Function getSubcolumnsOfNested

src/DataTypes/NestedUtils.cpp:464–491

using NameToDataType = std::map<String, DataTypePtr>;

NameToDataType getSubcolumnsOfNested(const NamesAndTypesList & names_and_types)
{
    std::unordered_map<String, NamesAndTypesList> nested;
    for (const auto & name_type : names_and_types)
    {
        /// Skip subcolumns (e.g. `c0.c2.null` derived from `c0.c2 Array(Nullable(Tuple()))`).
        /// They are not real flat-nested columns like `n.a Array(T)`, `n.b Array(T)`.
        if (name_type.isSubcolumn())
            continue;

        const auto * type_arr = typeid_cast<const DataTypeArray *>(name_type.type.get());

        /// Ignore true Nested type, but try to unite flatten arrays to Nested type.
        if (!isNested(name_type.type) && type_arr)
        {
            auto split = splitName(name_type.name);
            if (!split.second.empty())
                nested[split.first].emplace_back(split.second, type_arr->getNestedType());
        }
    }

    std::map<String, DataTypePtr> nested_types;

    for (const auto & [name, elems] : nested)
        nested_types.emplace(name, createNested(elems.getTypes(), elems.getNames()));

    return nested_types;
}

}