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Question: Why is my MongoDB group operation too slow?


MongoDB's $group stage in aggregation operations can sometimes perform slower than expected due to various reasons. Understanding and mitigating these factors can help improve performance.

1. Lack of Indexes

One common reason for slow $group operations is the absence of appropriate indexes. If your query could benefit from an index before grouping, ensure to create it. For example, if you're grouping documents by the category field:

db.collection.aggregate([ { $match: { category: 'Books' } }, { $group: { _id: '$author', count: { $sum: 1 } } } ]);

Creating an index on category can significantly speed up the query:

db.collection.createIndex({ category: 1 });

2. Large Datasets

Grouping a very large dataset can naturally lead to slower performance, especially if the operation has to process many documents or the group keys are highly unique. Consider whether all data is necessary for the operation, or if it can be filtered beforehand with $match.

3. Not Using AllowDiskUse

For operations that require more memory than the aggregation pipeline allows (by default, 100 MB), MongoDB will return an error. However, you can enable the allowDiskUse option to allow stages to spill over to disk:

db.collection.aggregate([ // Your stages here ], { allowDiskUse: true });

While this can allow your operation to complete, using disk space can be significantly slower than memory. Optimizing your query or infrastructure may provide better solutions.

4. Hardware Limitations

The hardware running your MongoDB instance can also impact performance. Slow disks, limited CPU, or insufficient RAM can all contribute to slow operations. Upgrading your hardware or moving to a cloud provider with better resources can help.

5. Sharding and Distribution

In a sharded environment, consider how your data is distributed across shards. Poor shard key choice or unbalanced data distribution can lead to inefficient queries, as the aggregation operation might need to gather data from multiple shards.

Optimization Strategies

  • Projection: Use $project to limit fields before grouping.
  • Pre-aggregation: Store aggregated results periodically if real-time results are not required.
  • Incremental Aggregation: For frequently accessed aggregations, consider maintaining a separate collection that stores intermediate results.


Slow MongoDB $group operations can often be mitigated by ensuring proper indexing, considering data size and complexity, optimizing hardware, and designing queries with performance in mind.

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