The queryStats top-K sort optimization (SERVER-129387) sets a TopKSortSpec hint on $queryStats for {$queryStats, $sort, $limit:K} pipelines, and the exec stage materializes only the K candidates we want to return. We left the $sort and $limit stages in the pipeline for correctness reasons. When a query stats entry fails to reparse, the exec stage falls back to a full store scan so we can guarantee we will always return at least K documents. 

However, this ticket should investigate approaches to remove this fall back while guaranteeing correctness, so we could remove the $sort/$limit stages, since we already perform the sort/limit inside the exec stage.

See TODO in the code for more details.