> It might be worth to investigate this tradeoff, or just to provide an
> option to rebuild indices without using temporary sort spaces so others
> can measure it for themselves (or to have a workaround if disk space is
> not large enough to accommodate the rebuild).

You already can do it by yourself. Run and compare

1. good and fast
- create table
- insert a lot of records
- create index

and

2. bad and slow
- create table
- create index
- insert a lot of records

Regards,
Vlad