SERVER-70395 Change spilling for SBE HashAggStage to use a more efficient algorithm

The new algorithm spills the entire hash table to a
RecordStore whenever the memory budget is exceeded. Once all
the input is consumed, it switches to a streaming approach,
merging the partial aggregates recovered from disk.

22 files changed, 847 insertions, 397 deletions

diff --git a/src/mongo/db/exec/sbe/abt/abt_lower.cpp b/src/mongo/db/exec/sbe/abt/abt_lower.cpp
index 17b13b26db1..72206e75512 100644
--- a/src/mongo/db/exec/sbe/abt/abt_lower.cpp
+++ b/src/mongo/db/exec/sbe/abt/abt_lower.cpp
@@ -683,14 +683,15 @@ std::unique_ptr<sbe::PlanStage> SBENodeLowering::walk(const GroupByNode& n,
     tassert(6624227, "refs expected", refsAgg);
     auto& exprs = refsAgg->nodes();
 
-    sbe::value::SlotMap<std::unique_ptr<sbe::EExpression>> aggs;
+    sbe::SlotExprPairVector aggs;
+    aggs.reserve(exprs.size());
 
     for (size_t idx = 0; idx < exprs.size(); ++idx) {
         auto expr = lowerExpression(exprs[idx]);
         auto slot = _slotIdGenerator.generate();
 
         mapProjToSlot(names[idx], slot);
-        aggs.emplace(slot, std::move(expr));
+        aggs.push_back({slot, std::move(expr)});
     }
 
     boost::optional<sbe::value::SlotId> collatorSlot = _namedSlots.getSlotIfExists("collator"_sd);
@@ -705,6 +706,11 @@ std::unique_ptr<sbe::PlanStage> SBENodeLowering::walk(const GroupByNode& n,
                                          true /*optimizedClose*/,
                                          collatorSlot,
                                          false /*allowDiskUse*/,
+                                         // Since we are always disallowing disk use for this stage,
+                                         // we need not provide merging expressions. Once spilling
+                                         // is permitted here, we will need to generate merging
+                                         // expressions during lowering.
+                                         sbe::makeSlotExprPairVec() /*mergingExprs*/,
                                          planNodeId);
 }
 
diff --git a/src/mongo/db/exec/sbe/expressions/expression.cpp b/src/mongo/db/exec/sbe/expressions/expression.cpp
index 77f69e8cbca..a96a559cd2d 100644
--- a/src/mongo/db/exec/sbe/expressions/expression.cpp
+++ b/src/mongo/db/exec/sbe/expressions/expression.cpp
@@ -701,6 +701,7 @@ static stdx::unordered_map<std::string, BuiltinFn> kBuiltinFunctions = {
      BuiltinFn{[](size_t n) { return n == 3; }, vm::Builtin::collSetDifference, false}},
     {"collSetEquals",
      BuiltinFn{[](size_t n) { return n >= 3; }, vm::Builtin::collSetEquals, false}},
+    {"aggSetUnion", BuiltinFn{[](size_t n) { return n == 1; }, vm::Builtin::aggSetUnion, true}},
     {"aggSetUnionCapped",
      BuiltinFn{[](size_t n) { return n == 2; }, vm::Builtin::aggSetUnionCapped, true}},
     {"aggCollSetUnionCapped",
diff --git a/src/mongo/db/exec/sbe/expressions/expression.h b/src/mongo/db/exec/sbe/expressions/expression.h
index ee0f997445f..433aa995146 100644
--- a/src/mongo/db/exec/sbe/expressions/expression.h
+++ b/src/mongo/db/exec/sbe/expressions/expression.h
@@ -127,6 +127,8 @@ protected:
     }
 };
 
+using SlotExprPairVector = std::vector<std::pair<value::SlotId, std::unique_ptr<EExpression>>>;
+
 template <typename T, typename... Args>
 inline std::unique_ptr<EExpression> makeE(Args&&... args) {
     return std::make_unique<T>(std::forward<Args>(args)...);
@@ -143,20 +145,30 @@ inline auto makeEs(Ts&&... pack) {
 
 namespace detail {
 // base case
-inline void makeEM_unwind(value::SlotMap<std::unique_ptr<EExpression>>& result,
-                          value::SlotId slot,
-                          std::unique_ptr<EExpression> expr) {
-    result.emplace(slot, std::move(expr));
+template <typename R>
+inline void makeSlotExprPairHelper(R& result,
+                                   value::SlotId slot,
+                                   std::unique_ptr<EExpression> expr) {
+    if constexpr (std::is_same_v<R, value::SlotMap<std::unique_ptr<EExpression>>>) {
+        result.emplace(slot, std::move(expr));
+    } else {
+        result.push_back({slot, std::move(expr)});
+    }
 }
 
 // recursive case
-template <typename... Ts>
-inline void makeEM_unwind(value::SlotMap<std::unique_ptr<EExpression>>& result,
-                          value::SlotId slot,
-                          std::unique_ptr<EExpression> expr,
-                          Ts&&... rest) {
-    result.emplace(slot, std::move(expr));
-    makeEM_unwind(result, std::forward<Ts>(rest)...);
+template <typename R, typename... Ts>
+inline void makeSlotExprPairHelper(R& result,
+                                   value::SlotId slot,
+                                   std::unique_ptr<EExpression> expr,
+                                   Ts&&... rest) {
+    if constexpr (std::is_same_v<R, value::SlotMap<std::unique_ptr<EExpression>>>) {
+        result.emplace(slot, std::move(expr));
+    } else {
+        static_assert(std::is_same_v<R, SlotExprPairVector>);
+        result.push_back({slot, std::move(expr)});
+    }
+    makeSlotExprPairHelper(result, std::forward<Ts>(rest)...);
 }
 }  // namespace detail
 
@@ -165,7 +177,7 @@ auto makeEM(Ts&&... pack) {
     value::SlotMap<std::unique_ptr<EExpression>> result;
     if constexpr (sizeof...(pack) > 0) {
         result.reserve(sizeof...(Ts) / 2);
-        detail::makeEM_unwind(result, std::forward<Ts>(pack)...);
+        detail::makeSlotExprPairHelper(result, std::forward<Ts>(pack)...);
     }
     return result;
 }
@@ -178,6 +190,16 @@ auto makeSV(Args&&... args) {
     return v;
 }
 
+template <typename... Ts>
+auto makeSlotExprPairVec(Ts&&... pack) {
+    SlotExprPairVector v;
+    if constexpr (sizeof...(pack) > 0) {
+        v.reserve(sizeof...(Ts) / 2);
+        detail::makeSlotExprPairHelper(v, std::forward<Ts>(pack)...);
+    }
+    return v;
+}
+
 /**
  * This is a constant expression. It assumes the ownership of the input constant.
  */
diff --git a/src/mongo/db/exec/sbe/expressions/sbe_set_expressions_test.cpp b/src/mongo/db/exec/sbe/expressions/sbe_set_expressions_test.cpp
index 74cd7dce343..60380c5438e 100644
--- a/src/mongo/db/exec/sbe/expressions/sbe_set_expressions_test.cpp
+++ b/src/mongo/db/exec/sbe/expressions/sbe_set_expressions_test.cpp
@@ -102,6 +102,35 @@ TEST_F(SBEBuiltinSetOpTest, ReturnsNothingSetUnion) {
     runAndAssertNothing(compiledExpr.get());
 }
 
+TEST_F(SBEBuiltinSetOpTest, AggSetUnion) {
+    value::OwnedValueAccessor aggAccessor, inputAccessor;
+    auto inputSlot = bindAccessor(&inputAccessor);
+    auto setUnionExpr =
+        stage_builder::makeFunction("aggSetUnion", stage_builder::makeVariable(inputSlot));
+    auto compiledExpr = compileAggExpression(*setUnionExpr, &aggAccessor);
+
+    auto [arrTag1, arrVal1] = makeArray(BSON_ARRAY(1 << 2));
+    inputAccessor.reset(arrTag1, arrVal1);
+    auto [resTag1, resVal1] = makeArraySet(BSON_ARRAY(1 << 2));
+    runAndAssertExpression(compiledExpr.get(), {resTag1, resVal1});
+    aggAccessor.reset(resTag1, resVal1);
+
+    auto [arrTag2, arrVal2] = makeArraySet(BSON_ARRAY(1 << 3 << 2 << 6));
+    inputAccessor.reset(arrTag2, arrVal2);
+    auto [resTag2, resVal2] = makeArraySet(BSON_ARRAY(1 << 2 << 3 << 6));
+    runAndAssertExpression(compiledExpr.get(), {resTag2, resVal2});
+    aggAccessor.reset(resTag2, resVal2);
+
+    auto [arrTag3, arrVal3] = makeArray(BSONArray{});
+    inputAccessor.reset(arrTag3, arrVal3);
+    auto [resTag3, resVal3] = makeArraySet(BSON_ARRAY(1 << 2 << 3 << 6));
+    runAndAssertExpression(compiledExpr.get(), {resTag3, resVal3});
+    aggAccessor.reset(resTag3, resVal3);
+
+    inputAccessor.reset(value::TypeTags::Nothing, 0);
+    runAndAssertNothing(compiledExpr.get());
+}
+
 TEST_F(SBEBuiltinSetOpTest, ComputesSetIntersection) {
     value::OwnedValueAccessor slotAccessor1, slotAccessor2;
     auto arrSlot1 = bindAccessor(&slotAccessor1);
diff --git a/src/mongo/db/exec/sbe/sbe_hash_agg_test.cpp b/src/mongo/db/exec/sbe/sbe_hash_agg_test.cpp
index 603d71f08ea..b48ed496798 100644
--- a/src/mongo/db/exec/sbe/sbe_hash_agg_test.cpp
+++ b/src/mongo/db/exec/sbe/sbe_hash_agg_test.cpp
@@ -82,18 +82,23 @@ void HashAggStageTest::performHashAggWithSpillChecking(
     auto makeStageFn = [this, collatorSlot, shouldUseCollator, shouldSpill](
                            value::SlotId scanSlot, std::unique_ptr<PlanStage> scanStage) {
         auto countsSlot = generateSlotId();
+        auto spillSlot = generateSlotId();
 
         auto hashAggStage = makeS<HashAggStage>(
             std::move(scanStage),
             makeSV(scanSlot),
-            makeEM(countsSlot,
-                   stage_builder::makeFunction("sum",
-                                               makeE<EConstant>(value::TypeTags::NumberInt64,
-                                                                value::bitcastFrom<int64_t>(1)))),
+            makeSlotExprPairVec(
+                countsSlot,
+                stage_builder::makeFunction("sum",
+                                            makeE<EConstant>(value::TypeTags::NumberInt64,
+                                                             value::bitcastFrom<int64_t>(1)))),
             makeSV(),
             true,
             boost::optional<value::SlotId>{shouldUseCollator, collatorSlot},
             shouldSpill,
+            makeSlotExprPairVec(
+                spillSlot,
+                stage_builder::makeFunction("sum", stage_builder::makeVariable(spillSlot))),
             kEmptyPlanNodeId);
 
         return std::make_pair(countsSlot, std::move(hashAggStage));
@@ -179,20 +184,21 @@ TEST_F(HashAggStageTest, HashAggMinMaxTest) {
         auto hashAggStage = makeS<HashAggStage>(
             std::move(scanStage),
             makeSV(),
-            makeEM(minSlot,
-                   stage_builder::makeFunction("min", makeE<EVariable>(scanSlot)),
-                   maxSlot,
-                   stage_builder::makeFunction("max", makeE<EVariable>(scanSlot)),
-                   collMinSlot,
-                   stage_builder::makeFunction(
-                       "collMin", collExpr->clone(), makeE<EVariable>(scanSlot)),
-                   collMaxSlot,
-                   stage_builder::makeFunction(
-                       "collMax", collExpr->clone(), makeE<EVariable>(scanSlot))),
+            makeSlotExprPairVec(minSlot,
+                                stage_builder::makeFunction("min", makeE<EVariable>(scanSlot)),
+                                maxSlot,
+                                stage_builder::makeFunction("max", makeE<EVariable>(scanSlot)),
+                                collMinSlot,
+                                stage_builder::makeFunction(
+                                    "collMin", collExpr->clone(), makeE<EVariable>(scanSlot)),
+                                collMaxSlot,
+                                stage_builder::makeFunction(
+                                    "collMax", collExpr->clone(), makeE<EVariable>(scanSlot))),
             makeSV(),
             true,
             boost::none,
             false /* allowDiskUse */,
+            makeSlotExprPairVec() /* mergingExprs */,
             kEmptyPlanNodeId);
 
         auto outSlot = generateSlotId();
@@ -243,13 +249,15 @@ TEST_F(HashAggStageTest, HashAggAddToSetTest) {
         auto hashAggStage = makeS<HashAggStage>(
             std::move(scanStage),
             makeSV(),
-            makeEM(hashAggSlot,
-                   stage_builder::makeFunction(
-                       "collAddToSet", std::move(collExpr), makeE<EVariable>(scanSlot))),
+            makeSlotExprPairVec(hashAggSlot,
+                                stage_builder::makeFunction("collAddToSet",
+                                                            std::move(collExpr),
+                                                            makeE<EVariable>(scanSlot))),
             makeSV(),
             true,
             boost::none,
             false /* allowDiskUse */,
+            makeSlotExprPairVec() /* mergingExprs */,
             kEmptyPlanNodeId);
 
         return std::make_pair(hashAggSlot, std::move(hashAggStage));
@@ -340,14 +348,16 @@ TEST_F(HashAggStageTest, HashAggSeekKeysTest) {
         auto hashAggStage = makeS<HashAggStage>(
             std::move(scanStage),
             makeSV(scanSlot),
-            makeEM(countsSlot,
-                   stage_builder::makeFunction("sum",
-                                               makeE<EConstant>(value::TypeTags::NumberInt64,
-                                                                value::bitcastFrom<int64_t>(1)))),
+            makeSlotExprPairVec(
+                countsSlot,
+                stage_builder::makeFunction("sum",
+                                            makeE<EConstant>(value::TypeTags::NumberInt64,
+                                                             value::bitcastFrom<int64_t>(1)))),
             makeSV(seekSlot),
             true,
             boost::none,
             false /* allowDiskUse */,
+            makeSlotExprPairVec() /* mergingExprs */,
             kEmptyPlanNodeId);
 
         return std::make_pair(countsSlot, std::move(hashAggStage));
@@ -400,17 +410,21 @@ TEST_F(HashAggStageTest, HashAggBasicCountNoSpill) {
 
