DOCSP-13698 Index Intersection is rarely chosen by the query planner

Author: jmd-mongo

source/core/index-intersection.txt

@@ -10,10 +10,29 @@ Index Intersection
    :depth: 1
    :class: singlecol
 
+.. important::
+
+   This page documents cases where the :doc:`query optimizer 
+   </core/query-plans>` **may** be able to use index intersection. 
+   
+   In practice, the :doc:`query optimizer </core/query-plans>` rarely 
+   selects plans that use index intersection.
+
+   Hash-based index intersection is disabled by default and sort-based 
+   index intersection is disfavored in plan selection. The optimizer 
+   behaves in this fashion in order to prevent bad plan selection. 
+   
+   Schema designs should not rely on index intersection. Instead,
+   :doc:`compound indexes </core/index-compound>` should be used.
+   
+   Future improvements to the query optimizer may allow the system to 
+   better identify cases where an index intersection plan would be 
+   beneficial.
+
 MongoDB can use the intersection of multiple indexes to fulfill
-queries. In general, each index intersection
-involves two indexes; however, MongoDB can employ multiple/nested index
-intersections to resolve a query.
+queries. In general, each index intersection involves two indexes; 
+however, MongoDB can employ multiple/nested index intersections to 
+resolve a query.
 
 To illustrate index intersection, consider a collection ``orders`` that
 has the following indexes: