SERVER-69707 Fix undefined slot error for $group with conditional expression

(cherry picked from commit 8e990303de72e04600770082e41aec92f6221c8b)

2 files changed, 263 insertions, 17 deletions

diff --git a/jstests/noPassthroughWithMongod/group_pushdown.js b/jstests/noPassthroughWithMongod/group_pushdown.js
index d53176b4a55..a4d6bd4b7f8 100644
--- a/jstests/noPassthroughWithMongod/group_pushdown.js
+++ b/jstests/noPassthroughWithMongod/group_pushdown.js
@@ -229,6 +229,194 @@ assertResultsMatchWithAndWithoutPushdown(
               assertResultsMatchWithAndWithoutPushdown(
                   coll, pipeline, [{_id: "a", ss: 30}, {_id: "b", ss: 60}, {_id: "c", ss: 10}], 2));
 
+// The second $group stage refers to a top-field below a $switch
+assertResultsMatchWithAndWithoutPushdown(coll,
+                                         [
+                                             {$group: {_id: {$divide: ["$price", 5]}}},
+                                             {
+                                                 $group: {
+                                                     _id: null,
+                                                     lowp: {
+                                                         $sum: {
+                                                             $switch: {
+                                                                 branches: [{
+                                                                     case: {$lte: ["$_id", 3]},
+                                                                     then: 1
+                                                                 }],
+                                                                 default: 0
+                                                             }
+                                                         }
+                                                     },
+                                                     highp: {
+                                                         $sum: {
+                                                             $switch: {
+                                                                 branches: [{
+                                                                     case: {$gt: ["$_id", 3]},
+                                                                     then: 1
+                                                                 }],
+                                                                 default: 0
+                                                             }
+                                                         }
+                                                     }
+                                                 }
+                                             }
+                                         ],
+                                         [{"_id": null, "lowp": 2, "highp": 1}],
+                                         2);
+
+// The second $group stage refers to a top-field below a $cond
+assertResultsMatchWithAndWithoutPushdown(
+    coll,
+    [
+        {$group: {_id: {$divide: ["$price", 5]}}},
+        {
+            $group: {
+                _id: null,
+                lowp: {$sum: {$cond: [{$lte: ["$_id", 3]}, 1, 0]}},
+                highp: {$sum: {$cond: [{$gt: ["$_id", 3]}, 1, 0]}}
+            }
+        }
+    ],
+    [{"_id": null, "lowp": 2, "highp": 1}],
+    2);
+
+// The second $group stage refers to a top-field below a nested $cond / $ifNull
+assertResultsMatchWithAndWithoutPushdown(coll,
+                                         [
+                                             {$group: {_id: {$divide: ["$price", 5]}}},
+                                             {
+                                                 $group: {
+                                                     _id: null,
+                                                     lowp: {
+                                                         $sum: {
+                                                             $cond: [
+                                                                 {
+                                                                     $lte:
+                                                                         [{$ifNull: ["$_id", 0]}, 3]
+                                                                 },
+                                                                 1,
+                                                                 0
+                                                             ]
+                                                         }
+                                                     },
+                                                     highp: {
+                                                         $sum: {
+                                                             $cond: [
+                                                                 {$gt: [{$ifNull: ["$_id", 0]}, 3]},
+                                                                 1,
+                                                                 0
+                                                             ]
+                                                         }
+                                                     }
+                                                 }
+                                             }
+                                         ],
+                                         [{"_id": null, "lowp": 2, "highp": 1}],
+                                         2);
+
+// The second $group stage refers to top-fields below a $filter
+assertResultsMatchWithAndWithoutPushdown(
+    coll,
+    [
+        {$group: {_id: "$item", prices: {$push: "$price"}}},
+        {
+            $group: {
+                _id: "$_id",
+                o: {$push: {$filter: {input: "$prices", as: "p", cond: {$gte: ["$$p", 5]}}}}
+            }
+        }
+    ],
+    [{"_id": "a", "o": [[10, 5]]}, {"_id": "b", "o": [[20, 10]]}, {"_id": "c", "o": [[5]]}],
+    2);
+
+// The second $group stage refers to top-fields below a $let
+assertResultsMatchWithAndWithoutPushdown(
+    coll,
+    [
+        {$group: {
+            _id: "$item", maxp: {$max: "$price"}, minp: {$min: "$price"}, count: {$count: {}}
+        }},
+        {$group: {_id: "$_id", o: {$sum: {
+            $let: {
+                vars: {
+                    minPlusMax: {$add: ["$maxp", "$minp"]},
+                    count: "$count"
+                },
+                in: {$multiply: ["$$minPlusMax", "$$count"]}
+            }
+        }}}}
+    ],
+    [{ "_id" : "a", "o" : 30 }, { "_id" : "c", "o" : 10 }, { "_id" : "b", "o" : 60 }],
+    2);
+
+// The second $group stage refers to top-fields below a $and
+assertResultsMatchWithAndWithoutPushdown(coll,
+                                         [
+                                             {
+                                                 $group: {
+                                                     _id: "$item",
+                                                     maxp: {$max: "$price"},
+                                                     minp: {$min: "$price"}
+                                                 }
+                                             },
+                                             {
+                                                 $group:
+                                                     {
+                                                         _id: "$_id",
+                                                         o:
+                                                             {
+                                                                 $sum:
+                                                                     {
+                                                                         $and: [
+                                                                             {$gt: ["$maxp", 15]},
+                                                                             {$lt: ["$minp", 10]}
+                                                                         ]
+                                                                     }
+                                                             }
+                                                     }
+                                             }
+                                         ],
+                                         [
+                                             {"_id": "a", "o": 0},
+                                             {"_id": "c", "o": 0},
+                                             {"_id": "b", "o": 0}
+                                         ],
+                                         2);
+
+// The second $group stage refers to top-fields below a $or
+assertResultsMatchWithAndWithoutPushdown(coll,
+                                         [
+                                             {
+                                                 $group: {
+                                                     _id: "$item",
+                                                     maxp: {$max: "$price"},
+                                                     minp: {$min: "$price"}
+                                                 }
+                                             },
+                                             {
+                                                 $group:
+                                                     {
+                                                         _id: "$_id",
+                                                         o:
+                                                             {
+                                                                 $sum:
+                                                                     {
+                                                                         $or: [
+                                                                             {$gt: ["$maxp", 15]},
+                                                                             {$lt: ["$minp", 10]}
+                                                                         ]
+                                                                     }
+                                                             }
+                                                     }
+                                             }
+                                         ],
+                                         [
+                                             {"_id": "a", "o": 0},
+                                             {"_id": "c", "o": 0},
+                                             {"_id": "b", "o": 0}
+                                         ],
+                                         2);
+
 // The second $group stage refers to both a top-level field and a sub-field twice which does not
 // exist.
 assertResultsMatchWithAndWithoutPushdown(
diff --git a/src/mongo/db/query/sbe_stage_builder.cpp b/src/mongo/db/query/sbe_stage_builder.cpp
index d16cf3ce880..78fee0dc2a0 100644
--- a/src/mongo/db/query/sbe_stage_builder.cpp
+++ b/src/mongo/db/query/sbe_stage_builder.cpp
@@ -2100,44 +2100,104 @@ std::pair<std::unique_ptr<sbe::PlanStage>, PlanStageSlots> SlotBasedStageBuilder
 
