[Improve](variant) optimize cast performance when variant only contains null values

be/src/vec/columns/column_variant.cpp

@@ -2090,8 +2090,13 @@ void ColumnVariant::ensure_root_node_type(const DataTypePtr& expected_root_type)
     }
 }
 
-bool ColumnVariant::empty() const {
-    return subcolumns.empty() || subcolumns.begin()->get()->path.get_path() == COLUMN_NAME_DUMMY;
+bool ColumnVariant::only_have_default_values() const {
+    for (const auto& entry : subcolumns) {
+        if (entry->data.least_common_type.get_base_type_id() != PrimitiveType::INVALID_TYPE) {
+            return false;
+        }
+    }
+    return true;
 }
 
 ColumnPtr ColumnVariant::filter(const Filter& filter, ssize_t count) const {

be/src/vec/columns/column_variant.h

@@ -490,7 +490,7 @@ class ColumnVariant final : public COWHelper<IColumn, ColumnVariant> {
     template <typename Func>
     MutableColumnPtr apply_for_columns(Func&& func) const;
 
-    bool empty() const;
+    bool only_have_default_values() const;
 
     // Check if all columns and types are aligned, only in debug mode
     Status sanitize() const;

be/src/vec/functions/cast/cast_to_variant.h

@@ -70,8 +70,7 @@ struct CastFromVariant {
                         {0}, 1, input_rows_count);
             }
         } else {
-            if (variant.empty()) {
-                // TODO not found root cause, a tmp fix
+            if (variant.only_have_default_values()) {
                 col_to->assume_mutable()->insert_many_defaults(input_rows_count);
                 col_to = make_nullable(col_to, true);
             } else if (is_string_type(data_type_to->get_primitive_type())) {

be/test/vec/columns/column_variant_test.cpp

@@ -3799,4 +3799,11 @@ TEST_F(ColumnVariantTest, subcolumn_insert_range_from_test_advanced) {
     }
 }
 
+TEST_F(ColumnVariantTest, test_variant_no_data_insert) {
+    auto variant = ColumnVariant::create(1);
+    variant->insert_many_defaults(10);
+    EXPECT_EQ(variant->size(), 10);
+    EXPECT_TRUE(variant->only_have_default_values());
+}
+
 } // namespace doris::vectorized