     // Build a HashAggStage, group by the scanSlot and compute a simple count.
     auto countsSlot = generateSlotId();
+    auto spillSlot = generateSlotId();
     auto stage = makeS<HashAggStage>(
         std::move(scanStage),
         makeSV(scanSlot),
-        makeEM(countsSlot,
-               stage_builder::makeFunction(
-                   "sum",
-                   makeE<EConstant>(value::TypeTags::NumberInt64, value::bitcastFrom<int64_t>(1)))),
+        makeSlotExprPairVec(
+            countsSlot,
+            stage_builder::makeFunction(
+                "sum",
+                makeE<EConstant>(value::TypeTags::NumberInt64, value::bitcastFrom<int64_t>(1)))),
         makeSV(),  // Seek slot
         true,
         boost::none,
         true /* allowDiskUse */,
+        makeSlotExprPairVec(
+            spillSlot, stage_builder::makeFunction("sum", stage_builder::makeVariable(spillSlot))),
         kEmptyPlanNodeId);
 
     // Prepare the tree and get the 'SlotAccessor' for the output slot.
@@ -433,6 +447,7 @@ TEST_F(HashAggStageTest, HashAggBasicCountNoSpill) {
     // Check that the spilling behavior matches the expected.
     auto stats = static_cast<const HashAggStats*>(stage->getSpecificStats());
     ASSERT_FALSE(stats->usedDisk);
+    ASSERT_EQ(0, stats->numSpills);
     ASSERT_EQ(0, stats->spilledRecords);
 
     stage->close();
@@ -441,7 +456,6 @@ TEST_F(HashAggStageTest, HashAggBasicCountNoSpill) {
 TEST_F(HashAggStageTest, HashAggBasicCountSpill) {
     // We estimate the size of result row like {int64, int64} at 50B. Set the memory threshold to
     // 64B so that exactly one row fits in memory.
-    const int expectedRowsToFitInMemory = 1;
     auto defaultInternalQuerySBEAggApproxMemoryUseInBytesBeforeSpill =
         internalQuerySBEAggApproxMemoryUseInBytesBeforeSpill.load();
     internalQuerySBEAggApproxMemoryUseInBytesBeforeSpill.store(64);
@@ -459,17 +473,21 @@ TEST_F(HashAggStageTest, HashAggBasicCountSpill) {
 
     // Build a HashAggStage, group by the scanSlot and compute a simple count.
     auto countsSlot = generateSlotId();
+    auto spillSlot = generateSlotId();
     auto stage = makeS<HashAggStage>(
         std::move(scanStage),
         makeSV(scanSlot),
-        makeEM(countsSlot,
-               stage_builder::makeFunction(
-                   "sum",
-                   makeE<EConstant>(value::TypeTags::NumberInt64, value::bitcastFrom<int64_t>(1)))),
+        makeSlotExprPairVec(
+            countsSlot,
+            stage_builder::makeFunction(
+                "sum",
+                makeE<EConstant>(value::TypeTags::NumberInt64, value::bitcastFrom<int64_t>(1)))),
         makeSV(),  // Seek slot
         true,
         boost::none,
         true /* allowDiskUse */,
+        makeSlotExprPairVec(
+            spillSlot, stage_builder::makeFunction("sum", stage_builder::makeVariable(spillSlot))),
         kEmptyPlanNodeId);
 
     // Prepare the tree and get the 'SlotAccessor' for the output slot.
@@ -492,7 +510,14 @@ TEST_F(HashAggStageTest, HashAggBasicCountSpill) {
     // Check that the spilling behavior matches the expected.
     auto stats = static_cast<const HashAggStats*>(stage->getSpecificStats());
     ASSERT_TRUE(stats->usedDisk);
-    ASSERT_EQ(results.size() - expectedRowsToFitInMemory, stats->spilledRecords);
+    // Memory usage is estimated only every two rows at the most frequent. Also, we only start
+    // spilling after estimating that the memory budget is exceeded. These two factors result in
+    // fewer expected spills than there are input records, even though only one record fits in
+    // memory at a time.
+    ASSERT_EQ(stats->numSpills, 3);
+    // The input has one run of two consecutive values, so we expect to spill as many records as
+    // there are input values minus one.
+    ASSERT_EQ(stats->spilledRecords, 8);
 
     stage->close();
 }
@@ -526,17 +551,21 @@ TEST_F(HashAggStageTest, HashAggBasicCountNoSpillIfNoMemCheck) {
 
     // Build a HashAggStage, group by the scanSlot and compute a simple count.
     auto countsSlot = generateSlotId();
+    auto spillSlot = generateSlotId();
     auto stage = makeS<HashAggStage>(
         std::move(scanStage),
         makeSV(scanSlot),
-        makeEM(countsSlot,
-               stage_builder::makeFunction(
-                   "sum",
-                   makeE<EConstant>(value::TypeTags::NumberInt64, value::bitcastFrom<int64_t>(1)))),
+        makeSlotExprPairVec(
+            countsSlot,
+            stage_builder::makeFunction(
+                "sum",
+                makeE<EConstant>(value::TypeTags::NumberInt64, value::bitcastFrom<int64_t>(1)))),
         makeSV(),  // Seek slot
         true,
         boost::none,
         true /* allowDiskUse */,
+        makeSlotExprPairVec(
+            spillSlot, stage_builder::makeFunction("sum", stage_builder::makeVariable(spillSlot))),
         kEmptyPlanNodeId);
 
     // Prepare the tree and get the 'SlotAccessor' for the output slot.
@@ -559,6 +588,7 @@ TEST_F(HashAggStageTest, HashAggBasicCountNoSpillIfNoMemCheck) {
     // Check that it did not spill.
     auto stats = static_cast<const HashAggStats*>(stage->getSpecificStats());
     ASSERT_FALSE(stats->usedDisk);
+    ASSERT_EQ(0, stats->numSpills);
     ASSERT_EQ(0, stats->spilledRecords);
 
     stage->close();
@@ -567,7 +597,6 @@ TEST_F(HashAggStageTest, HashAggBasicCountNoSpillIfNoMemCheck) {
 TEST_F(HashAggStageTest, HashAggBasicCountSpillDouble) {
     // We estimate the size of result row like {double, int64} at 50B. Set the memory threshold to
     // 64B so that exactly one row fits in memory.
-    const int expectedRowsToFitInMemory = 1;
     auto defaultInternalQuerySBEAggApproxMemoryUseInBytesBeforeSpill =
         internalQuerySBEAggApproxMemoryUseInBytesBeforeSpill.load();
     internalQuerySBEAggApproxMemoryUseInBytesBeforeSpill.store(64);
@@ -585,17 +614,21 @@ TEST_F(HashAggStageTest, HashAggBasicCountSpillDouble) {
 
     // Build a HashAggStage, group by the scanSlot and compute a simple count.
     auto countsSlot = generateSlotId();
+    auto spillSlot = generateSlotId();
     auto stage = makeS<HashAggStage>(
         std::move(scanStage),
         makeSV(scanSlot),
-        makeEM(countsSlot,
-               stage_builder::makeFunction(
-                   "sum",
-                   makeE<EConstant>(value::TypeTags::NumberInt64, value::bitcastFrom<int64_t>(1)))),
+        makeSlotExprPairVec(
+            countsSlot,
+            stage_builder::makeFunction(
+                "sum",
+                makeE<EConstant>(value::TypeTags::NumberInt64, value::bitcastFrom<int64_t>(1)))),
         makeSV(),  // Seek slot
         true,
         boost::none,
         true /* allowDiskUse */,
+        makeSlotExprPairVec(
+            spillSlot, stage_builder::makeFunction("sum", stage_builder::makeVariable(spillSlot))),
         kEmptyPlanNodeId);
 
     // Prepare the tree and get the 'SlotAccessor' for the output slot.
@@ -618,7 +651,14 @@ TEST_F(HashAggStageTest, HashAggBasicCountSpillDouble) {
     // Check that the spilling behavior matches the expected.
     auto stats = static_cast<const HashAggStats*>(stage->getSpecificStats());
     ASSERT_TRUE(stats->usedDisk);
-    ASSERT_EQ(results.size() - expectedRowsToFitInMemory, stats->spilledRecords);
+    // Memory usage is estimated only every two rows at the most frequent. Also, we only start
+    // spilling after estimating that the memory budget is exceeded. These two factors result in
+    // fewer expected spills than there are input records, even though only one record fits in
+    // memory at a time.
+    ASSERT_EQ(stats->numSpills, 3);
+    // The input has one run of two consecutive values, so we expect to spill as many records as
+    // there are input values minus one.
+    ASSERT_EQ(stats->spilledRecords, 8);
 
     stage->close();
 }
@@ -640,17 +680,21 @@ TEST_F(HashAggStageTest, HashAggBasicCountNoSpillWithNoGroupByDouble) {
 
     // Build a HashAggStage, with an empty group by slot and compute a simple count.
     auto countsSlot = generateSlotId();
+    auto spillSlot = generateSlotId();
     auto stage = makeS<HashAggStage>(
         std::move(scanStage),
         makeSV(),
-        makeEM(countsSlot,
-               stage_builder::makeFunction(
-                   "sum",
-                   makeE<EConstant>(value::TypeTags::NumberInt64, value::bitcastFrom<int64_t>(1)))),
+        makeSlotExprPairVec(
+            countsSlot,
+            stage_builder::makeFunction(
+                "sum",
+                makeE<EConstant>(value::TypeTags::NumberInt64, value::bitcastFrom<int64_t>(1)))),
         makeSV(),  // Seek slot
         true,
         boost::none,
         true /* allowDiskUse */,
+        makeSlotExprPairVec(
+            spillSlot, stage_builder::makeFunction("sum", stage_builder::makeVariable(spillSlot))),
         kEmptyPlanNodeId);
 
     // Prepare the tree and get the 'SlotAccessor' for the output slot.
@@ -671,6 +715,7 @@ TEST_F(HashAggStageTest, HashAggBasicCountNoSpillWithNoGroupByDouble) {
     // Check that the spilling behavior matches the expected.
     auto stats = static_cast<const HashAggStats*>(stage->getSpecificStats());
     ASSERT_FALSE(stats->usedDisk);
+    ASSERT_EQ(0, stats->numSpills);
     ASSERT_EQ(0, stats->spilledRecords);
 
     stage->close();
@@ -679,7 +724,6 @@ TEST_F(HashAggStageTest, HashAggBasicCountNoSpillWithNoGroupByDouble) {
 TEST_F(HashAggStageTest, HashAggMultipleAccSpill) {
     // We estimate the size of result row like {double, int64} at 59B. Set the memory threshold to
     // 128B so that two rows fit in memory.
-    const int expectedRowsToFitInMemory = 2;
     auto defaultInternalQuerySBEAggApproxMemoryUseInBytesBeforeSpill =
         internalQuerySBEAggApproxMemoryUseInBytesBeforeSpill.load();
     internalQuerySBEAggApproxMemoryUseInBytesBeforeSpill.store(128);
@@ -698,19 +742,27 @@ TEST_F(HashAggStageTest, HashAggMultipleAccSpill) {
     // Build a HashAggStage, group by the scanSlot and compute a simple count.
     auto countsSlot = generateSlotId();
     auto sumsSlot = generateSlotId();
+    auto spillSlot1 = generateSlotId();
+    auto spillSlot2 = generateSlotId();
     auto stage = makeS<HashAggStage>(
         std::move(scanStage),
         makeSV(scanSlot),
-        makeEM(countsSlot,
-               stage_builder::makeFunction(
-                   "sum",
-                   makeE<EConstant>(value::TypeTags::NumberInt64, value::bitcastFrom<int64_t>(1))),
-               sumsSlot,
-               stage_builder::makeFunction("sum", makeE<EVariable>(scanSlot))),
+        makeSlotExprPairVec(
+            countsSlot,
+            stage_builder::makeFunction(
+                "sum",
+                makeE<EConstant>(value::TypeTags::NumberInt64, value::bitcastFrom<int64_t>(1))),
+            sumsSlot,
+            stage_builder::makeFunction("sum", makeE<EVariable>(scanSlot))),
         makeSV(),  // Seek slot
         true,
         boost::none,
         true /* allowDiskUse */,
+        makeSlotExprPairVec(
+            spillSlot1,
+            stage_builder::makeFunction("sum", stage_builder::makeVariable(spillSlot1)),
+            spillSlot2,
+            stage_builder::makeFunction("sum", stage_builder::makeVariable(spillSlot2))),
         kEmptyPlanNodeId);
 
     // Prepare the tree and get the 'SlotAccessor' for the output slot.
@@ -740,7 +792,10 @@ TEST_F(HashAggStageTest, HashAggMultipleAccSpill) {
     // Check that the spilling behavior matches the expected.
     auto stats = static_cast<const HashAggStats*>(stage->getSpecificStats());
     ASSERT_TRUE(stats->usedDisk);
-    ASSERT_EQ(results.size() - expectedRowsToFitInMemory, stats->spilledRecords);
+    ASSERT_EQ(stats->numSpills, 3);
+    // The input has one run of two consecutive values, so we expect to spill as many records as
+    // there are input values minus one.
+    ASSERT_EQ(stats->spilledRecords, 8);
 
     stage->close();
 }
@@ -765,19 +820,27 @@ TEST_F(HashAggStageTest, HashAggMultipleAccSpillAllToDisk) {
     // Build a HashAggStage, group by the scanSlot and compute a simple count.
     auto countsSlot = generateSlotId();
     auto sumsSlot = generateSlotId();
+    auto spillSlot1 = generateSlotId();
+    auto spillSlot2 = generateSlotId();
     auto stage = makeS<HashAggStage>(
         std::move(scanStage),
         makeSV(scanSlot),
-        makeEM(countsSlot,
-               stage_builder::makeFunction(
-                   "sum",
-                   makeE<EConstant>(value::TypeTags::NumberInt64, value::bitcastFrom<int64_t>(1))),
-               sumsSlot,
-               stage_builder::makeFunction("sum", makeE<EVariable>(scanSlot))),
+        makeSlotExprPairVec(
+            countsSlot,
+            stage_builder::makeFunction(
+                "sum",
+                makeE<EConstant>(value::TypeTags::NumberInt64, value::bitcastFrom<int64_t>(1))),
+            sumsSlot,
+            stage_builder::makeFunction("sum", makeE<EVariable>(scanSlot))),
         makeSV(),  // Seek slot
         true,
         boost::none,
         true,  // allowDiskUse=true
+        makeSlotExprPairVec(
+            spillSlot1,
+            stage_builder::makeFunction("sum", stage_builder::makeVariable(spillSlot1)),
+            spillSlot2,
+            stage_builder::makeFunction("sum", stage_builder::makeVariable(spillSlot2))),
         kEmptyPlanNodeId);
 
