Avoid Aggregate 16MB Limit

问题:

I have a collection of about 1M documents. Each document has internalNumber property and I need to get all internalNumbers in my node.js code.

Previously I was using

db.docs.distinct("internalNumber")

or

collection.distinct('internalNumber', {}, {},(err, result) => { /* ... */ })

in Node.

But with the growth of the collection I started to get the error: distinct is too big, 16m cap.

Now I want to use aggregation. It consumes a lot of memory and it is slow, but it is OK since I need to do it only once at the script startup. I've tried following in Robo 3T GUI tool:

db.docs.aggregate([{$group: {_id: '$internalNumber'} }]); 

It works, and I wanted to use it in node.js code the following way:

collection.aggregate([{$group: {_id: '$internalNumber'} }],
  (err, docs) => { /* ... * });

But in Node I get an error: "MongoError: aggregation result exceeds maximum document size (16MB) at Function.MongoError.create".

Please help to overcome that limit.

回答1:

The problem is that the native driver differs from how the shell method is working by default in that the "shell" is actually returning a "cursor" object where the native driver needs this option "explicitly".

Without a "cursor", .aggregate() returns a single BSON document as an array of documents, so we turn it into a cursor to avoid the limitation:

let cursor = collection.aggregate(
  [{ "$group": { "_id": "$internalNumber" } }],
  { "cursor": { "batchSize": 500 } }
);

cursor.toArray((err,docs) => {
   // work with resuls
});

Then you can use regular methods like .toArray() to make the results a JavaScript array which on the 'client' does not share the same limitations, or other methods for iterating a "cursor".

回答2:

For Casbah users:

val pipeline = ...
collection.aggregate(pipeline, AggregationOptions(batchSize = 500, outputMode = AggregationOptions.CURSOR)