What does the $sort stage do?

The $sort stage sorts the stream of documents inside the aggregation pipeline - by one or several fields, ascending (1) or descending (-1). Conceptually, this is the analog of ORDER BY in SQL.

#How it works

• receives documents from the previous stage
• sorts them in memory or using temporary storage
• passes the already-sorted stream further down the pipeline

#Example

First it sorts by createdAt descending, and where values match, by price ascending.

#An important nuance

$sort works most efficiently after $match, once there's already less data. If the sorted fields are indexed (in a plain find), that speeds up sorting, but inside a pipeline, sorting more often happens in memory.

#Summary

$sort is the stage that controls the order of documents, so the next stages (or the final result) get the data in the sequence you need.