SERVER-64403 Encode the sort key from a document with collation only if value is present when comparing a sort key from index with one from the document in SORT_MERGE stage.

2 files changed, 21 insertions, 17 deletions

diff --git a/jstests/core/merge_sort_collation.js b/jstests/core/merge_sort_collation.js
index a49dffc765a..ae571e43477 100644
--- a/jstests/core/merge_sort_collation.js
+++ b/jstests/core/merge_sort_collation.js
@@ -263,33 +263,33 @@ const testCases = [
         // collation of the indexes. Also, the second/third operands to $or queries on fields
         // 'e'/'g' that are not covered by the indexes therefore triggers addition of a FETCH stage
         // between MERGE_SORT and IXSCAN. This tests comparison of index versus fetched document
-        // provided sort keys. In addition to that, some documents have objects as sort attribute
-        // values.
+        // provided sort keys. In addition to that, some documents have missing values and objects
+        // as sort attribute values.
         indexes: [{a: 1, b: 1, c: 1}, {d: 1, c: 1}, {f: 1, c: 1}],
         indexOptions: {collation: {locale: "en", strength: 2}},
         filter: {$or: [{a: {$in: ["1", "2"]}, b: "1"}, {d: "3", e: "3"}, {f: "4", g: "3"}]},
         sort: {c: 1},
         findCollation: {locale: "en", strength: 2},
         inputDocuments: [
-            {_id: 0, a: "1", b: "1", c: "a"},
-            {_id: 1, a: "1", b: "1", c: "d"},
-            {_id: 2, a: "2", b: "1", c: "b"},
-            {_id: 3, a: "2", b: "1", c: "e"},
-            {_id: 6, a: "2", b: "1", c: {a: "B"}},
+            {_id: 0, d: "3", e: "3", c: "a"},
+            {_id: 1, d: "3", e: "3", c: "b"},
+            {_id: 2, d: "3", e: "3"},
+            {_id: 3, d: "3", e: "3", c: {}},
             {_id: 4, d: "3", e: "3", c: "c"},
-            {_id: 5, d: "3", e: "3", c: "f"},
+            {_id: 5, d: "3", e: "3", c: 1},
+            {_id: 6, a: "2", b: "1", c: {a: "B"}},
             {_id: 7, d: "3", e: "3", c: {a: "A"}},
             {_id: 8, d: "3", e: "3", c: {a: "C"}},
-            {_id: 9, f: "4", g: "3", c: "g"},
+            {_id: 9, f: "4", g: "3", c: "d"},
         ],
         expectedResults: [
-            {_id: 0, a: "1", b: "1", c: "a"},
-            {_id: 2, a: "2", b: "1", c: "b"},
+            {_id: 2, d: "3", e: "3"},
+            {_id: 5, d: "3", e: "3", c: 1},
+            {_id: 0, d: "3", e: "3", c: "a"},
+            {_id: 1, d: "3", e: "3", c: "b"},
             {_id: 4, d: "3", e: "3", c: "c"},
-            {_id: 1, a: "1", b: "1", c: "d"},
-            {_id: 3, a: "2", b: "1", c: "e"},
-            {_id: 5, d: "3", e: "3", c: "f"},
-            {_id: 9, f: "4", g: "3", c: "g"},
+            {_id: 9, f: "4", g: "3", c: "d"},
+            {_id: 3, d: "3", e: "3", c: {}},
             {_id: 7, d: "3", e: "3", c: {a: "A"}},
             {_id: 6, a: "2", b: "1", c: {a: "B"}},
             {_id: 8, d: "3", e: "3", c: {a: "C"}},
diff --git a/src/mongo/db/exec/merge_sort.cpp b/src/mongo/db/exec/merge_sort.cpp
index 443a6ed59e1..8520a959475 100644
--- a/src/mongo/db/exec/merge_sort.cpp
+++ b/src/mongo/db/exec/merge_sort.cpp
@@ -207,8 +207,12 @@ bool MergeSortStage::StageWithValueComparison::operator()(const MergingRef& lhs,
             // One of the sort key parts was extracted from fetched documents. Encode that part
             // using the query's collation.
             auto& keyPartFetchedFromDocument = rhsIsFromIndexKey ? lhsElt : rhsElt;
-            collationEncodedKeyPart = encodeKeyPartWithCollation(keyPartFetchedFromDocument);
-            keyPartFetchedFromDocument = collationEncodedKeyPart.firstElement();
+
+            // Encode the sort key part only if it contains some value.
+            if (keyPartFetchedFromDocument.ok()) {
+                collationEncodedKeyPart = encodeKeyPartWithCollation(keyPartFetchedFromDocument);
+                keyPartFetchedFromDocument = collationEncodedKeyPart.firstElement();
+            }
         }
 
         // false means don't compare field name.