     // Prepare the tree and get the 'SlotAccessor' for the output slot.
@@ -807,7 +870,9 @@ TEST_F(HashAggStageTest, HashAggMultipleAccSpillAllToDisk) {
     // Check that the spilling behavior matches the expected.
     auto stats = static_cast<const HashAggStats*>(stage->getSpecificStats());
     ASSERT_TRUE(stats->usedDisk);
-    ASSERT_EQ(results.size(), stats->spilledRecords);
+    // We expect each incoming value to result in a spill of a single record.
+    ASSERT_EQ(stats->numSpills, 9);
+    ASSERT_EQ(stats->spilledRecords, 9);
 
     stage->close();
 }
@@ -844,14 +909,18 @@ TEST_F(HashAggStageTest, HashAggSum10Groups) {
 
     // Build a HashAggStage, group by the scanSlot and compute a sum for each group.
     auto sumsSlot = generateSlotId();
+    auto spillSlot = generateSlotId();
     auto stage = makeS<HashAggStage>(
         std::move(scanStage),
         makeSV(scanSlot),
-        makeEM(sumsSlot, stage_builder::makeFunction("sum", makeE<EVariable>(scanSlot))),
+        makeSlotExprPairVec(sumsSlot,
+                            stage_builder::makeFunction("sum", makeE<EVariable>(scanSlot))),
         makeSV(),  // Seek slot
         true,
         boost::none,
         true,  // allowDiskUse=true
+        makeSlotExprPairVec(
+            spillSlot, stage_builder::makeFunction("sum", stage_builder::makeVariable(spillSlot))),
         kEmptyPlanNodeId);
 
     // Prepare the tree and get the 'SlotAccessor' for the output slot.
@@ -885,17 +954,21 @@ TEST_F(HashAggStageTest, HashAggBasicCountWithRecordIds) {
 
     // Build a HashAggStage, group by the scanSlot and compute a simple count.
     auto countsSlot = generateSlotId();
+    auto spillSlot = generateSlotId();
     auto stage = makeS<HashAggStage>(
         std::move(scanStage),
         makeSV(scanSlot),
-        makeEM(countsSlot,
-               stage_builder::makeFunction(
-                   "sum",
-                   makeE<EConstant>(value::TypeTags::NumberInt64, value::bitcastFrom<int64_t>(1)))),
+        makeSlotExprPairVec(
+            countsSlot,
+            stage_builder::makeFunction(
+                "sum",
+                makeE<EConstant>(value::TypeTags::NumberInt64, value::bitcastFrom<int64_t>(1)))),
         makeSV(),  // Seek slot
         true,
         boost::none,
         true,  // allowDiskUse=true
+        makeSlotExprPairVec(
+            spillSlot, stage_builder::makeFunction("sum", stage_builder::makeVariable(spillSlot))),
         kEmptyPlanNodeId);
 
     // Prepare the tree and get the 'SlotAccessor' for the output slot.
diff --git a/src/mongo/db/exec/sbe/sbe_plan_size_test.cpp b/src/mongo/db/exec/sbe/sbe_plan_size_test.cpp
index 6446ab57845..ecd22c9df81 100644
--- a/src/mongo/db/exec/sbe/sbe_plan_size_test.cpp
+++ b/src/mongo/db/exec/sbe/sbe_plan_size_test.cpp
@@ -129,11 +129,12 @@ TEST_F(PlanSizeTest, Filter) {
 TEST_F(PlanSizeTest, HashAgg) {
     auto stage = makeS<HashAggStage>(mockS(),
                                      mockSV(),
-                                     makeEM(generateSlotId(), mockE()),
+                                     makeSlotExprPairVec(generateSlotId(), mockE()),
                                      makeSV(),
                                      true,
                                      generateSlotId(),
                                      false,
+                                     makeSlotExprPairVec(),
                                      kEmptyPlanNodeId);
     assertPlanSize(*stage);
 }
diff --git a/src/mongo/db/exec/sbe/sbe_trial_run_tracker_test.cpp b/src/mongo/db/exec/sbe/sbe_trial_run_tracker_test.cpp
index 06097b3a3b2..40f9ca9a9e6 100644
--- a/src/mongo/db/exec/sbe/sbe_trial_run_tracker_test.cpp
+++ b/src/mongo/db/exec/sbe/sbe_trial_run_tracker_test.cpp
@@ -141,14 +141,16 @@ TEST_F(TrialRunTrackerTest, TrialEndsDuringOpenPhaseOfBlockingStage) {
     auto hashAggStage = makeS<HashAggStage>(
         std::move(scanStage),
         makeSV(scanSlot),
-        makeEM(countsSlot,
-               stage_builder::makeFunction(
-                   "sum",
-                   makeE<EConstant>(value::TypeTags::NumberInt64, value::bitcastFrom<int64_t>(1)))),
-        makeSV(),  // Seek slot
+        makeSlotExprPairVec(
+            countsSlot,
+            stage_builder::makeFunction(
+                "sum",
+                makeE<EConstant>(value::TypeTags::NumberInt64, value::bitcastFrom<int64_t>(1)))),
+        makeSV(), /* Seek slot */
         true,
         boost::none,
         false /* allowDiskUse */,
+        makeSlotExprPairVec(), /* mergingExprs */
         kEmptyPlanNodeId);
 
     auto tracker = std::make_unique<TrialRunTracker>(numResultsLimit, size_t{0});
@@ -210,14 +212,16 @@ TEST_F(TrialRunTrackerTest, OnlyDeepestNestedBlockingStageHasTrialRunTracker) {
     auto hashAggStage = makeS<HashAggStage>(
         std::move(unionStage),
         makeSV(unionSlot),
-        makeEM(countsSlot,
-               stage_builder::makeFunction(
-                   "sum",
-                   makeE<EConstant>(value::TypeTags::NumberInt64, value::bitcastFrom<int64_t>(1)))),
-        makeSV(),  // Seek slot
+        makeSlotExprPairVec(
+            countsSlot,
+            stage_builder::makeFunction(
+                "sum",
+                makeE<EConstant>(value::TypeTags::NumberInt64, value::bitcastFrom<int64_t>(1)))),
+        makeSV(), /* Seek slot */
         true,
         boost::none,
         false /* allowDiskUse */,
+        makeSlotExprPairVec(), /* mergingExprs */
         kEmptyPlanNodeId);
 
     hashAggStage->prepare(*ctx);
@@ -277,14 +281,16 @@ TEST_F(TrialRunTrackerTest, SiblingBlockingStagesBothGetTrialRunTracker) {
         auto hashAggStage = makeS<HashAggStage>(
             std::move(scanStage),
             makeSV(scanSlot),
-            makeEM(countsSlot,
-                   stage_builder::makeFunction("sum",
-                                               makeE<EConstant>(value::TypeTags::NumberInt64,
-                                                                value::bitcastFrom<int64_t>(1)))),
-            makeSV(),  // Seek slot
+            makeSlotExprPairVec(
+                countsSlot,
+                stage_builder::makeFunction("sum",
+                                            makeE<EConstant>(value::TypeTags::NumberInt64,
+                                                             value::bitcastFrom<int64_t>(1)))),
+            makeSV(), /* Seek slot */
             true,
             boost::none,
             false /* allowDiskUse */,
+            makeSlotExprPairVec(), /* mergingExprs */
             kEmptyPlanNodeId);
 
         return std::make_pair(countsSlot, std::move(hashAggStage));
@@ -407,14 +413,16 @@ TEST_F(TrialRunTrackerTest, DisablingTrackingForAChildStagePreventsEarlyExit) {
         auto hashAggStage = makeS<HashAggStage>(
             std::move(scanStage),
             makeSV(scanSlot),
-            makeEM(countsSlot,
-                   stage_builder::makeFunction("sum",
-                                               makeE<EConstant>(value::TypeTags::NumberInt64,
-                                                                value::bitcastFrom<int64_t>(1)))),
-            makeSV(),  // Seek slot
+            makeSlotExprPairVec(
+                countsSlot,
+                stage_builder::makeFunction("sum",
+                                            makeE<EConstant>(value::TypeTags::NumberInt64,
+                                                             value::bitcastFrom<int64_t>(1)))),
+            makeSV(), /* Seek slot */
             true,
             boost::none,
             false /* allowDiskUse */,
+            makeSlotExprPairVec(), /* mergingExprs */
             kEmptyPlanNodeId);
 
         return std::make_pair(countsSlot, std::move(hashAggStage));
diff --git a/src/mongo/db/exec/sbe/size_estimator.h b/src/mongo/db/exec/sbe/size_estimator.h
index fb6684eea52..bbb92328331 100644
--- a/src/mongo/db/exec/sbe/size_estimator.h
+++ b/src/mongo/db/exec/sbe/size_estimator.h
@@ -92,6 +92,11 @@ inline size_t estimate(const S& stats) {
     return stats.estimateObjectSizeInBytes() - sizeof(S);
 }
 
+template <typename A, typename B>
+inline size_t estimate(const std::pair<A, B>& pair) {
+    return estimate(pair.first) + estimate(pair.second);
+}
+
 // Calculate the size of the inlined vector's elements.
 template <typename T, size_t N, typename A>
 size_t estimate(const absl::InlinedVector<T, N, A>& vector) {
diff --git a/src/mongo/db/exec/sbe/stages/hash_agg.cpp b/src/mongo/db/exec/sbe/stages/hash_agg.cpp
index 74b7625ede6..67c1b46d8cb 100644
--- a/src/mongo/db/exec/sbe/stages/hash_agg.cpp
+++ b/src/mongo/db/exec/sbe/stages/hash_agg.cpp
@@ -43,32 +43,54 @@ namespace mongo {
 namespace sbe {
 HashAggStage::HashAggStage(std::unique_ptr<PlanStage> input,
                            value::SlotVector gbs,
-                           value::SlotMap<std::unique_ptr<EExpression>> aggs,
+                           SlotExprPairVector aggs,
                            value::SlotVector seekKeysSlots,
                            bool optimizedClose,
                            boost::optional<value::SlotId> collatorSlot,
                            bool allowDiskUse,
+                           SlotExprPairVector mergingExprs,
                            PlanNodeId planNodeId,
-                           bool participateInTrialRunTracking)
+                           bool participateInTrialRunTracking,
+                           bool forceIncreasedSpilling)
     : PlanStage("group"_sd, planNodeId, participateInTrialRunTracking),
       _gbs(std::move(gbs)),
       _aggs(std::move(aggs)),
       _collatorSlot(collatorSlot),
       _allowDiskUse(allowDiskUse),
       _seekKeysSlots(std::move(seekKeysSlots)),
-      _optimizedClose(optimizedClose) {
+      _optimizedClose(optimizedClose),
+      _mergingExprs(std::move(mergingExprs)),
+      _forceIncreasedSpilling(forceIncreasedSpilling) {
     _children.emplace_back(std::move(input));
     invariant(_seekKeysSlots.empty() || _seekKeysSlots.size() == _gbs.size());
     tassert(5843100,
             "HashAgg stage was given optimizedClose=false and seek keys",
             _seekKeysSlots.empty() || _optimizedClose);
+
+    if (_allowDiskUse) {
+        tassert(7039549,
+                "disk use enabled for HashAggStage but incorrect number of merging expresssions",
+                _aggs.size() == _mergingExprs.size());
+    }
+
+    if (_forceIncreasedSpilling) {
+        tassert(7039554, "'forceIncreasedSpilling' set but disk use not allowed", _allowDiskUse);
+    }
 }
 
 std::unique_ptr<PlanStage> HashAggStage::clone() const {
-    value::SlotMap<std::unique_ptr<EExpression>> aggs;
+    SlotExprPairVector aggs;
+    aggs.reserve(_aggs.size());
     for (auto& [k, v] : _aggs) {
-        aggs.emplace(k, v->clone());
+        aggs.push_back({k, v->clone()});
     }
+
+    SlotExprPairVector mergingExprsClone;
+    mergingExprsClone.reserve(_mergingExprs.size());
+    for (auto&& [k, v] : _mergingExprs) {
+        mergingExprsClone.push_back({k, v->clone()});
+    }
+
     return std::make_unique<HashAggStage>(_children[0]->clone(),
                                           _gbs,
                                           std::move(aggs),
@@ -76,8 +98,10 @@ std::unique_ptr<PlanStage> HashAggStage::clone() const {
                                           _optimizedClose,
                                           _collatorSlot,
                                           _allowDiskUse,
+                                          std::move(mergingExprsClone),
                                           _commonStats.nodeId,
-                                          _participateInTrialRunTracking);
+                                          _participateInTrialRunTracking,
+                                          _forceIncreasedSpilling);
 }
 
 void HashAggStage::doSaveState(bool relinquishCursor) {
@@ -122,34 +146,36 @@ void HashAggStage::prepare(CompileCtx& ctx) {
     }
 
     value::SlotSet dupCheck;
+    auto throwIfDupSlot = [&dupCheck](value::SlotId slot) {
+        auto [_, inserted] = dupCheck.emplace(slot);
+        tassert(7039551, "duplicate slot id", inserted);
+    };
+
     size_t counter = 0;
     // Process group by columns.
     for (auto& slot : _gbs) {
-        auto [it, inserted] = dupCheck.emplace(slot);
-        uassert(4822827, str::stream() << "duplicate field: " << slot, inserted);
+        throwIfDupSlot(slot);
 
