fix serialize with root

be/src/vec/columns/column_object.cpp

@@ -1836,8 +1836,25 @@ struct Prefix {
     bool root_is_first_flag = true;
 };
 
+bool ColumnObject::is_visible_root_value(size_t nrow) const {
+    if (is_null_root()) {
+        return false;
+    }
+    const auto& nullable = assert_cast<const ColumnNullable&>(*get_root());
+    if (nullable.is_null_at(nrow)) {
+        return false;
+    }
+    const auto& value_column = assert_cast<const ColumnString&>(nullable.get_nested_column());
+    return !value_column.get_data_at(nrow).empty();
+}
+
 Status ColumnObject::serialize_one_row_to_json_format(int64_t row_num, BufferWritable& output,
                                                       bool* is_null) const {
+    // root is not eighther null or empty, we should only process root value
+    if (is_visible_root_value(row_num)) {
+        subcolumns.get_root()->data.serialize_text_json(row_num, output);
+        return Status::OK();
+    }
     const auto& column_map = assert_cast<const ColumnMap&>(*serialized_sparse_column);
     const auto& sparse_data_offsets = column_map.get_offsets();
     const auto [sparse_data_paths, sparse_data_values] = get_sparse_data_paths_and_values();
@@ -1850,9 +1867,13 @@ Status ColumnObject::serialize_one_row_to_json_format(int64_t row_num, BufferWri
     // For example:
     // b.c, a.b, a.a, b.e, g, h.u.t -> a.a, a.b, b.c, b.e, g, h.u.t -> {"a" : {"a" : ..., "b" : ...}, "b" : {"c" : ..., "e" : ...}, "g" : ..., "h" : {"u" : {"t" : ...}}}.
     std::vector<String> sorted_paths;
-    std::map<std::string, Subcolumn> subcolumn_path_map;
+    std::unordered_map<std::string, Subcolumn> subcolumn_path_map;
     sorted_paths.reserve(get_subcolumns().size() + (sparse_data_end - sparse_data_offset));
     for (const auto& subcolumn : get_subcolumns()) {
+        // Skip root value, we have already processed it
+        if (subcolumn->data.is_root) {
+            continue;
+        }
         /// We consider null value and absence of the path in a row as equivalent cases, because we cannot actually distinguish them.
         /// So, we don't output null values at all.
         if (!subcolumn->data.is_null_at(row_num)) {

be/src/vec/columns/column_object.h

@@ -631,6 +631,8 @@ class ColumnObject final : public COWHelper<IColumn, ColumnObject> {
             size_t start, size_t length);
 
     bool try_add_new_subcolumn(const PathInData& path);
+
+    bool is_visible_root_value(size_t nrow) const;
 };
 
 } // namespace doris::vectorized