 namespace {
 template <typename F>
-struct FieldPathVisitor : public SelectiveConstExpressionVisitorBase {
+struct FieldPathAndCondPreVisitor : public SelectiveConstExpressionVisitorBase {
     // To avoid overloaded-virtual warnings.
     using SelectiveConstExpressionVisitorBase::visit;
 
-    FieldPathVisitor(const F& fn) : _fn(fn) {}
+    FieldPathAndCondPreVisitor(const F& fn, int32_t& nestedCondLevel)
+        : _fn(fn), _nestedCondLevel(nestedCondLevel) {}
 
     void visit(const ExpressionFieldPath* expr) final {
-        _fn(expr);
+        _fn(expr, _nestedCondLevel);
+    }
+
+    void visit(const ExpressionCond* expr) final {
+        ++_nestedCondLevel;
+    }
+
+    void visit(const ExpressionSwitch* expr) final {
+        ++_nestedCondLevel;
+    }
+
+    void visit(const ExpressionIfNull* expr) final {
+        ++_nestedCondLevel;
+    }
+
+    void visit(const ExpressionAnd* expr) final {
+        ++_nestedCondLevel;
+    }
+
+    void visit(const ExpressionOr* expr) final {
+        ++_nestedCondLevel;
     }
 
     F _fn;
+    // Tracks the number of conditional expressions like $cond or $ifNull that are above us in the
+    // tree.
+    int32_t& _nestedCondLevel;
+};
+
+struct CondPostVisitor : public SelectiveConstExpressionVisitorBase {
+    // To avoid overloaded-virtual warnings.
+    using SelectiveConstExpressionVisitorBase::visit;
+
+    CondPostVisitor(int32_t& nestedCondLevel) : _nestedCondLevel(nestedCondLevel) {}
+
+    void visit(const ExpressionCond* expr) final {
+        --_nestedCondLevel;
+    }
+
+    void visit(const ExpressionSwitch* expr) final {
+        --_nestedCondLevel;
+    }
+
+    void visit(const ExpressionIfNull* expr) final {
+        --_nestedCondLevel;
+    }
+
+    void visit(const ExpressionAnd* expr) final {
+        --_nestedCondLevel;
+    }
+
+    void visit(const ExpressionOr* expr) final {
+        --_nestedCondLevel;
+    }
+
+    int32_t& _nestedCondLevel;
 };
 
