Existing: (I realize this was already here.)

I think it would be cheaper to use a new `JoinedRow` here instead of `buildRow`.  `buildRow` is going to allocate a new Array and copy all the values instead of just the values for `streamedRow`.  You will also need to call `copy()` on `streamedRow`.

A similar trick could be done below to avoid creating a new Array of nulls for each output row.