I am eager to know whether MongoDB’s Aggregation Framework processes each sub-pipeline inside $facet in parallel.
It is specified in the doc that:
Each sub-pipeline within
$facetis passed the exact same set of input documents. These sub-pipelines are completely independent of one another.
Does that mean each sub-pipeline in $facet is send to a separate thread and processed asynchronously?
No, all aggregation pipelines are executed sequentially, including $facet.
If you’re prepared to add some complexity and overhead to your client code, there’s a blog post exploring using custom code to execute facet sub-pipelines in parallel and then manually merging the results: http://pauldone.blogspot.com/2018/02/parallel-mongodb-aggregation-facets.html
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