Does $limit stage optimize an aggregation query that has no filter stage?

[Vladimir (Vladimir R.)]

Is aggregation stage $limit used on the whole data retrieved after it is required, or does MongoDB optimize the query? For example, if I make an aggregation query without a filter stage ($match or the like) and use $limit, will it first retrieve all the documents in the collection and then apply $limit or can it optimize and retrieve the limited amount immediately therefore not stressing the database as much?

[Akshat_Gupta3 (Akshat Gupta)]

Hi Vladimir,

MongoDB has an internal query optimiser which reshapes the pipeline in a way to efficiently retrieve the results. You can read more about it here -

https://www.mongodb.com/docs/manual/core/aggregation-pipeline-optimization/#:~:text=Aggregation%20pipeline%20operations%20have%20an,collection.

As to answer your specific question -
Even if you don’t use a $match stage and only use $limit , MongoDB still optimizes the operation. It won’t retrieve all documents in the collection; instead, it will directly limit the number of documents it processes to the specified limit.