SERVER-42291 generate correct bounds for $nin null queries

1 files changed, 9 insertions, 2 deletions

diff --git a/src/mongo/db/query/planner_ixselect.cpp b/src/mongo/db/query/planner_ixselect.cpp
index f15b87b470c..84b7616a24f 100644
--- a/src/mongo/db/query/planner_ixselect.cpp
+++ b/src/mongo/db/query/planner_ixselect.cpp
@@ -440,8 +440,9 @@ bool QueryPlannerIXSelect::_compatible(const BSONElement& keyPatternElt,
 
             // Most of the time we can't use a multikey index for a $ne: null query, however there
             // are a few exceptions around $elemMatch.
-            if (isNotEqualsNull &&
-                !notEqualsNullCanUseIndex(index, keyPatternElt, keyPatternIdx, elemMatchContext)) {
+            const bool canUseIndexForNeNull =
+                notEqualsNullCanUseIndex(index, keyPatternElt, keyPatternIdx, elemMatchContext);
+            if (isNotEqualsNull && !canUseIndexForNeNull) {
                 return false;
             }
 
@@ -451,6 +452,12 @@ bool QueryPlannerIXSelect::_compatible(const BSONElement& keyPatternElt,
                 if (!ime->getRegexes().empty()) {
                     return false;
                 }
+
+                // If we can't use the index for $ne to null, then we cannot use it for the
+                // case {$nin: [null, <...>]}.
+                if (!canUseIndexForNeNull && ime->hasNull()) {
+                    return false;
+                }
             }
         }