When a :pipeline:`$sort` immediately precedes a :pipeline:`$limit` in
the pipeline, the :pipeline:`$sort` operation only maintains the top
n results as it progresses, where n is the specified limit, and
MongoDB only needs to store n items in memory. This optimization
still applies when allowDiskUse is true and the n items
exceed the :ref:`aggregation memory limit <agg-memory-restrictions>`.
.. versionchanged:: 2.4 Before MongoDB 2.4, :pipeline:`$sort` would sort all the results in memory, and then limit the results to n results.