Feature Request: Support columnar for execute_iter

[KenKi0]

It would be nice to implement columnar logic for execute_iter if we have chunk_size > 1. I get chunks of 1 mln values and then I have to go through them again to turn them into a columnar view, in which they are initially and are obtained as I understood from the code, because for each chunk from the socket there is called transposed, which is an unnecessary action for me and the work of this function takes almost half of the query time.