Aggregation operations with the
aggregate command have the
aggregate command can either return a cursor or store
the results in a collection. Each document in the result set is subject
to the 16 megabyte BSON Document Size limit. If any single document exceeds the BSON Document Size
limit, the aggregation produces an error. The
limit only applies to the returned documents. During the pipeline
processing, the documents may exceed this size. The
db.collection.aggregate() method returns a cursor by default.
Each individual pipeline stage has a limit of 100 megabytes of RAM. By default, if a stage exceeds this limit, MongoDB produces an error. For some pipeline stages you can allow pipeline processing to take up more space by using the allowDiskUse option to enable aggregation pipeline stages to write data to temporary files.
$search aggregation stage is not restricted to
100 megabytes of RAM because it runs in a separate process.
Examples of stages that can spill to disk when allowDiskUse is
Pipeline stages operate on streams of documents with each pipeline stage taking in documents, processing them, and then outputing the resulting documents.
Some stages can't output any documents until they have processed all incoming documents. These pipeline stages must keep their stage output in RAM until all incoming documents are processed. As a result, these pipeline stages may require more space than the 100 MB limit.