Programming Tutorials

  You migrated from PostgreSQL to MongoDB for flexibility and scale, but your dashboard analytics are timing out. The culprit is almost always the   $lookup   stage in your aggregation pipeline. It works perfectly on your local machine with 500 documents. In production, with 5 million documents, it triggers memory cap errors ( Sort exceeded memory limit of 104857600 bytes ) or massive latency spikes. The issue isn’t MongoDB; it’s attempting to force relational theory into a document store. Here is the architectural root cause of slow joins and three concrete implementation patterns to fix them. The Root Cause: Cartesian Products and Memory Caps When you execute a standard  $lookup , MongoDB performs a left outer join. Without specific optimizations, the query execution engine must: Scan the entire input collection  (unless filtered by a preceding  $match ). Perform a query against the "from" collection  for  every single document  passed thro...