> I'm reading release notes, and I saw this about GTT:
> "references between persistent and temporary tables are forbidden"
>
> So it isn't possible to do something like this? if yes, is it possible
> to know the reason?
>
> create table t1 (
> field integer unique
> );
>
> create global temporary table gtt (
> field integer references t1(field)
> );
>
> or is there only a restriction about static tables referencing temporary?

No, this is not possible in both directions.
Reasons is : (a) SQL Standard and (b) practical.

Regards,
Vlad