help you avoid that! Before we dive in, I’ve written another article on using the MongoDB Query Planner to check if your queries are efficient. It’s a great way to see what’s really happening under the hood and catch problems early.

Why Is This Happening?

When you sort a big chunk of data, MongoDB tries to load it all into memory. If the dataset is too large, it might run out of memory and crash. Here’s how you can avoid that and make sure your queries are running smoothly.

Simple Fixes:

• 1. Use Indexes This is the most important step! Always index the fields you’re sorting by. It helps MongoDB sort data much faster without loading everything into memory. Think of an index like a library card catalog — it helps MongoDB find what it needs without scanning the whole bookshelf.

Example:

  db.users.createIndex({ createdAt: 1 });

Query Planner Output:

  {
    "stage": "FETCH",
    "inputStage": {
      "stage": "IXSCAN",
      "keyPattern": { "createdAt": 1 }
    }
  }

Here you can see MongoDB is using an index scan (IXSCAN), meaning it’s being smart and efficient!

• 2. Check Your Query Efficiency Use the MongoDB Query Planner to see if your query is using indexes or doing a slow collection scan. This is like having a map of your query’s journey — it shows you if things are going the right way or if there’s a traffic jam. (Want more info on using the Query Planner? Check out my article!)

Example:

  db.users.find().sort({ createdAt: 1 });

Query Planner Output:

  {
    "stage": "FETCH",
    "inputStage": {
      "stage": "IXSCAN",
      "keyPattern": { "createdAt": 1 }
    }
  }

Nice! MongoDB is using the index correctly, so it’s not bogging down your system.

• 3. Limit and Paginate Don't try to load everything at once — that’s just asking for trouble. Use pagination to grab smaller chunks of data and keep things manageable. It’s like taking one bite of pizza at a time instead of eating the whole thing in one go.

Example:

  db.users.find().sort({ createdAt: 1 }).limit(100).skip(0);

Query Planner Output:

  {
    "stage": "LIMIT",
    "inputStage": {
      "stage": "IXSCAN",
      "keyPattern": { "createdAt": 1 }
    }
  }

MongoDB is being smart here by limiting the result set, so it’s only fetching what you need right now.

• 4. Let MongoDB Use Disk Space If memory is still an issue, you can tell MongoDB to temporarily use disk space for sorting. This can slow things down a bit but may prevent crashes — like borrowing a bigger table when you can’t fit everything on your desk. However, how well this works depends on the MongoDB plan you’re using. If you’re on a stronger plan with more resources, this option can help a lot more. If you’re on a limited plan, it might not perform as well.

Example:

  db.users.aggregate([{ $sort: { createdAt: 1 } }], { allowDiskUse: true });

Query Planner Output:

  {
    "stage": "SORT",
    "diskUsed": true,
    "inputStage": {
      "stage": "COLLSCAN"
    }
  }

MongoDB is now using disk space to help with the sorting. If you're on a more powerful plan, this could be a great way to avoid memory crashes.

• 5. Watch Out for Operations on Fields You're Sorting By

Here’s a tip to keep things running smoothly: if you do anything to the field you’re trying to sort by (like converting it, adding something to it, or adjusting it) in an aggregation pipeline, MongoDB will drop the index. Once the index is dropped, MongoDB can’t use it to help sort, which means your query will get slower and use more memory.

For example, let’s say you want to sort users by the year they signed up. If you do something like this:

  db.users.aggregate([
    { $project: { year: { $year: "$createdAt" } } },
    { $sort: { year: 1 } }
  ]);

Because you’re converting the createdAt field into a year, MongoDB drops the index on createdAt. Now it has to scan through the whole collection to sort the data, which can slow things down a lot.

Thanks for Reading!

Hope these tips keep your MongoDB running smoothly! If you have questions, feel free to reach out. Happy coding! 🚀