MongoDB Aggregation Performance: From High CPU to High Performance

MongoDB's aggregation framework is incredibly versatile, but it can also lead to significant performance issues in modern Node.js applications, manifesting as "CPU Spikes" and "Gateway Timeouts." If your dashboards take longer than 500ms to load, it's likely that your aggregation pipeline is executing a Full Collection Scan. In this article, we’ll explore optimization techniques that can help you achieve production-scale performance.

The Importance of Pipeline Order

The sequence of stages in your aggregation pipeline is crucial for performance. Always position the $match and $sort stages at the beginning of your pipeline. Why is this important? These stages can leverage database indexes, which significantly enhances query performance. If you place a $lookup (join) or $project (field modification) before filtering your data, MongoDB may need to load every document into RAM for processing, risking server crashes as your dataset grows.

Analyzing Performance with the 'Explain' Plan

To identify the root cause of slow queries, avoid guessing and instead use the command db.collection.aggregate([...]).explain("executionStats"). Look for the label stage: "COLLSCAN"—this is an indication that MongoDB is scanning every document on disk. Your goal is to achieve stage: "IXSCAN", which signifies that the query is utilizing a Winning Index. If you're filtering based on company_id and created_at, ensure that you have a Compound Index on these fields for optimal performance.

Memory Limits and the Use of $facet

Keep in mind that aggregation stages have a default memory limit of 100MB. If this limit is exceeded, your query will fail. While you can enable allowDiskUse: true to bypass this restriction, it often leads to slower performance. A more efficient strategy is to implement Incremental Aggregation. Instead of recalculating complex reports each time a user refreshes the page, consider using a Cron job to "pre-calculate" the results and store them in a reporting_cache collection every 10 minutes.

Expert Tip: Avoiding the $lookup Trap

Be cautious with $lookup stages, as each instance essentially creates a nested loop. For example, joining a collection with 1 million rows can drastically reduce performance. In the world of NoSQL, Denormalization is your ally. If you require the customer_name in every order report, consider storing a copy of the name directly within the order document. This small redundancy can yield performance improvements of up to 100x.