Problem is not about intervals and doesn't actually cause 'full table scan'.
We have an optimization for DISTINCT when we have
'DISTINCT field_from_first_join_table' we don't need to read all the
rows from the JOIN-ed table if we found one conforming row.
It stopped working in 5.0 as we return NESTED_LOOP_OK if we came upon
that case in the evaluate_join_record() and that doesn't break the
recordreading loop in sub_select().

Fixed by returning NESTED_LOOP_NO_MORE_ROWS in this case.