         _inKeyAccessors.emplace_back(_children[0]->getAccessor(ctx, slot));
 
-        // Construct accessors for the key to be processed from either the '_ht' or the
-        // '_recordStore'. Before the memory limit is reached the '_outHashKeyAccessors' will carry
-        // the group-by keys, otherwise the '_outRecordStoreKeyAccessors' will carry the group-by
-        // keys.
+        // Construct accessors for obtaining the key values from either the hash table '_ht' or the
+        // '_recordStore'.
         _outHashKeyAccessors.emplace_back(std::make_unique<HashKeyAccessor>(_htIt, counter));
         _outRecordStoreKeyAccessors.emplace_back(
-            std::make_unique<value::MaterializedSingleRowAccessor>(_aggKeyRecordStore, counter));
+            std::make_unique<value::MaterializedSingleRowAccessor>(_outKeyRowRecordStore, counter));
 
-        counter++;
-
-        // A SwitchAccessor is used to point the '_outKeyAccessors' to the key coming from the '_ht'
-        // or the '_recordStore' when draining the HashAgg stage in getNext. The group-by key will
-        // either be in the '_ht' or the '_recordStore' if the key lives in memory, or if the key
-        // has been spilled to disk, respectively. The SwitchAccessor allows toggling between the
-        // two so the parent stage can read it through the '_outAccessors'.
+        // A 'SwitchAccessor' is used to point the '_outKeyAccessors' to the key coming from the
+        // '_ht' or the '_recordStore' when draining the HashAgg stage in getNext(). If no spilling
+        // occurred, the keys will be obtained from the hash table. If spilling kicked in, then all
+        // of the data is written out to the record store, so the 'SwitchAccessor' is reconfigured
+        // to obtain all of the keys from the spill table.
         _outKeyAccessors.emplace_back(
             std::make_unique<value::SwitchAccessor>(std::vector<value::SlotAccessor*>{
                 _outHashKeyAccessors.back().get(), _outRecordStoreKeyAccessors.back().get()}));
 
         _outAccessors[slot] = _outKeyAccessors.back().get();
+
+        ++counter;
     }
 
     // Process seek keys (if any). The keys must come from outside of the subtree (by definition) so
@@ -160,26 +186,18 @@ void HashAggStage::prepare(CompileCtx& ctx) {
 
     counter = 0;
     for (auto& [slot, expr] : _aggs) {
-        auto [it, inserted] = dupCheck.emplace(slot);
-        // Some compilers do not allow to capture local bindings by lambda functions (the one
-        // is used implicitly in uassert below), so we need a local variable to construct an
-        // error message.
-        const auto slotId = slot;
-        uassert(4822828, str::stream() << "duplicate field: " << slotId, inserted);
-
-        // Construct accessors for the agg state to be processed from either the '_ht' or the
-        // '_recordStore' by the SwitchAccessor owned by '_outAggAccessors' below.
+        throwIfDupSlot(slot);
+
+        // Just like with the output accessors for the keys, we construct output accessors for the
+        // aggregate values that read from either the hash table '_ht' or the '_recordStore'.
         _outRecordStoreAggAccessors.emplace_back(
-            std::make_unique<value::MaterializedSingleRowAccessor>(_aggValueRecordStore, counter));
+            std::make_unique<value::MaterializedSingleRowAccessor>(_outAggRowRecordStore, counter));
         _outHashAggAccessors.emplace_back(std::make_unique<HashAggAccessor>(_htIt, counter));
-        counter++;
-
-        // A SwitchAccessor is used to toggle the '_outAggAccessors' between the '_ht' and the
-        // '_recordStore' when updating the agg state via the bytecode. By compiling the agg
-        // EExpressions with a SwitchAccessor we can load the agg value into the memory of
-        // '_aggValueRecordStore' if the value comes from the '_recordStore' or we can use the
-        // agg value referenced through '_htIt' and run the bytecode to mutate the value through the
-        // SwitchAccessor.
+
+        // A 'SwitchAccessor' is used to toggle the '_outAggAccessors' between the '_ht' and the
+        // '_recordStore'. Just like the key values, the aggregate values are always obtained from
+        // the hash table if no spilling occurred and are always obtained from the record store if
+        // spilling occurred.
         _outAggAccessors.emplace_back(
             std::make_unique<value::SwitchAccessor>(std::vector<value::SlotAccessor*>{
                 _outHashAggAccessors.back().get(), _outRecordStoreAggAccessors.back().get()}));
@@ -189,10 +207,32 @@ void HashAggStage::prepare(CompileCtx& ctx) {
         ctx.root = this;
         ctx.aggExpression = true;
         ctx.accumulator = _outAggAccessors.back().get();
-
         _aggCodes.emplace_back(expr->compile(ctx));
         ctx.aggExpression = false;
+
+        ++counter;
+    }
+
+    // If disk use is allowed, then we need to compile the merging expressions as well.
+    if (_allowDiskUse) {
+        counter = 0;
+        for (auto&& [spillSlot, mergingExpr] : _mergingExprs) {
+            throwIfDupSlot(spillSlot);
+
+            _spilledAggsAccessors.push_back(
+                std::make_unique<value::MaterializedSingleRowAccessor>(_spilledAggRow, counter));
+            _spilledAggsAccessorMap[spillSlot] = _spilledAggsAccessors.back().get();
+
+            ctx.root = this;
+            ctx.aggExpression = true;
+            ctx.accumulator = _outAggAccessors[counter].get();
+            _mergingExprCodes.emplace_back(mergingExpr->compile(ctx));
+            ctx.aggExpression = false;
+
+            ++counter;
+        }
     }
+
     _compiled = true;
 }
 
@@ -202,6 +242,15 @@ value::SlotAccessor* HashAggStage::getAccessor(CompileCtx& ctx, value::SlotId sl
             return it->second;
         }
     } else {
+        // The slots into which we read spilled partial aggregates, accessible via
+        // '_spilledAggsAccessors', should only be visible to this stage. They are used internally
+        // when merging spilled partial aggregates and should never be read by ancestor stages.
+        // Therefore, they are only made visible when this stage is in the process of compiling
+        // itself.
+        if (auto it = _spilledAggsAccessorMap.find(slot); it != _spilledAggsAccessorMap.end()) {
+            return it->second;
+        }
+
         return _children[0]->getAccessor(ctx, slot);
     }
 
@@ -227,89 +276,82 @@ void HashAggStage::spillRowToDisk(const value::MaterializedRow& key,
                                   const value::MaterializedRow& val) {
     KeyString::Builder kb{KeyString::Version::kLatestVersion};
     key.serializeIntoKeyString(kb);
+    // Add a unique integer to the end of the key, since record ids must be unique. We want equal
+    // keys to be adjacent in the 'RecordStore' so that we can merge the partial aggregates with a
+    // single pass.
+    kb.appendNumberLong(_ridCounter++);
     auto typeBits = kb.getTypeBits();
-
     auto rid = RecordId(kb.getBuffer(), kb.getSize());
-    boost::optional<value::MaterializedRow> valFromRs =
-        readFromRecordStore(_opCtx, _recordStore->rs(), rid);
-    tassert(6031100, "Spilling a row doesn't support updating it in the store.", !valFromRs);
 
-    spillValueToDisk(rid, val, typeBits, false /*update*/);
+    upsertToRecordStore(_opCtx, _recordStore->rs(), rid, val, typeBits, false /*update*/);
+    _specificStats.spilledRecords++;
 }
 
-void HashAggStage::spillValueToDisk(const RecordId& key,
-                                    const value::MaterializedRow& val,
-                                    const KeyString::TypeBits& typeBits,
-                                    bool update) {
-    auto nBytes = upsertToRecordStore(_opCtx, _recordStore->rs(), key, val, typeBits, update);
-    if (!update) {
-        _specificStats.spilledRecords++;
+void HashAggStage::spill(MemoryCheckData& mcd) {
+    uassert(ErrorCodes::QueryExceededMemoryLimitNoDiskUseAllowed,
+            "Exceeded memory limit for $group, but didn't allow external spilling;"
+            " pass allowDiskUse:true to opt in",
+            _allowDiskUse);
+
+    // Since we flush the entire hash table to disk, we also clear any state related to estimating
+    // memory consumption.
+    mcd.reset();
+
+    if (!_recordStore) {
+        makeTemporaryRecordStore();
     }
-    _specificStats.lastSpilledRecordSize = nBytes;
+
+    for (auto&& it : *_ht) {
+        spillRowToDisk(it.first, it.second);
+    }
+    _ht->clear();
+
+    ++_specificStats.numSpills;
 }
 
 // Checks memory usage. Ideally, we'd want to know the exact size of already accumulated data, but
 // we cannot, so we estimate it based on the last updated/inserted row, if we have one, or the first
 // row in the '_ht' table. If the estimated memory usage exceeds the allowed, this method initiates
-// spilling (if haven't been done yet) and evicts some records from the '_ht' table into the temp
-// store to keep the memory usage under the limit.
+// spilling.
 void HashAggStage::checkMemoryUsageAndSpillIfNecessary(MemoryCheckData& mcd) {
-    // The '_ht' table might become empty in the degenerate case when all rows had to be evicted to
-    // meet the memory constraint during a previous check -- we don't need to keep checking memory
-    // usage in this case because the table will never get new rows.
-    if (_ht->empty()) {
-        return;
-    }
+    invariant(!_ht->empty());
 
     // If the group-by key is empty we will only ever aggregate into a single row so no sense in
-    // spilling since we will just be moving a single row back and forth from disk to main memory.
+    // spilling.
     if (_inKeyAccessors.size() == 0) {
         return;
     }
 
     mcd.memoryCheckpointCounter++;
-    if (mcd.memoryCheckpointCounter >= mcd.nextMemoryCheckpoint) {
-        if (_htIt == _ht->end()) {
-            _htIt = _ht->begin();
-        }
-        const long estimatedRowSize =
-            _htIt->first.memUsageForSorter() + _htIt->second.memUsageForSorter();
-        long long estimatedTotalSize = _ht->size() * estimatedRowSize;
+    if (mcd.memoryCheckpointCounter < mcd.nextMemoryCheckpoint) {
+        // We haven't reached the next checkpoint at which we estimate memory usage and decide if we
+        // should spill.
+        return;
+    }
+
+    const long estimatedRowSize =
+        _htIt->first.memUsageForSorter() + _htIt->second.memUsageForSorter();
+    long long estimatedTotalSize = _ht->size() * estimatedRowSize;
+
+    if (estimatedTotalSize >= _approxMemoryUseInBytesBeforeSpill) {
+        spill(mcd);
+    } else {
+        // Calculate the next memory checkpoint. We estimate it based on the prior growth of the
+        // '_ht' and the remaining available memory. If 'estimatedGainPerChildAdvance' suggests that
+        // the hash table is growing, then the checkpoint is estimated as some configurable
+        // percentage of the number of additional input rows that we would have to process to
+        // consume the remaining memory. On the other hand, a value of 'estimtedGainPerChildAdvance'
+        // close to zero indicates a stable hash stable size, in which case we can delay the next
+        // check progressively.
         const double estimatedGainPerChildAdvance =
             (static_cast<double>(estimatedTotalSize - mcd.lastEstimatedMemoryUsage) /
              mcd.memoryCheckpointCounter);
 
-        if (estimatedTotalSize >= _approxMemoryUseInBytesBeforeSpill) {
-            uassert(ErrorCodes::QueryExceededMemoryLimitNoDiskUseAllowed,
-                    "Exceeded memory limit for $group, but didn't allow external spilling."
-                    " Pass allowDiskUse:true to opt in.",
-                    _allowDiskUse);
-            if (!_recordStore) {
-                makeTemporaryRecordStore();
-            }
-
-            // Evict enough rows into the temporary store to drop below the memory constraint.
-            const long rowsToEvictCount =
-                1 + (estimatedTotalSize - _approxMemoryUseInBytesBeforeSpill) / estimatedRowSize;
-            for (long i = 0; !_ht->empty() && i < rowsToEvictCount; i++) {
-                spillRowToDisk(_htIt->first, _htIt->second);
-                _ht->erase(_htIt);
-                _htIt = _ht->begin();
-            }
-            estimatedTotalSize = _ht->size() * estimatedRowSize;
-        }
-
-        // Calculate the next memory checkpoint. We estimate it based on the prior growth of the
-        // '_ht' and the remaining available memory. We have to keep doing this even after starting
-        // to spill because some accumulators can grow in size inside '_ht' (with no bounds).
-        // Value of 'estimatedGainPerChildAdvance' can be negative if the previous checkpoint
-        // evicted any records. And a value close to zero indicates a stable size of '_ht' so can
-        // delay the next check progressively.
         const long nextCheckpointCandidate = (estimatedGainPerChildAdvance > 0.1)
             ? mcd.checkpointMargin * (_approxMemoryUseInBytesBeforeSpill - estimatedTotalSize) /
                 estimatedGainPerChildAdvance
-            : (estimatedGainPerChildAdvance < -0.1) ? mcd.atMostCheckFrequency
-                                                    : mcd.nextMemoryCheckpoint * 2;
+            : mcd.nextMemoryCheckpoint * 2;
+
         mcd.nextMemoryCheckpoint =
             std::min<long>(mcd.memoryCheckFrequency,
                            std::max<long>(mcd.atMostCheckFrequency, nextCheckpointCandidate));
@@ -335,16 +377,28 @@ void HashAggStage::open(bool reOpen) {
                 5402503, "collatorSlot must be of collator type", tag == value::TypeTags::collator);
             auto collatorView = value::getCollatorView(collatorVal);
             const value::MaterializedRowHasher hasher(collatorView);
-            const value::MaterializedRowEq equator(collatorView);
-            _ht.emplace(0, hasher, equator);
+            _keyEq = value::MaterializedRowEq(collatorView);
+            _ht.emplace(0, hasher, _keyEq);
         } else {
             _ht.emplace();
         }
 
