>> No, there is no guarantee. It's just that UNION ALL works this way today
>> (preserving the order of the subselects) - and I'm not even sure about
>> that, it may not preserve the order in all cases, with different indexes or
>> partitioning or a parallel plan, etc.
>
> Yeah, that. You can get a parallelized plan today for UNION ALL:
...
> Since the documentation doesn't make a guarantee there is none.
Thanks all for the confirmation.
I'd still suggest documenting the lack of guarantee; yes, mathematically it may be correct to not document lack of guarantees, but users can come with various expectations and misunderstandings (I also wasn't clear on this specifically for UNION ALL), and it's always good to say this kind of thing explicitly.