Function flattenTupleRecursiveImpl

src/DataTypes/NestedUtils.cpp:306–342

/// with the sole ancestor `t`, never the unrelated path `t.p`.
template <typename LeafCallback>
static void flattenTupleRecursiveImpl(
    const ColumnPtr & column,
    const DataTypePtr & data_type,
    LeafCallback && emit_leaf,
    const String & name_prefix,
    const Strings & ancestors)
{
    const auto * tuple_type = tryGetFlattenableTuple(data_type);
    if (!tuple_type)
    {
        emit_leaf(column, data_type, name_prefix, ancestors);
        return;
    }

    /// If the column is ColumnConst, expand it to a full column first,
    /// so that all leaf columns are always non-const after flattening.
    ColumnPtr materialized_column = column->convertToFullColumnIfConst();
    const auto * column_tuple = assert_cast<const ColumnTuple *>(materialized_column.get());

    const DataTypes & element_types = tuple_type->getElements();
    const Strings & element_names = tuple_type->getElementNames();
    const auto & sub_columns = column_tuple->getColumns();

    /// `name_prefix` is empty only on the column-only flattening path (flattenTupleColumnsRecursive),
    /// where leaf names and ancestors are unused, so skip building them there.
    const bool build_metadata = !name_prefix.empty();

    Strings element_ancestors = ancestors;
    if (build_metadata)
        element_ancestors.push_back(name_prefix);

    for (size_t i = 0; i < element_types.size(); ++i)
    {
        String element_name = build_metadata ? concatenateName(name_prefix, element_names[i]) : String{};
        flattenTupleRecursiveImpl(sub_columns[i], element_types[i], emit_leaf, element_name, element_ancestors);
    }
}

Block flattenTupleRecursive(const Block & block, std::vector<Strings> * flattened_ancestors)
{