         _seekKeys.resize(_seekKeysAccessors.size());
 
-        // A default value for spilling a key to the record store.
-        value::MaterializedRow defaultVal{_outAggAccessors.size()};
+        // Reset state since this stage may have been previously opened.
+        for (auto&& accessor : _outKeyAccessors) {
+            accessor->setIndex(0);
+        }
+        for (auto&& accessor : _outAggAccessors) {
+            accessor->setIndex(0);
+        }
+        _rsCursor.reset();
+        _recordStore.reset();
+        _outKeyRowRecordStore = {0};
+        _outAggRowRecordStore = {0};
+        _spilledAggRow = {0};
+        _stashedNextRow = {0, 0};
+
         MemoryCheckData memoryCheckData;
 
         while (_children[0]->getNext() == PlanState::ADVANCED) {
@@ -356,53 +410,35 @@ void HashAggStage::open(bool reOpen) {
                 key.reset(idx++, false, tag, val);
             }
 
+            bool newKey = false;
+            _htIt = _ht->find(key);
+            if (_htIt == _ht->end()) {
+                // The key is not present in the hash table yet, so we insert it and initialize the
+                // corresponding accumulator. Note that as a future optimization, we could avoid
+                // doing a lookup both in the 'find()' call and in 'emplace()'.
+                newKey = true;
+                key.makeOwned();
+                auto [it, _] = _ht->emplace(std::move(key), value::MaterializedRow{0});
+                it->second.resize(_outAggAccessors.size());
+                _htIt = it;
+            }
+
+            // Accumulate state in '_ht'.
+            for (size_t idx = 0; idx < _outAggAccessors.size(); ++idx) {
+                auto [owned, tag, val] = _bytecode.run(_aggCodes[idx].get());
+                _outHashAggAccessors[idx]->reset(owned, tag, val);
+            }
 
-            if (_htIt = _ht->find(key); !_recordStore || _htIt != _ht->end()) {
-                if (_htIt == _ht->end()) {
-                    // The memory limit hasn't been reached yet, insert a new key in '_ht' by
-                    // copying the key. Note as a future optimization, we should avoid the lookup in
-                    // the find() call and the emplace.
-                    key.makeOwned();
-                    auto [it, _] = _ht->emplace(std::move(key), value::MaterializedRow{0});
-                    // Initialize accumulators.
-                    it->second.resize(_outAggAccessors.size());
-                    _htIt = it;
-                }
-                // Accumulate state in '_ht' by pointing the '_outAggAccessors' the
-                // '_outHashAggAccessors'.
-                for (size_t idx = 0; idx < _outAggAccessors.size(); ++idx) {
-                    _outAggAccessors[idx]->setIndex(0);
-                    auto [owned, tag, val] = _bytecode.run(_aggCodes[idx].get());
-                    _outHashAggAccessors[idx]->reset(owned, tag, val);
-                }
+            if (_forceIncreasedSpilling && !newKey) {
+                // If configured to spill more than usual, we spill after seeing the same key twice.
+                spill(memoryCheckData);
             } else {
-                // The memory limit has been reached and the key wasn't in the '_ht' so we need
-                // to spill it to the '_recordStore'.
-                KeyString::Builder kb{KeyString::Version::kLatestVersion};
-                key.serializeIntoKeyString(kb);
-                auto typeBits = kb.getTypeBits();
-
-                auto rid = RecordId(kb.getBuffer(), kb.getSize());
-
-                boost::optional<value::MaterializedRow> valFromRs =
-                    readFromRecordStore(_opCtx, _recordStore->rs(), rid);
-                if (!valFromRs) {
-                    _aggValueRecordStore = defaultVal;
-                } else {
-                    _aggValueRecordStore = *valFromRs;
-                }
-
-                for (size_t idx = 0; idx < _outAggAccessors.size(); ++idx) {
-                    _outAggAccessors[idx]->setIndex(1);
-                    auto [owned, tag, val] = _bytecode.run(_aggCodes[idx].get());
-                    _aggValueRecordStore.reset(idx, owned, tag, val);
-                }
-                spillValueToDisk(rid, _aggValueRecordStore, typeBits, valFromRs ? true : false);
+                // Estimates how much memory is being used. If we estimate that the hash table
+                // exceeds the allotted memory budget, its contents are spilled to the
+                // '_recordStore' and '_ht' is cleared.
+                checkMemoryUsageAndSpillIfNecessary(memoryCheckData);
             }
 
-            // Estimates how much memory is being used and might start spilling.
-            checkMemoryUsageAndSpillIfNecessary(memoryCheckData);
-
             if (_tracker && _tracker->trackProgress<TrialRunTracker::kNumResults>(1)) {
                 // During trial runs, we want to limit the amount of work done by opening a blocking
                 // stage, like this one. The blocking stage tracks the number of documents it has
@@ -418,8 +454,33 @@ void HashAggStage::open(bool reOpen) {
             _children[0]->close();
             _childOpened = false;
         }
-    }
 
+        // If we spilled at any point while consuming the input, then do one final spill to write
+        // any leftover contents of '_ht' to the record store. That way, when recovering the input
+        // from the record store and merging partial aggregates we don't have to worry about the
+        // possibility of some of the data being in the hash table and some being in the record
+        // store.
+        if (_recordStore) {
+            if (!_ht->empty()) {
+                spill(memoryCheckData);
+            }
+
+            _specificStats.spilledDataStorageSize = _recordStore->rs()->storageSize(_opCtx);
+
+            // Establish a cursor, positioned at the beginning of the record store.
+            _rsCursor = _recordStore->rs()->getCursor(_opCtx);
+
+            // Callers will be obtaining the results from the spill table, so set the
+            // 'SwitchAccessors' so that they refer to the rows recovered from the record store
+            // under the hood.
+            for (auto&& accessor : _outKeyAccessors) {
+                accessor->setIndex(1);
+            }
+            for (auto&& accessor : _outAggAccessors) {
+                accessor->setIndex(1);
+            }
+        }
+    }
 
     if (!_seekKeysAccessors.empty()) {
         // Copy keys in order to do the lookup.
@@ -431,20 +492,76 @@ void HashAggStage::open(bool reOpen) {
     }
 
     _htIt = _ht->end();
+}
 
-    // Set the SwitchAccessors to point to the '_ht' so we can drain it first before draining the
-    // '_recordStore' in getNext().
-    for (size_t idx = 0; idx < _outAggAccessors.size(); ++idx) {
-        _outAggAccessors[idx]->setIndex(0);
+HashAggStage::SpilledRow HashAggStage::deserializeSpilledRecord(const Record& record,
+                                                                BufBuilder& keyBuffer) {
+    // Read the values and type bits out of the value part of the record.
+    BufReader valReader(record.data.data(), record.data.size());
+    auto val = value::MaterializedRow::deserializeForSorter(valReader, {});
+    auto typeBits = KeyString::TypeBits::fromBuffer(KeyString::Version::kLatestVersion, &valReader);
+
+    keyBuffer.reset();
+    auto key = value::MaterializedRow::deserializeFromKeyString(
+        decodeKeyString(record.id, typeBits), &keyBuffer, _gbs.size() /*numPrefixValuesToRead*/);
+    return {std::move(key), std::move(val)};
+}
+
+PlanState HashAggStage::getNextSpilled() {
+    if (_stashedNextRow.first.isEmpty()) {
+        auto nextRecord = _rsCursor->next();
+        if (!nextRecord) {
+            return trackPlanState(PlanState::IS_EOF);
+        }
+
+        // We are just starting the process of merging the spilled file segments.
+        auto recoveredRow = deserializeSpilledRecord(*nextRecord, _outKeyRowRSBuffer);
+
+        _outKeyRowRecordStore = std::move(recoveredRow.first);
+        _outAggRowRecordStore = std::move(recoveredRow.second);
+    } else {
+        // We peeked at the next key last time around.
+        _outKeyRowRSBuffer = std::move(_stashedKeyBuffer);
+        _outKeyRowRecordStore = std::move(_stashedNextRow.first);
+        _outAggRowRecordStore = std::move(_stashedNextRow.second);
+        // Clear the stashed row.
+        _stashedNextRow = {0, 0};
     }
-    _drainingRecordStore = false;
+
+    // Find additional partial aggregates for the same key and merge them in order to compute the
+    // final output.
+    for (auto nextRecord = _rsCursor->next(); nextRecord; nextRecord = _rsCursor->next()) {
+        auto recoveredRow = deserializeSpilledRecord(*nextRecord, _stashedKeyBuffer);
+        if (!_keyEq(recoveredRow.first, _outKeyRowRecordStore)) {
+            // The newly recovered spilled row belongs to a new key, so we're done merging partial
+            // aggregates for the old key. Save the new row for later and return advanced.
+            _stashedNextRow = std::move(recoveredRow);
+            return trackPlanState(PlanState::ADVANCED);
+        }
+
+        // Merge in the new partial aggregate values.
+        _spilledAggRow = std::move(recoveredRow.second);
+        for (size_t idx = 0; idx < _mergingExprCodes.size(); ++idx) {
+            auto [owned, tag, val] = _bytecode.run(_mergingExprCodes[idx].get());
+            _outRecordStoreAggAccessors[idx]->reset(owned, tag, val);
+        }
+    }
+
+    return trackPlanState(PlanState::ADVANCED);
 }
 
 PlanState HashAggStage::getNext() {
     auto optTimer(getOptTimer(_opCtx));
 
-    if (_htIt == _ht->end() && !_drainingRecordStore) {
-        // First invocation of getNext() after open() when not draining the '_recordStore'.
+    // If we've spilled, then we need to produce the output by merging the spilled segments from the
+    // spill file.
+    if (_recordStore) {
+        return getNextSpilled();
+    }
+
+    // We didn't spill. Obtain the next output row from the hash table.
+    if (_htIt == _ht->end()) {
+        // First invocation of getNext() after open().
         if (!_seekKeysAccessors.empty()) {
             _htIt = _ht->find(_seekKeys);
         } else {
@@ -453,55 +570,13 @@ PlanState HashAggStage::getNext() {
     } else if (!_seekKeysAccessors.empty()) {
         // Subsequent invocation with seek keys. Return only 1 single row (if any).
         _htIt = _ht->end();
-    } else if (!_drainingRecordStore) {
-        // Returning the results of the entire hash table first before draining the '_recordStore'.
+    } else {
         ++_htIt;
     }
 
-    if (_htIt == _ht->end() && !_recordStore) {
-        // The hash table has been drained and nothing was spilled to disk.
+    if (_htIt == _ht->end()) {
+        // The hash table has been drained (and we never spilled to disk) so we're done.
         return trackPlanState(PlanState::IS_EOF);
-    } else if (_htIt != _ht->end()) {
-        // Drain the '_ht' on the next 'getNext()' call.
-        return trackPlanState(PlanState::ADVANCED);
-    } else if (_seekKeysAccessors.empty()) {
-        // A record store was created to spill to disk. Drain it then clean it up.
-        if (!_rsCursor) {
-            _rsCursor = _recordStore->rs()->getCursor(_opCtx);
-        }
-        auto nextRecord = _rsCursor->next();
-        if (nextRecord) {
-            // Point the out accessors to the recordStore accessors to allow parent stages to read
-            // the agg state from the '_recordStore'.
-            if (!_drainingRecordStore) {
-                for (size_t i = 0; i < _outKeyAccessors.size(); ++i) {
-                    _outKeyAccessors[i]->setIndex(1);
-                }
-                for (size_t i = 0; i < _outAggAccessors.size(); ++i) {
-                    _outAggAccessors[i]->setIndex(1);
-                }
-            }
-            _drainingRecordStore = true;
-
-            // Read the agg state value from the '_recordStore' and Reconstruct the key from the
-            // typeBits stored along side of the value.
-            BufReader valReader(nextRecord->data.data(), nextRecord->data.size());
-            auto val = value::MaterializedRow::deserializeForSorter(valReader, {});
-            auto typeBits =
-                KeyString::TypeBits::fromBuffer(KeyString::Version::kLatestVersion, &valReader);
-            _aggValueRecordStore = val;
-
-            _aggKeyRSBuffer.reset();
-            _aggKeyRecordStore = value::MaterializedRow::deserializeFromKeyString(
-                decodeKeyString(nextRecord->id, typeBits), &_aggKeyRSBuffer);
-            return trackPlanState(PlanState::ADVANCED);
-        } else {
-            _rsCursor.reset();
-            _specificStats.spilledRecordEstimatedStorageSize =
-                _recordStore->rs()->storageSize(_opCtx);
-            _recordStore.reset();
-            return trackPlanState(PlanState::IS_EOF);
-        }
     } else {
         return trackPlanState(PlanState::ADVANCED);
     }
@@ -521,13 +596,19 @@ std::unique_ptr<PlanStageStats> HashAggStage::getStats(bool includeDebugInfo) co
                 childrenBob.append(str::stream() << slot, printer.print(expr->debugPrint()));
             }
         }
+
+        if (!_mergingExprs.empty()) {
+            BSONObjBuilder nestedBuilder{bob.subobjStart("mergingExprs")};
+            for (auto&& [slot, expr] : _mergingExprs) {
+                nestedBuilder.append(str::stream() << slot, printer.print(expr->debugPrint()));
+            }
+        }
+
         // Spilling stats.
         bob.appendBool("usedDisk", _specificStats.usedDisk);
+        bob.appendNumber("numSpills", _specificStats.numSpills);
         bob.appendNumber("spilledRecords", _specificStats.spilledRecords);
-        bob.appendNumber("spilledBytesApprox",
-                         _specificStats.lastSpilledRecordSize * _specificStats.spilledRecords);
-        bob.appendNumber("spilledRecordEstimatedStorageSize",
-                         _specificStats.spilledRecordEstimatedStorageSize);
+        bob.appendNumber("spilledDataStorageSize", _specificStats.spilledDataStorageSize);
 
         ret->debugInfo = bob.obj();
     }
@@ -545,11 +626,12 @@ void HashAggStage::close() {
 
     trackClose();
     _ht = boost::none;
-    if (_recordStore) {
-        // A record store was created to spill to disk. Clean it up.
-        _recordStore.reset();
-        _drainingRecordStore = false;
-    }
+    _rsCursor.reset();
+    _recordStore.reset();
+    _outKeyRowRecordStore = {0};
+    _outAggRowRecordStore = {0};
+    _spilledAggRow = {0};
+    _stashedNextRow = {0, 0};
 
     if (_childOpened) {
         _children[0]->close();
@@ -572,7 +654,7 @@ std::vector<DebugPrinter::Block> HashAggStage::debugPrint() const {
 
     ret.emplace_back(DebugPrinter::Block("[`"));
     bool first = true;
-    value::orderedSlotMapTraverse(_aggs, [&](auto slot, auto&& expr) {
+    for (auto&& [slot, expr] : _aggs) {
         if (!first) {
             ret.emplace_back(DebugPrinter::Block("`,"));
         }
@@ -581,7 +663,7 @@ std::vector<DebugPrinter::Block> HashAggStage::debugPrint() const {
         ret.emplace_back("=");
         DebugPrinter::addBlocks(ret, expr->debugPrint());
         first = false;
-    });
+    }
     ret.emplace_back("`]");
 
     if (!_seekKeysSlots.empty()) {
@@ -596,6 +678,28 @@ std::vector<DebugPrinter::Block> HashAggStage::debugPrint() const {
         ret.emplace_back("`]");
     }
 
