(you don't have to strictly follow this form)

A lot of usecases for debugging and introspection.

A clear and concise description of what is the intended usage scenario is.

Thanks to @davenger we have some basic infrastructure for persistent virtual columns. First example is _row_exists column. We need to implement _block_number column which will be hidden by default, but can be queried any time with explicit name in SELECT query. Serialization of the column should be optional (with setting).