SERVER-108945 Handle $elemMatch with empty path under a contained $or correctly (#39824) (#40915)

GitOrigin-RevId: 2c7e210d9047bcd86a6f46e0768584b8ca70faa9

etc/backports_required_for_multiversion_tests.yml

@@ -21,6 +21,8 @@
 #
 last-continuous:
   all:
+    - test_file: jstests/core/index/elemmatch_index.js
+      ticket: SERVER-108945
     - test_file: jstests/replsets/malformed_heartbeat_request.js
       ticket: SERVER-104563
     - test_file: jstests/core/query/query_settings/query_settings_strict_api.js
@@ -473,6 +475,8 @@ last-continuous:
   suites: null
 last-lts:
   all:
+    - test_file: jstests/core/index/elemmatch_index.js
+      ticket: SERVER-108945
     - test_file: jstests/replsets/malformed_heartbeat_request.js
       ticket: SERVER-104563
     - test_file: jstests/core/query/query_settings/query_settings_strict_api.js

jstests/core/index/elemmatch_index.js

@@ -80,8 +80,19 @@ assertIndexResults(coll, {a: {$all: [{$elemMatch: {"b.c": "x"}}]}}, true, 1);
 // Tests that $elemMatch with path components that are empty strings are correctly handled in the
 // plan enumerator in case if an index is used on another predicate of the query.
 assertIndexResults(coll, {$and: [{"a.b.c": "x"}, {"": {$elemMatch: {"a.b.c": "x"}}}]}, true, 0);
-
 assertIndexResults(coll, {$and: [{"a.b.c": "x"}, {"a.b.c": {$elemMatch: {"": "x"}}}]}, true, 0);
+
+// Similar to the above but with the $elemMatch placed under a contained $or.
+assertIndexResults(
+    coll,
+    {$and: [{"a.b.c": "x"}, {$or: [{"a.b.c": "y"}, {"": {$elemMatch: {"a.b.c": "x"}}}]}]},
+    true,
+    0);
+assertIndexResults(
+    coll,
+    {$and: [{"a.b.c": "x"}, {$or: [{"a.b.c": "y"}, {"a.b.c": {$elemMatch: {"": "x"}}}]}]},
+    true,
+    0);
 })();
 
 (function() {

src/mongo/db/query/plan_enumerator/plan_enumerator.cpp

@@ -440,7 +440,7 @@ bool PlanEnumerator::prepMemo(MatchExpression* node, const PrepMemoContext& cont
             assign->assignment = std::move(orAssignment);
         }
         return true;
-    } else if (Indexability::arrayUsesIndexOnChildren(node)) {
+    } else if (Indexability::isBoundsGeneratingElemMatchObject(node)) {
         // Add each of our children as a subnode.  We enumerate through each subnode one at a
         // time until it's exhausted then we move on.
         ArrayAssignment aa;

src/mongo/db/query/planner_ixselect.cpp

@@ -810,7 +810,7 @@ void QueryPlannerIXSelect::rateIndices(MatchExpression* node,
             childRt->path = rt->path;
             node->getChild(0)->setTag(childRt);
         }
-    } else if (Indexability::arrayUsesIndexOnChildren(node) && !node->path().empty()) {
+    } else if (Indexability::isBoundsGeneratingElemMatchObject(node)) {
         // Note we skip empty path components since they are not allowed in index key patterns.
         const auto newPath = prefix + std::string{node->path()};
         ElemMatchContext newEMContext;