+    if (!_mergingExprs.empty()) {
+        ret.emplace_back("spillSlots[`");
+        for (size_t idx = 0; idx < _mergingExprs.size(); ++idx) {
+            if (idx) {
+                ret.emplace_back("`,");
+            }
+
+            DebugPrinter::addIdentifier(ret, _mergingExprs[idx].first);
+        }
+        ret.emplace_back("`]");
+
+        ret.emplace_back("mergingExprs[`");
+        for (size_t idx = 0; idx < _mergingExprs.size(); ++idx) {
+            if (idx) {
+                ret.emplace_back("`,");
+            }
+
+            DebugPrinter::addBlocks(ret, _mergingExprs[idx].second->debugPrint());
+        }
+        ret.emplace_back("`]");
+    }
+
     if (!_optimizedClose) {
         ret.emplace_back("reopen");
     }
@@ -616,6 +720,7 @@ size_t HashAggStage::estimateCompileTimeSize() const {
     size += size_estimator::estimate(_gbs);
     size += size_estimator::estimate(_aggs);
     size += size_estimator::estimate(_seekKeysSlots);
+    size += size_estimator::estimate(_mergingExprs);
     return size;
 }
 
diff --git a/src/mongo/db/exec/sbe/stages/hash_agg.h b/src/mongo/db/exec/sbe/stages/hash_agg.h
index d200c4b9c3d..86f7337b7f7 100644
--- a/src/mongo/db/exec/sbe/stages/hash_agg.h
+++ b/src/mongo/db/exec/sbe/stages/hash_agg.h
@@ -29,8 +29,6 @@
 
 #pragma once
 
-#include <unordered_map>
-
 #include "mongo/db/exec/sbe/expressions/expression.h"
 #include "mongo/db/exec/sbe/stages/stages.h"
 #include "mongo/db/exec/sbe/vm/vm.h"
@@ -61,22 +59,39 @@ namespace sbe {
  * determining whether two group-by keys are equal. For instance, the plan may require us to do a
  * case-insensitive group on a string field.
  *
+ * The 'allowDiskUse' flag controls whether this stage can spill. If false and the memory budget is
+ * exhausted, this stage throws a query-fatal error with code
+ * 'QueryExceededMemoryLimitNoDiskUseAllowed'. If true, then spilling is possible and the caller
+ * must provide a vector of 'mergingExprs'. This is a vector of (slot, expression) pairs which is
+ * symmetrical with 'aggs'. The slots are only visible internally and are used to store partial
+ * aggregate values that have been recovered from the spill table. Each of the expressions is an agg
+ * function which merges the partial aggregate value from this slot into the final aggregate value.
+ * In the debug string output, the internal slots used to house the partial aggregates are printed
+ * as a list of "spillSlots" and the expressions are printed as a parallel list of "mergingExprs".
+ *
+ * If 'forcedIncreasedSpilling' is true, then this stage will spill frequently even if the memory
+ * limit is not reached. This is intended to be used in test contexts to exercise the otherwise
+ * infrequently used spilling logic.
+ *
  * Debug string representation:
  *
- *  group [<group by slots>] [slot_1 = expr_1, ..., slot_n = expr_n] [<seek slots>]? reopen?
- * collatorSlot? childStage
+ *  group [<group by slots>] [slot_1 = expr_1, ..., slot_n = expr_n] [<seek slots>]?
+ *      spillSlots[slot_1, ..., slot_n] mergingExprs[expr_1, ..., expr_n] reopen? collatorSlot?
+ *  childStage
  */
 class HashAggStage final : public PlanStage {
 public:
     HashAggStage(std::unique_ptr<PlanStage> input,
                  value::SlotVector gbs,
-                 value::SlotMap<std::unique_ptr<EExpression>> aggs,
+                 SlotExprPairVector aggs,
                  value::SlotVector seekKeysSlots,
                  bool optimizedClose,
                  boost::optional<value::SlotId> collatorSlot,
                  bool allowDiskUse,
+                 SlotExprPairVector mergingExprs,
                  PlanNodeId planNodeId,
-                 bool participateInTrialRunTracking = true);
+                 bool participateInTrialRunTracking = true,
+                 bool forceIncreasedSpilling = false);
 
     std::unique_ptr<PlanStage> clone() const final;
 
@@ -109,99 +124,138 @@ private:
     using HashKeyAccessor = value::MaterializedRowKeyAccessor<TableType::iterator>;
     using HashAggAccessor = value::MaterializedRowValueAccessor<TableType::iterator>;
 
-    void makeTemporaryRecordStore();
-
-    /**
-     * Spills a key and value pair to the '_recordStore' where the semantics are insert or update
-     * depending on the 'update' flag. When the 'update' flag is true this method already expects
-     * the 'key' to be inserted into the '_recordStore', otherwise the 'key' and 'val' pair are
-     * fresh.
-     *
-     * This method expects the key to be seralized into a KeyString::Value so that the key is
-     * memcmp-able and lookups can be done to update the 'val' in the '_recordStore'. Note that the
-     * 'typeBits' are needed to reconstruct the spilled 'key' when calling 'getNext' to deserialize
-     * the 'key' to a MaterializedRow. Since the '_recordStore' only stores the memcmp-able part of
-     * the KeyString we need to carry the 'typeBits' separately, and we do this by appending the
-     * 'typeBits' to the end of the serialized 'val' buffer and store them at the leaves of the
-     * backing B-tree of the '_recordStore'. used as the RecordId.
-     */
-    void spillValueToDisk(const RecordId& key,
-                          const value::MaterializedRow& val,
-                          const KeyString::TypeBits& typeBits,
-                          bool update);
-    void spillRowToDisk(const value::MaterializedRow& key,
-                        const value::MaterializedRow& defaultVal);
+    using SpilledRow = std::pair<value::MaterializedRow, value::MaterializedRow>;
 
     /**
      * We check amount of used memory every T processed incoming records, where T is calculated
      * based on the estimated used memory and its recent growth. When the memory limit is exceeded,
-     * 'checkMemoryUsageAndSpillIfNecessary()' will create '_recordStore' and might spill some of
-     * the already accumulated data into it.
+     * 'checkMemoryUsageAndSpillIfNecessary()' will create '_recordStore' (if it hasn't already been
+     * created) and spill the contents of the hash table into this record store.
      */
     struct MemoryCheckData {
+        MemoryCheckData() {
+            reset();
+        }
+
+        void reset() {
+            memoryCheckFrequency = std::min(atMostCheckFrequency, atLeastMemoryCheckFrequency);
+            nextMemoryCheckpoint = 0;
+            memoryCheckpointCounter = 0;
+            lastEstimatedMemoryUsage = 0;
+        }
+
         const double checkpointMargin = internalQuerySBEAggMemoryUseCheckMargin.load();
-        const long atMostCheckFrequency = internalQuerySBEAggMemoryCheckPerAdvanceAtMost.load();
-        const long atLeastMemoryCheckFrequency =
+        const int64_t atMostCheckFrequency = internalQuerySBEAggMemoryCheckPerAdvanceAtMost.load();
+        const int64_t atLeastMemoryCheckFrequency =
             internalQuerySBEAggMemoryCheckPerAdvanceAtLeast.load();
 
         // The check frequency upper bound, which start at 'atMost' and exponentially backs off
         // to 'atLeast' as more data is accumulated. If 'atLeast' is less than 'atMost', the memory
         // checks will be done every 'atLeast' incoming records.
-        long memoryCheckFrequency = 1;
+        int64_t memoryCheckFrequency = 1;
 
         // The number of incoming records to process before the next memory checkpoint.
-        long nextMemoryCheckpoint = 0;
+        int64_t nextMemoryCheckpoint = 0;
 
         // The counter of the incoming records between memory checkpoints.
-        long memoryCheckpointCounter = 0;
+        int64_t memoryCheckpointCounter = 0;
 
-        long long lastEstimatedMemoryUsage = 0;
-
-        MemoryCheckData() {
-            memoryCheckFrequency = std::min(atMostCheckFrequency, atLeastMemoryCheckFrequency);
-        }
+        int64_t lastEstimatedMemoryUsage = 0;
     };
+
+    /**
+     * Inserts a key and value pair to the '_recordStore'. They key is serialized to a
+     * 'KeyString::Value' which becomes the 'RecordId'. This makes the keys memcmp-able and ensures
+     * that the record store ends up sorted by the group-by keys.
+     *
+     * Note that the 'typeBits' are needed to reconstruct the spilled 'key' to a 'MaterializedRow',
+     * but are not necessary for comparison purposes. Therefore, we carry the type bits separately
+     * from the record id, instead appending them to the end of the serialized 'val' buffer.
+     */
+    void spillRowToDisk(const value::MaterializedRow& key, const value::MaterializedRow& val);
+
     void checkMemoryUsageAndSpillIfNecessary(MemoryCheckData& mcd);
+    void spill(MemoryCheckData& mcd);
+
+    /**
+     * Given a 'record' from the record store, decodes it into a pair of materialized rows (one for
+     * the group-by keys and another for the agg values).
+     *
+     * The given 'keyBuffer' is cleared, and then used to hold data (e.g. long strings and other
+     * values that can't be inlined) obtained by decoding the 'RecordId' keystring to a
+     * 'MaterializedRow'. The values in the resulting 'MaterializedRow' may be pointers into
+     * 'keyBuffer', so it is important that 'keyBuffer' outlive the row.
+     */
+    SpilledRow deserializeSpilledRecord(const Record& record, BufBuilder& keyBuffer);
+
+    PlanState getNextSpilled();
+
+    void makeTemporaryRecordStore();
 
     const value::SlotVector _gbs;
-    const value::SlotMap<std::unique_ptr<EExpression>> _aggs;
+    const SlotExprPairVector _aggs;
     const boost::optional<value::SlotId> _collatorSlot;
     const bool _allowDiskUse;
     const value::SlotVector _seekKeysSlots;
     // When this operator does not expect to be reopened (almost always) then it can close the child
     // early.
     const bool _optimizedClose{true};
+
+    // Expressions used to merge partial aggregates that have been spilled to disk and their
+    // corresponding input slots. For example, imagine that this list contains a pair (s12,
+    // sum(s12)). This means that the partial aggregate values will be read into slot s12 after
+    // being recovered from the spill table and can be merged using the 'sum()' agg function.
+    //
+    // When disk use is allowed, this vector must have the same length as '_aggs'.
+    const SlotExprPairVector _mergingExprs;
+
+    // When true, we spill frequently without reaching the memory limit. This allows us to exercise
+    // the spilling logic more often in test contexts.
+    const bool _forceIncreasedSpilling;
+
     value::SlotAccessorMap _outAccessors;
+
+    // Accessors used to obtain the values of the group by slots when reading the input from the
+    // child.
     std::vector<value::SlotAccessor*> _inKeyAccessors;
 
-    // Accessors for the key stored in '_ht', a SwitchAccessor is used so we can produce the key
-    // from either the '_ht' or the '_recordStore'.
+    // This buffer stores values for '_outKeyRowRecordStore'; values in the '_outKeyRowRecordStore'
+    // can be pointers that point to data in this buffer.
+    BufBuilder _outKeyRowRSBuffer;
+    // Accessors for the key slots provided as output by this stage. The keys can either come from
+    // the hash table or recovered from a temporary record store. We use a 'SwitchAccessor' to
+    // switch between these two cases.
     std::vector<std::unique_ptr<HashKeyAccessor>> _outHashKeyAccessors;
+    // Row of key values to output used when recovering spilled data from the record store.
+    value::MaterializedRow _outKeyRowRecordStore{0};
+    std::vector<std::unique_ptr<value::MaterializedSingleRowAccessor>> _outRecordStoreKeyAccessors;
     std::vector<std::unique_ptr<value::SwitchAccessor>> _outKeyAccessors;
 
-    // Accessor for the agg state value stored in the '_recordStore' when data is spilled to disk.
-    value::MaterializedRow _aggKeyRecordStore{0};
-    value::MaterializedRow _aggValueRecordStore{0};
-    std::vector<std::unique_ptr<value::MaterializedSingleRowAccessor>> _outRecordStoreKeyAccessors;
+    // Accessors for the output aggregate results. The aggregates can either come from the hash
+    // table or can be computed after merging partial aggregates spilled to a record store. We use a
+    // 'SwitchAccessor' to switch between these two cases.
+    std::vector<std::unique_ptr<HashAggAccessor>> _outHashAggAccessors;
+    // Row of agg values to output used when recovering spilled data from the record store.
+    value::MaterializedRow _outAggRowRecordStore{0};
     std::vector<std::unique_ptr<value::MaterializedSingleRowAccessor>> _outRecordStoreAggAccessors;
-
-    // This buffer stores values for the spilled '_aggKeyRecordStore' that's loaded into memory from
-    // the '_recordStore'. Values in the '_aggKeyRecordStore' row are pointers that point to data in
-    // this buffer.
-    BufBuilder _aggKeyRSBuffer;
+    std::vector<std::unique_ptr<value::SwitchAccessor>> _outAggAccessors;
 
     std::vector<value::SlotAccessor*> _seekKeysAccessors;
     value::MaterializedRow _seekKeys;
 
