How to fix MongoServerError: PlanExecutor error during aggregation :: caused by :: Sort exceeded memory limit of 104857600 bytes, but did not opt in to external sorting. Aborting operation. in MongoDB

Pass { allowDiskUse: true } when calling aggregate so MongoDB can spill to disk instead of keeping everything in RAM.

// mongosh or mongo shell
const pipeline = [...];
printjson(db.collection.aggregate(pipeline, { allowDiskUse: true }).toArray());

// Node.js driver
const cursor = collection.aggregate(pipeline, { allowDiskUse: true });
await cursor.toArray();

// PyMongo
result = list(collection.aggregate(pipeline, allowDiskUse=True))

If you rely on find().sort(), convert to an aggregation with a final $sort and the same flag (modern drivers also expose .allowDiskUse(true) on cursors). Enabling disk use is the quickest fix, but monitor I/O as spilling can slow other queries.

Trim the data before MongoDB has to sort it. Push $match, $project, and $limit ahead of $sort or $group, and only carry the fields you actually need.

const pipeline = [
{ $match: { status: 'active' } },
{ $project: { _id: 0, batteryId: 1, time: 1 } },
{ $sort: { time: -1 } },
{ $limit: 1000 },
// rest of the stages
];

Also make sure indexes support the sort/group keys so MongoDB can stream results instead of buffering huge sets. If arrays or lookups inflate each document, consider splitting the pipeline or using $setWindowFields to operate in chunks.

Run .explain('executionStats') on the pipeline to see which stage is using the most RAM and how far over the limit it went.

const plan = await collection.aggregate(pipeline, { allowDiskUse: false }).explain('executionStats');
console.log('Max blocking sort bytes:', plan.maxBlockingSortBytes);
console.log(plan.stages);

The output shows whether SORT or GROUP is buffering documents, how many docs it processed, and whether allowDiskUse was false. Use this information to rearrange stages, narrow the input, or confirm that you need to raise the limit before rerunning with allowDiskUse enabled.

Self-managed mongod instances let you increase the default 100 MB buffer via the internalQueryExecMaxBlockingSortBytes parameter.

// mongosh
db.adminCommand({
setParameter: 1,
internalQueryExecMaxBlockingSortBytes: 335544320,
});

Or put this in mongod.conf and restart:
setParameter:
internalQueryExecMaxBlockingSortBytes: 335544320

Atlas free/shared tiers and serverless clusters stay limited to 32 MB, so in those environments you must optimize the query or upgrade to a dedicated tier. After raising the limit, watch for memory pressure because very large buffers can hurt concurrency.