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. 

protected:
    Chunk generate() override
    {
        Block res;
        start();
        if (block_in)
        {
            res = block_in->read();
            
            if (!res)
            {
                block_in.reset();
                data_in.reset();
                indices.reset();
            }
        }