Method prefetchForMapColumn

src/Storages/MergeTree/IMergeTreeReader.cpp:432–461

}

void IMergeTreeReader::prefetchForMapColumn(
    Priority priority,
    const NameAndTypePair & name_and_type,
    size_t from_mark,
    bool continue_reading,
    size_t current_task_last_mark)
{
    const auto & [name, type] = name_and_type;
    // collect all the substreams based on map column's name and its keys substream.
    // and somehow construct runtime two implicit columns(key&value) representation.
    auto keys_iter = map_column_keys.equal_range(name);
    const DataTypeMap & type_map = typeid_cast<const DataTypeMap &>(*type);
    DataTypePtr impl_value_type = type_map.getValueTypeForImplicitColumn();

    for (auto kit = keys_iter.first; kit != keys_iter.second; ++kit)
    {
        String impl_key_name = getImplicitColNameForMapKey(name, kit->second);
        NameAndTypePair implicit_key_name_and_type{impl_key_name, impl_value_type};
        auto cache = caches[implicit_key_name_and_type.getNameInStorage()];

        // If MAP implicit column and MAP column co-exist in columns, implicit column should
        // only read only once.
        if (dup_implicit_keys.count(impl_key_name) != 0)
            continue;

        const SerializationPtr & serialization = serializations[impl_key_name];
        prefetchForColumn(priority, implicit_key_name_and_type, serialization, from_mark,
            continue_reading, current_task_last_mark, cache);
    }
}

void IMergeTreeReader::prefetchForColumn(
    Priority priority,