YQL-19535: Fix wrong scalar conversion

commit_hash:997d731b3d106421ee68aa2e3e18f18b03ea7a66

2 files changed, 26 insertions, 2 deletions

diff --git a/yql/essentials/minikql/comp_nodes/ut/mkql_blocks_ut.cpp b/yql/essentials/minikql/comp_nodes/ut/mkql_blocks_ut.cpp
index f5c6f86884..1ad2c3c5c5 100644
--- a/yql/essentials/minikql/comp_nodes/ut/mkql_blocks_ut.cpp
+++ b/yql/essentials/minikql/comp_nodes/ut/mkql_blocks_ut.cpp
@@ -264,6 +264,24 @@ Y_UNIT_TEST(TestScalar) {
     UNIT_ASSERT_VALUES_EQUAL(TArrowBlock::From(value).GetDatum().scalar_as<arrow::UInt64Scalar>().value, testValue);
 }
 
+template<auto Type, typename ArrowType>
+void TestContainerForStringType() {
+    TSetup<false> setup;
+    auto dataLiteral = setup.PgmBuilder->NewDataLiteral<Type>("\"Just a string\"");
+    const auto dataAfterBlocks = setup.PgmBuilder->AsScalar(dataLiteral);
+    const auto graph = setup.BuildGraph(dataAfterBlocks);
+    const auto value = graph->GetValue();
+
+    UNIT_ASSERT(typeid(*TArrowBlock::From(value).GetDatum().scalar()) == typeid(ArrowType));
+}
+
+Y_UNIT_TEST(TestStringTypesHasAppropriateContainer) {
+    TestContainerForStringType<NUdf::EDataSlot::Utf8, arrow::StringScalar>();
+    TestContainerForStringType<NUdf::EDataSlot::Json, arrow::StringScalar>();
+    TestContainerForStringType<NUdf::EDataSlot::Yson, arrow::BinaryScalar>();
+    TestContainerForStringType<NUdf::EDataSlot::String, arrow::BinaryScalar>();
+}
+
 Y_UNIT_TEST_LLVM(TestReplicateScalar) {
     const ui64 count = 1000;
     const ui32 value = 42;
diff --git a/yql/essentials/minikql/computation/mkql_block_impl.cpp b/yql/essentials/minikql/computation/mkql_block_impl.cpp
index 2920a1ac3c..49d52eb0b8 100644
--- a/yql/essentials/minikql/computation/mkql_block_impl.cpp
+++ b/yql/essentials/minikql/computation/mkql_block_impl.cpp
@@ -114,8 +114,14 @@ arrow::Datum DoConvertScalar(TType* type, const T& value, arrow::MemoryPool& poo
             const auto& str = value.AsStringRef();
             std::shared_ptr<arrow::Buffer> buffer(ARROW_RESULT(arrow::AllocateBuffer(str.Size(), &pool)));
             std::memcpy(buffer->mutable_data(), str.Data(), str.Size());
-            auto type = (slot == NUdf::EDataSlot::String || slot == NUdf::EDataSlot::Yson || slot == NUdf::EDataSlot::JsonDocument) ? arrow::binary() : arrow::utf8();
-            std::shared_ptr<arrow::Scalar> scalar = std::make_shared<arrow::BinaryScalar>(buffer, type);
+            std::shared_ptr<arrow::Scalar> scalar;
+            if (slot == NUdf::EDataSlot::String || slot == NUdf::EDataSlot::Yson || slot == NUdf::EDataSlot::JsonDocument) {
+                scalar = std::make_shared<arrow::BinaryScalar>(buffer, arrow::binary());
+            } else {
+                // NOTE: Do not use |arrow::BinaryScalar| for utf8 and json types directly.
+                // This is necessary so that the type of the scalar is clearly preserved at runtime.
+                scalar = std::make_shared<arrow::StringScalar>(buffer);
+            }
             return arrow::Datum(scalar);
         }
         case NUdf::EDataSlot::TzDate: {