-    // Accesors for the agg state in '_ht', a SwitchAccessor is used so we can produce the agg state
-    // from either the '_ht' or the '_recordStore' when draining the HashAgg stage.
-    std::vector<std::unique_ptr<value::SwitchAccessor>> _outAggAccessors;
-    std::vector<std::unique_ptr<HashAggAccessor>> _outHashAggAccessors;
+    // Bytecode which gets executed to aggregate incoming rows into the hash table.
     std::vector<std::unique_ptr<vm::CodeFragment>> _aggCodes;
+    // Bytecode for the merging expressions, executed if partial aggregates are spilled to a record
+    // store and need to be subsequently combined.
+    std::vector<std::unique_ptr<vm::CodeFragment>> _mergingExprCodes;
 
     // Only set if collator slot provided on construction.
     value::SlotAccessor* _collatorAccessor = nullptr;
 
+    // Function object which can be used to check whether two materialized rows of key values are
+    // equal. This comparison is collation-aware if the query has a non-simple collation.
+    value::MaterializedRowEq _keyEq;
+
     boost::optional<TableType> _ht;
     TableType::iterator _htIt;
 
@@ -213,10 +267,31 @@ private:
     // Memory tracking and spilling to disk.
     const long long _approxMemoryUseInBytesBeforeSpill =
         internalQuerySBEAggApproxMemoryUseInBytesBeforeSpill.load();
+
+    // A record store which is instantiated and written to in the case of spilling.
     std::unique_ptr<TemporaryRecordStore> _recordStore;
-    bool _drainingRecordStore{false};
     std::unique_ptr<SeekableRecordCursor> _rsCursor;
 
+    // A monotically increasing counter used to ensure uniqueness of 'RecordId' values. When
+    // spilling, the key is encoding into the 'RecordId' of the '_recordStore'. Record ids must be
+    // unique by definition, but we might end up spilling multiple partial aggregates for the same
+    // key. We ensure uniqueness by appending a unique integer to the end of this key, which is
+    // simply ignored during deserialization.
+    int64_t _ridCounter = 0;
+
+    // Partial aggregates that have been spilled are read into '_spilledAggRow' and read using
+    // '_spilledAggsAccessors' so that they can be merged to compute the final aggregate value.
+    value::MaterializedRow _spilledAggRow{0};
+    std::vector<std::unique_ptr<value::MaterializedSingleRowAccessor>> _spilledAggsAccessors;
+    value::SlotAccessorMap _spilledAggsAccessorMap;
+
+    // Buffer to hold data for the deserialized key values from '_stashedNextRow'.
+    BufBuilder _stashedKeyBuffer;
+    // Place to stash the next keys and values during the streaming phase. The record store cursor
+    // doesn't offer a "peek" API, so we need to hold onto the next row between getNext() calls when
+    // the key value advances.
+    SpilledRow _stashedNextRow;
+
     HashAggStats _specificStats;
 
     // If provided, used during a trial run to accumulate certain execution stats. Once the trial
diff --git a/src/mongo/db/exec/sbe/stages/plan_stats.h b/src/mongo/db/exec/sbe/stages/plan_stats.h
index 360731a13cb..2801c8abb07 100644
--- a/src/mongo/db/exec/sbe/stages/plan_stats.h
+++ b/src/mongo/db/exec/sbe/stages/plan_stats.h
@@ -319,9 +319,13 @@ struct HashAggStats : public SpecificStats {
     }
 
     bool usedDisk{false};
+    // The number of times that the entire hash table was spilled.
+    long long numSpills{0};
+    // The number of individual records spilled to disk.
     long long spilledRecords{0};
-    long long lastSpilledRecordSize{0};
-    long long spilledRecordEstimatedStorageSize{0};
+    // An estimate, in bytes, of the size of the final spill table after all spill events have taken
+    // place.
+    long long spilledDataStorageSize{0};
 };
 
 struct HashLookupStats : public SpecificStats {
diff --git a/src/mongo/db/exec/sbe/util/spilling.cpp b/src/mongo/db/exec/sbe/util/spilling.cpp
index c54f3bfe956..6e8c027d7fe 100644
--- a/src/mongo/db/exec/sbe/util/spilling.cpp
+++ b/src/mongo/db/exec/sbe/util/spilling.cpp
@@ -85,7 +85,6 @@ int upsertToRecordStore(OperationContext* opCtx,
                         BufBuilder& buf,
                         const KeyString::TypeBits& typeBits,  // recover type of value.
                         bool update) {
-
     // Append the 'typeBits' to the end of the val's buffer so the 'key' can be reconstructed when
     // draining HashAgg.
     buf.appendBuf(typeBits.getBuffer(), typeBits.getSize());
diff --git a/src/mongo/db/exec/sbe/util/spilling.h b/src/mongo/db/exec/sbe/util/spilling.h
index 95f73e2b02e..0a562d7e864 100644
--- a/src/mongo/db/exec/sbe/util/spilling.h
+++ b/src/mongo/db/exec/sbe/util/spilling.h
@@ -55,9 +55,12 @@ boost::optional<value::MaterializedRow> readFromRecordStore(OperationContext* op
                                                             RecordStore* rs,
                                                             const RecordId& rid);
 
-/** Inserts or updates a key/value into 'rs'. The 'update' flag controls whether or not an update
+/**
+ * Inserts or updates a key/value into 'rs'. The 'update' flag controls whether or not an update
  * will be performed. If a key/value pair is inserted into the 'rs' that already exists and
  * 'update' is false, this function will tassert.
+ *
+ * Returns the size of the new record in bytes, including the record id and value portions.
  */
 int upsertToRecordStore(OperationContext* opCtx,
                         RecordStore* rs,
@@ -65,7 +68,6 @@ int upsertToRecordStore(OperationContext* opCtx,
                         const value::MaterializedRow& val,
                         const KeyString::TypeBits& typeBits,
                         bool update);
-
 int upsertToRecordStore(OperationContext* opCtx,
                         RecordStore* rs,
                         const RecordId& key,
diff --git a/src/mongo/db/exec/sbe/values/slot.cpp b/src/mongo/db/exec/sbe/values/slot.cpp
index e9f6e08f313..32a4275ff5f 100644
--- a/src/mongo/db/exec/sbe/values/slot.cpp
+++ b/src/mongo/db/exec/sbe/values/slot.cpp
@@ -565,8 +565,10 @@ void MaterializedRow::serializeIntoKeyString(KeyString::Builder& buf) const {
     }
 }
 
-MaterializedRow MaterializedRow::deserializeFromKeyString(const KeyString::Value& keyString,
-                                                          BufBuilder* valueBufferBuilder) {
+MaterializedRow MaterializedRow::deserializeFromKeyString(
+    const KeyString::Value& keyString,
+    BufBuilder* valueBufferBuilder,
+    boost::optional<size_t> numPrefixValsToRead) {
     BufReader reader(keyString.getBuffer(), keyString.getSize());
     KeyString::TypeBits typeBits(keyString.getTypeBits());
     KeyString::TypeBits::Reader typeBitsReader(typeBits);
@@ -578,7 +580,8 @@ MaterializedRow MaterializedRow::deserializeFromKeyString(const KeyString::Value
             &reader, &typeBitsReader, false /* inverted */, typeBits.version, &valBuilder);
     } while (keepReading);
 
-    MaterializedRow result{valBuilder.numValues()};
+    size_t sizeOfRow = numPrefixValsToRead ? *numPrefixValsToRead : valBuilder.numValues();
+    MaterializedRow result{sizeOfRow};
     valBuilder.readValues(result);
 
     return result;
diff --git a/src/mongo/db/exec/sbe/values/slot.h b/src/mongo/db/exec/sbe/values/slot.h
index efd92255c9f..49020874bf6 100644
--- a/src/mongo/db/exec/sbe/values/slot.h
+++ b/src/mongo/db/exec/sbe/values/slot.h
@@ -503,10 +503,16 @@ public:
      * intended for spilling key values used in the HashAgg stage. The format is not guaranteed to
      * be stable between versions, so it should not be used for long-term storage or communication
      * between instances.
+     *
+     * If 'numPrefixValsToRead' is provided, then only the given number of values from 'keyString'
+     * are decoded into the resulting 'MaterializedRow'. The remaining suffix values in the
+     * 'keyString' are ignored.
      */
-    static MaterializedRow deserializeFromKeyString(const KeyString::Value& keyString,
+    static MaterializedRow deserializeFromKeyString(
+        const KeyString::Value& keyString,
+        BufBuilder* valueBufferBuilder,
+        boost::optional<size_t> numPrefixValsToRead = boost::none);
 
-                                                    BufBuilder* valueBufferBuilder);
     void serializeIntoKeyString(KeyString::Builder& builder) const;
 
 private:
@@ -597,9 +603,9 @@ private:
 };
 
 struct MaterializedRowEq {
-    using ComparatorType = StringData::ComparatorInterface*;
+    using ComparatorType = StringData::ComparatorInterface;
 
-    explicit MaterializedRowEq(const ComparatorType comparator = nullptr)
+    explicit MaterializedRowEq(const ComparatorType* comparator = nullptr)
         : _comparator(comparator) {}
 
     bool operator()(const MaterializedRow& lhs, const MaterializedRow& rhs) const {
@@ -617,7 +623,7 @@ struct MaterializedRowEq {
     }
 
 private:
-    const ComparatorType _comparator = nullptr;
+    const ComparatorType* _comparator = nullptr;
 };
 
 struct MaterializedRowLess {
diff --git a/src/mongo/db/exec/sbe/values/value_builder.h b/src/mongo/db/exec/sbe/values/value_builder.h
index 9ad2b511242..a39432a4429 100644
--- a/src/mongo/db/exec/sbe/values/value_builder.h
+++ b/src/mongo/db/exec/sbe/values/value_builder.h
@@ -323,7 +323,10 @@ public:
         auto bufferLen = _valueBufferBuilder->len();
         size_t bufIdx = 0;
         size_t rowIdx = 0;
-        while (bufIdx < _numValues) {
+        // The 'row' output parameter might be smaller than the number of values owned by this
+        // builder. Be careful to only read as many values into 'row' as this output 'row' has space
+        // for.
+        while (rowIdx < row.size()) {
             invariant(rowIdx < row.size());
             auto [_, tagNothing, valNothing] = getValue(bufIdx++, bufferLen);
             tassert(6136200, "sbe tag must be 'Boolean'", tagNothing == TypeTags::Boolean);
diff --git a/src/mongo/db/exec/sbe/vm/vm.cpp b/src/mongo/db/exec/sbe/vm/vm.cpp
index 6d0bc2f38e7..3a8e48daf0a 100644
--- a/src/mongo/db/exec/sbe/vm/vm.cpp
+++ b/src/mongo/db/exec/sbe/vm/vm.cpp
@@ -4219,6 +4219,41 @@ FastTuple<bool, value::TypeTags, value::Value> ByteCode::aggSetUnionCappedImpl(
     return {ownAcc, tagAcc, valAcc};
 }
 
+FastTuple<bool, value::TypeTags, value::Value> ByteCode::builtinAggSetUnion(ArityType arity) {
+    auto [ownAcc, tagAcc, valAcc] = getFromStack(0);
+
+    if (tagAcc == value::TypeTags::Nothing) {
+        // Initialize the accumulator.
+        ownAcc = true;
+        std::tie(tagAcc, valAcc) = value::makeNewArraySet();
+    } else {
+        // Take ownership of the accumulator.
+        topStack(false, value::TypeTags::Nothing, 0);
+    }
+
+    tassert(7039552, "accumulator must be owned", ownAcc);
+    value::ValueGuard guardAcc{tagAcc, valAcc};
+    tassert(7039553, "accumulator must be of type ArraySet", tagAcc == value::TypeTags::ArraySet);
+    auto acc = value::getArraySetView(valAcc);
+
+    auto [tagNewSet, valNewSet] = moveOwnedFromStack(1);
+    value::ValueGuard guardNewSet{tagNewSet, valNewSet};
+    if (!value::isArray(tagNewSet)) {
+        return {false, value::TypeTags::Nothing, 0};
+    }
+
+    auto i = value::ArrayEnumerator{tagNewSet, valNewSet};
+    while (!i.atEnd()) {
+        auto [elTag, elVal] = i.getViewOfValue();
+        auto [copyTag, copyVal] = value::copyValue(elTag, elVal);
+        acc->push_back(copyTag, copyVal);
+        i.advance();
+    }
+
+    guardAcc.reset();
+    return {ownAcc, tagAcc, valAcc};
+}
+
 FastTuple<bool, value::TypeTags, value::Value> ByteCode::builtinAggSetUnionCapped(ArityType arity) {
     auto [tagNewElem, valNewElem] = moveOwnedFromStack(1);
     value::ValueGuard guardNewElem{tagNewElem, valNewElem};
@@ -5354,6 +5389,8 @@ FastTuple<bool, value::TypeTags, value::Value> ByteCode::dispatchBuiltin(Builtin
             return builtinConcatArrays(arity);
         case Builtin::aggConcatArraysCapped:
             return builtinAggConcatArraysCapped(arity);
+        case Builtin::aggSetUnion:
+            return builtinAggSetUnion(arity);
         case Builtin::aggSetUnionCapped:
             return builtinAggSetUnionCapped(arity);
         case Builtin::aggCollSetUnionCapped:
@@ -5551,6 +5588,8 @@ std::string builtinToString(Builtin b) {
             return "concatArrays";
         case Builtin::aggConcatArraysCapped:
             return "aggConcatArraysCapped";
+        case Builtin::aggSetUnion:
+            return "aggSetUnion";
         case Builtin::aggSetUnionCapped:
             return "aggSetUnionCapped";
         case Builtin::aggCollSetUnionCapped:
diff --git a/src/mongo/db/exec/sbe/vm/vm.h b/src/mongo/db/exec/sbe/vm/vm.h
index 0ed76cefad0..b6ee6d2dc75 100644
--- a/src/mongo/db/exec/sbe/vm/vm.h
+++ b/src/mongo/db/exec/sbe/vm/vm.h
@@ -669,6 +669,8 @@ enum class Builtin : uint8_t {
     // specified size.
     aggSetUnionCapped,
     aggCollSetUnionCapped,
+    // Agg function for a simple set union (with no size cap or collation).
+    aggSetUnion,
 
     acos,
     acosh,
@@ -1289,6 +1291,7 @@ private:
     FastTuple<bool, value::TypeTags, value::Value> builtinConcat(ArityType arity);
     FastTuple<bool, value::TypeTags, value::Value> builtinConcatArrays(ArityType arity);
     FastTuple<bool, value::TypeTags, value::Value> builtinAggConcatArraysCapped(ArityType arity);
+    FastTuple<bool, value::TypeTags, value::Value> builtinAggSetUnion(ArityType arity);
     FastTuple<bool, value::TypeTags, value::Value> builtinAggSetUnionCapped(ArityType arity);
     FastTuple<bool, value::TypeTags, value::Value> builtinAggCollSetUnionCapped(ArityType arity);
     FastTuple<bool, value::TypeTags, value::Value> aggSetUnionCappedImpl(
diff --git a/src/mongo/db/query/sbe_stage_builder.cpp b/src/mongo/db/query/sbe_stage_builder.cpp
index d48ee06aa50..186b877c5ec 100644
--- a/src/mongo/db/query/sbe_stage_builder.cpp
+++ b/src/mongo/db/query/sbe_stage_builder.cpp
@@ -2248,12 +2248,11 @@ std::tuple<sbe::value::SlotVector, EvalStage, std::unique_ptr<sbe::EExpression>>
     return {sbe::value::SlotVector{slot}, std::move(stage), nullptr};
 }
 
