GitLab

Doesn't allow instant alter of a field which is a prefix part of some key

is_part_of_a_key_prefix(): I hope the name of the function is clear

ha_innobase::can_convert_string()
ha_innobase::can_convert_varstring()
ha_innobase::can_convert_blob(): it can't when field is_part_of_a_key_prefix()