DOCS-13918 aggregation optimizer (#481) (#487)

* DOCS-13918-aggregation-optimizer

* DOCS-13918-aggregation-optimizer

* DOCS-13918-aggregation-optimizer

* DOCS-13918-aggregation-optimizer

* DOCS-13918-aggregation-optimizer

Co-authored-by: jason-price-mongodb <jshfjghsdfgjsdjh@aolsdjfhkjsdhfkjsdf.com>

Co-authored-by: jason-price-mongodb <jshfjghsdfgjsdjh@aolsdjfhkjsdhfkjsdf.com>

Author: jason-price-mongodb

source/core/aggregation-pipeline-optimization.txt

@@ -454,6 +454,10 @@ The following list shows some pipeline stages that can use indexes:
   :pipeline:`$geoNear` can use a geospatial index. :pipeline:`$geoNear`
   must be the first stage in an aggregation pipeline.
 
+Starting in MongoDB 4.2, in some cases, an aggregation pipeline can use
+a ``DISTINCT_SCAN`` index plan, which typically has higher performance
+than ``IXSCAN``.
+
 Indexes can :ref:`cover <read-operations-covered-query>` queries in an
 aggregation pipeline. A covered query uses an index to return all of the
 documents and has high performance.