 /**
  * Walks through the 'expr' expression tree and whenever finds an 'ExpressionFieldPath', calls
  * the 'fn' function. Type requirement for 'fn' is it must have a const 'ExpressionFieldPath'
- * pointer parameter.
+ * pointer parameter and 'nestedCondLevel' parameter.
  */
 template <typename F>
 void walkAndActOnFieldPaths(Expression* expr, const F& fn) {
-    FieldPathVisitor<F> visitor(fn);
-    ExpressionWalker walker(&visitor, nullptr /*inVisitor*/, nullptr /*postVisitor*/);
+    int32_t nestedCondLevel = 0;
+    FieldPathAndCondPreVisitor<F> preVisitor(fn, nestedCondLevel);
+    CondPostVisitor postVisitor(nestedCondLevel);
+    ExpressionWalker walker(&preVisitor, nullptr /*inVisitor*/, &postVisitor);
     expression_walker::walk(expr, &walker);
 }
 
 /**
  * Checks whether all field paths in 'idExpr' and all accumulator expressions are top-level ones.
  */
-bool checkAllFieldPathsAreTopLevel(const boost::intrusive_ptr<Expression>& idExpr,
-                                   const std::vector<AccumulationStatement>& accStmts) {
-    auto areAllTopLevelFields = true;
+bool areAllFieldPathsOptimizable(const boost::intrusive_ptr<Expression>& idExpr,
+                                 const std::vector<AccumulationStatement>& accStmts) {
+    auto areFieldPathsOptimizable = true;
 
-    auto checkFieldPath = [&](const ExpressionFieldPath* fieldExpr) {
+    auto checkFieldPath = [&](const ExpressionFieldPath* fieldExpr, int32_t nestedCondLevel) {
         // We optimize neither a field path for the top-level document itself (getPathLength() == 1)
         // nor a field path that refers to a variable. We can optimize only top-level fields
         // (getPathLength() == 2).
-        if (fieldExpr->getFieldPath().getPathLength() != 2 || fieldExpr->isVariableReference()) {
-            areAllTopLevelFields = false;
+        //
+        // The 'nestedCondLevel' being > 0 means that a field path is refered to below conditional
+        // expressions at the parent $group node, when we cannot optimize field path access and
+        // therefore, cannot avoid materialization.
+        if (nestedCondLevel > 0 || fieldExpr->getFieldPath().getPathLength() != 2 ||
+            fieldExpr->isVariableReference()) {
+            areFieldPathsOptimizable = false;
             return;
         }
     };
@@ -2150,7 +2210,7 @@ bool checkAllFieldPathsAreTopLevel(const boost::intrusive_ptr<Expression>& idExp
         walkAndActOnFieldPaths(accStmt.expr.argument.get(), checkFieldPath);
     }
 
-    return areAllTopLevelFields;
+    return areFieldPathsOptimizable;
 }
 
 /**
@@ -2176,7 +2236,7 @@ EvalStage optimizeFieldPaths(StageBuilderState& state,
     auto searchInChildOutputs = !optionalRootSlot.has_value();
     auto retEvalStage = std::move(childEvalStage);
 
-    walkAndActOnFieldPaths(expr.get(), [&](const ExpressionFieldPath* fieldExpr) {
+    walkAndActOnFieldPaths(expr.get(), [&](const ExpressionFieldPath* fieldExpr, int32_t) {
         // We optimize neither a field path for the top-level document itself nor a field path that
         // refers to a variable instead of calling getField().
         if (fieldExpr->getFieldPath().getPathLength() == 1 || fieldExpr->isVariableReference()) {
@@ -2470,10 +2530,8 @@ std::pair<std::unique_ptr<sbe::PlanStage>, PlanStageSlots> SlotBasedStageBuilder
     const auto& accStmts = groupNode->accumulators;
     auto childStageType = childNode->getType();
 
-    auto areAllTopLevelFields = checkAllFieldPathsAreTopLevel(idExpr, accStmts);
-
     auto childReqs = reqs.copy();
-    if (childStageType == StageType::STAGE_GROUP && areAllTopLevelFields) {
+    if (childStageType == StageType::STAGE_GROUP && areAllFieldPathsOptimizable(idExpr, accStmts)) {
         // Does not ask the GROUP child for the result slot to avoid unnecessary materialization if
         // all fields are top-level fields. See the end of this function. For example, GROUP - GROUP
         // - COLLSCAN case.