Ordinary database engine is deprecated but there is no simple mean to change database engine to Atomic for existing databases.

Possible options:
- ALTER DATABASE MODIFY ENGINE query.
- Flag that triggers migration at ClickHouse startup.

The latter should be easier to implement. 

{
    if (isFinished())
    {
        
        
        
        streams.clear();
        return {};
    }
    
    size_t max_rows_to_read = std::min(block_size, rows_limit - rows_read);
    std::unordered_map<String, ISerialization::SubstreamsCache> caches;
    Block res;
    for (const auto & name_type : columns)