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What causes a “Sort operation used more than the maximum 33554432 bytes of RAM” error and how can it be fixed?

 The MongoDB error:

“Sort operation used more than the maximum 33554432 bytes of RAM”

means that your query's in-memory sort exceeded the 32MB limit, and MongoDB couldn’t use disk space to complete the operation — usually because allowDiskUse was not enabled.

What causes a “Sort operation used more than the maximum 33554432 bytes of RAM” error and how can it be fixed?

🔍 Why This Happens

  • MongoDB allows up to 32MB of memory for in-memory sort operations.

  • If the result set being sorted is too large, and you're not using allowDiskUse: true, MongoDB aborts the query and throws this error.

🧪 Typical Example

js

db.collection.aggregate([
  { $sort: { someLargeField: 1 } }
])

🔴 If this sort uses more than 32MB and allowDiskUse is not enabled, you'll get the error.

✅ How to Fix It

✅ 1. Enable allowDiskUse in Aggregation

This lets MongoDB spill large sorts to disk:

js

db.collection.aggregate([
  { $sort: { someLargeField: 1 } }
], { allowDiskUse: true })

In MongoDB drivers:

  • Node.js:

    js
    collection.aggregate(pipeline, { allowDiskUse: true })

  • Python (PyMongo):

    python
    collection.aggregate(pipeline, allowDiskUse=True)

✅ 2. Add an Index to Support the Sort

If your sort matches an index, MongoDB can use the index to sort efficiently, avoiding memory usage.

Example:

db.collection.createIndex({ someLargeField: 1 })

Then your sort query:

db.collection.find().sort({ someLargeField: 1 })

will use the index and not memory.

✅ 3. Use limit() to Reduce Result Set

If you only need a subset of sorted results:

db.collection.find().sort({ field: 1 }).limit(100)

This limits memory usage during sort.

✅ 4. Reduce the Document Size or Fields Returned

Use projections to avoid sorting large documents:

db.collection.find({}, { _id: 0, smallField: 1 }).sort({ smallField: 1 })

🚫 Don’t Try This

  • Don’t increase memory limit — MongoDB does not allow increasing the 32MB cap.

  • Don’t disable allowDiskUse if your sort exceeds memory.

🧠 Summary

SolutionDescription
allowDiskUse: trueEnables sorting using disk if memory is exceeded
Add appropriate indexPrevents sorting in memory entirely
Use limit()Reduces amount of data being sorted
Return only needed fieldsShrinks sort payload

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