-sbe::value::SlotVector generateAccumulator(
-    StageBuilderState& state,
-    const AccumulationStatement& accStmt,
-    const PlanStageSlots& outputs,
-    sbe::value::SlotIdGenerator* slotIdGenerator,
-    sbe::value::SlotMap<std::unique_ptr<sbe::EExpression>>& accSlotToExprMap) {
+sbe::value::SlotVector generateAccumulator(StageBuilderState& state,
+                                           const AccumulationStatement& accStmt,
+                                           const PlanStageSlots& outputs,
+                                           sbe::value::SlotIdGenerator* slotIdGenerator,
+                                           sbe::SlotExprPairVector& accSlotExprPairs) {
     auto rootSlot = outputs.getIfExists(PlanStageSlots::kResult);
     auto argExpr = generateExpression(state, accStmt.expr.argument.get(), rootSlot, &outputs);
 
@@ -2268,12 +2267,48 @@ sbe::value::SlotVector generateAccumulator(
     for (auto& accExpr : accExprs) {
         auto slot = slotIdGenerator->generate();
         aggSlots.push_back(slot);
-        accSlotToExprMap.emplace(slot, std::move(accExpr));
+        accSlotExprPairs.push_back({slot, std::move(accExpr)});
     }
 
     return aggSlots;
 }
 
+/**
+ * Generate a vector of (inputSlot, mergingExpression) pairs. The slot (whose id is allocated by
+ * this function) will be used to store spilled partial aggregate values that have been recovered
+ * from disk and deserialized. The merging expression is an agg function which combines these
+ * partial aggregates.
+ *
+ * Usually the returned vector will be of length 1, but in some cases the MQL accumulation statement
+ * is implemented by calculating multiple separate aggregates in the SBE plan, which are finalized
+ * by a subsequent project stage to produce the ultimate value.
+ */
+sbe::SlotExprPairVector generateMergingExpressions(StageBuilderState& state,
+                                                   const AccumulationStatement& accStmt,
+                                                   int numInputSlots) {
+    tassert(7039555, "'numInputSlots' must be positive", numInputSlots > 0);
+    auto slotIdGenerator = state.slotIdGenerator;
+    tassert(7039556, "expected non-null 'slotIdGenerator' pointer", slotIdGenerator);
+    auto frameIdGenerator = state.frameIdGenerator;
+    tassert(7039557, "expected non-null 'frameIdGenerator' pointer", frameIdGenerator);
+
+    auto spillSlots = slotIdGenerator->generateMultiple(numInputSlots);
+    auto collatorSlot = state.data->env->getSlotIfExists("collator"_sd);
+    auto mergingExprs =
+        buildCombinePartialAggregates(accStmt, spillSlots, collatorSlot, *frameIdGenerator);
+
+    // Zip the slot vector and expression vector into a vector of pairs.
+    tassert(7039550,
+            "expected same number of slots and input exprs",
+            spillSlots.size() == mergingExprs.size());
+    sbe::SlotExprPairVector result;
+    result.reserve(spillSlots.size());
+    for (size_t i = 0; i < spillSlots.size(); ++i) {
+        result.push_back({spillSlots[i], std::move(mergingExprs[i])});
+    }
+    return result;
+}
+
 std::tuple<std::vector<std::string>, sbe::value::SlotVector, EvalStage> generateGroupFinalStage(
     StageBuilderState& state,
     EvalStage groupEvalStage,
@@ -2517,11 +2552,23 @@ std::pair<std::unique_ptr<sbe::PlanStage>, PlanStageSlots> SlotBasedStageBuilder
     // Translates accumulators which are executed inside the group stage and gets slots for
     // accumulators.
     stage_builder::EvalStage currentStage = std::move(groupByEvalStage);
-    sbe::value::SlotMap<std::unique_ptr<sbe::EExpression>> accSlotToExprMap;
+    sbe::SlotExprPairVector accSlotExprPairs;
     std::vector<sbe::value::SlotVector> aggSlotsVec;
+    // Since partial accumulator state may be spilled to disk and then merged, we must construct not
+    // only the basic agg expressions for each accumulator, but also agg expressions that are used
+    // to combine partial aggregates that have been spilled to disk.
+    sbe::SlotExprPairVector mergingExprs;
     for (const auto& accStmt : accStmts) {
-        aggSlotsVec.emplace_back(generateAccumulator(
-            _state, accStmt, childOutputs, &_slotIdGenerator, accSlotToExprMap));
+        sbe::value::SlotVector curAggSlots =
+            generateAccumulator(_state, accStmt, childOutputs, &_slotIdGenerator, accSlotExprPairs);
+
+        sbe::SlotExprPairVector curMergingExprs =
+            generateMergingExpressions(_state, accStmt, curAggSlots.size());
+
+        aggSlotsVec.emplace_back(std::move(curAggSlots));
+        mergingExprs.insert(mergingExprs.end(),
+                            std::make_move_iterator(curMergingExprs.begin()),
+                            std::make_move_iterator(curMergingExprs.end()));
     }
 
     // There might be duplicated expressions and slots. Dedup them before creating a HashAgg
@@ -2531,9 +2578,10 @@ std::pair<std::unique_ptr<sbe::PlanStage>, PlanStageSlots> SlotBasedStageBuilder
     // Builds a group stage with accumulator expressions and group-by slot(s).
     auto groupEvalStage = makeHashAgg(std::move(currentStage),
                                       dedupedGroupBySlots,
-                                      std::move(accSlotToExprMap),
+                                      std::move(accSlotExprPairs),
                                       _state.data->env->getSlotIfExists("collator"_sd),
                                       _cq.getExpCtx()->allowDiskUse,
+                                      std::move(mergingExprs),
                                       nodeId);
 
     tassert(
diff --git a/src/mongo/db/query/sbe_stage_builder_helpers.cpp b/src/mongo/db/query/sbe_stage_builder_helpers.cpp
index 29da8e82b08..70e4693866c 100644
--- a/src/mongo/db/query/sbe_stage_builder_helpers.cpp
+++ b/src/mongo/db/query/sbe_stage_builder_helpers.cpp
@@ -485,15 +485,20 @@ EvalStage makeUnion(std::vector<EvalStage> inputStages,
 
 EvalStage makeHashAgg(EvalStage stage,
                       sbe::value::SlotVector gbs,
-                      sbe::value::SlotMap<std::unique_ptr<sbe::EExpression>> aggs,
+                      sbe::SlotExprPairVector aggs,
                       boost::optional<sbe::value::SlotId> collatorSlot,
                       bool allowDiskUse,
+                      sbe::SlotExprPairVector mergingExprs,
                       PlanNodeId planNodeId) {
     stage.setOutSlots(gbs);
     for (auto& [slot, _] : aggs) {
         stage.addOutSlot(slot);
     }
 
+    // In debug builds, we artificially force frequent spilling. This makes sure that our tests
+    // exercise the spilling algorithm and the associated logic for merging partial aggregates which
+    // otherwise would require large data sizes to exercise.
+    const bool forceIncreasedSpilling = kDebugBuild && allowDiskUse;
     stage.setStage(sbe::makeS<sbe::HashAggStage>(stage.extractStage(planNodeId),
                                                  std::move(gbs),
                                                  std::move(aggs),
@@ -501,7 +506,10 @@ EvalStage makeHashAgg(EvalStage stage,
                                                  true /* optimized close */,
                                                  collatorSlot,
                                                  allowDiskUse,
-                                                 planNodeId));
+                                                 std::move(mergingExprs),
+                                                 planNodeId,
+                                                 true /* participateInTrialRunTracking */,
+                                                 forceIncreasedSpilling));
     return stage;
 }
 
diff --git a/src/mongo/db/query/sbe_stage_builder_helpers.h b/src/mongo/db/query/sbe_stage_builder_helpers.h
index 06e67456b11..fdf0ea39a02 100644
--- a/src/mongo/db/query/sbe_stage_builder_helpers.h
+++ b/src/mongo/db/query/sbe_stage_builder_helpers.h
@@ -37,6 +37,7 @@
 #include "mongo/db/exec/sbe/expressions/expression.h"
 #include "mongo/db/exec/sbe/match_path.h"
 #include "mongo/db/exec/sbe/stages/filter.h"
+#include "mongo/db/exec/sbe/stages/hash_agg.h"
 #include "mongo/db/exec/sbe/stages/makeobj.h"
 #include "mongo/db/exec/sbe/stages/project.h"
 #include "mongo/db/pipeline/expression.h"
@@ -424,9 +425,10 @@ EvalStage makeUnion(std::vector<EvalStage> inputStages,
 
 EvalStage makeHashAgg(EvalStage stage,
                       sbe::value::SlotVector gbs,
-                      sbe::value::SlotMap<std::unique_ptr<sbe::EExpression>> aggs,
+                      sbe::SlotExprPairVector aggs,
                       boost::optional<sbe::value::SlotId> collatorSlot,
                       bool allowDiskUse,
+                      sbe::SlotExprPairVector mergingExprs,
                       PlanNodeId planNodeId);
 
 EvalStage makeMkBsonObj(EvalStage stage,
diff --git a/src/mongo/db/query/sbe_stage_builder_lookup.cpp b/src/mongo/db/query/sbe_stage_builder_lookup.cpp
index 4a4a5d396b6..2c57fe6374f 100644
--- a/src/mongo/db/query/sbe_stage_builder_lookup.cpp
+++ b/src/mongo/db/query/sbe_stage_builder_lookup.cpp
@@ -364,12 +364,15 @@ std::pair<SlotId /* keyValuesSetSlot */, std::unique_ptr<sbe::PlanStage>> buildK
     // Re-pack the individual key values into a set. We don't cap "addToSet" here because its size
     // is bounded by the size of the record.
     SlotId keyValuesSetSlot = slotIdGenerator.generate();
+    SlotId spillSlot = slotIdGenerator.generate();
     EvalStage packedKeyValuesStage = makeHashAgg(
         EvalStage{std::move(keyValuesStage), SlotVector{}},
         makeSV(), /* groupBy slots - "none" means creating a single group */
-        makeEM(keyValuesSetSlot, makeFunction("addToSet"_sd, makeVariable(keyValueSlot))),
+        makeSlotExprPairVec(keyValuesSetSlot,
+                            makeFunction("addToSet"_sd, makeVariable(keyValueSlot))),
         boost::none /* we group _all_ key values into a single set, so collator is irrelevant */,
         allowDiskUse,
+        makeSlotExprPairVec(spillSlot, makeFunction("aggSetUnion"_sd, makeVariable(spillSlot))),
         nodeId);
 
     // The set in 'keyValuesSetSlot' might end up empty if the localField contained only missing and
@@ -411,15 +414,20 @@ std::pair<SlotId /* resultSlot */, std::unique_ptr<sbe::PlanStage>> buildForeign
     // are no matches, return an empty array.
     const int sizeCap = internalLookupStageIntermediateDocumentMaxSizeBytes.load();
     SlotId accumulatorSlot = slotIdGenerator.generate();
+    SlotId spillSlot = slotIdGenerator.generate();
     innerBranch = makeHashAgg(
         std::move(innerBranch),
         makeSV(), /* groupBy slots */
-        makeEM(accumulatorSlot,
-               makeFunction("addToArrayCapped"_sd,
-                            makeVariable(foreignRecordSlot),
-                            makeConstant(TypeTags::NumberInt32, sizeCap))),
+        makeSlotExprPairVec(accumulatorSlot,
+                            makeFunction("addToArrayCapped"_sd,
+                                         makeVariable(foreignRecordSlot),
+                                         makeConstant(TypeTags::NumberInt32, sizeCap))),
         {} /* collatorSlot, no collation here because we want to return all matches "as is" */,
         allowDiskUse,
+        makeSlotExprPairVec(spillSlot,
+                            makeFunction("aggConcatArraysCapped",
+                                         makeVariable(spillSlot),
+                                         makeConstant(TypeTags::NumberInt32, sizeCap))),
         nodeId);
 
     // 'accumulatorSlot' is either Nothing or contains an array of size two, where the front element