1 files changed, 8 insertions, 0 deletions
diff --git a/src/mongo/db/query/get_executor.cpp b/src/mongo/db/query/get_executor.cpp
index 9be58ae42b2..f610b940db4 100644
--- a/src/mongo/db/query/get_executor.cpp
+++ b/src/mongo/db/query/get_executor.cpp
@@ -1021,6 +1021,14 @@ bool turnIxscanIntoCount(QuerySolution* soln) {
         return false;
     }
 
+    // Since count scans return no data, they are always forward scans. Index scans, on the other
+    // hand, may need to scan the index in reverse order in order to obtain a sort. If the index
+    // scan direction is backwards, then we need to swap the start and end of the count scan bounds.
+    if (isn->direction < 0) {
+        startKey.swap(endKey);
+        std::swap(startKeyInclusive, endKeyInclusive);
+    }
+
     // Make the count node that we replace the fetch + ixscan with.
     CountScanNode* csn = new CountScanNode(isn->index);
     csn->startKey = startKey;