Re: Do we want a hashset type?

On Tue, Jun 20, 2023, at 14:10, Tomas Vondra wrote:
> On 6/20/23 12:59, Joel Jacobson wrote:
>> On Mon, Jun 19, 2023, at 02:00, jian he wrote:
>>> select hashset_contains('{1,2}'::int4hashset,NULL::int);
>>> should return null?
>> 
>> I agree, it should.
>> 
>> I've now changed all functions except int4hashset() (the init function)
>> and the aggregate functions to be STRICT.
>
> I don't think this is correct / consistent with what we do elsewhere.
> IMHO it's perfectly fine to have a hashset containing a NULL value,

The reference to consistency with what we do elsewhere might not be entirely
applicable in this context, since the set feature we're designing is a new beast
in the SQL landscape.

I think adhering to the theoretical purity of sets by excluding NULLs aligns us
with set theory, simplifies our code, and parallels set implementations in other
languages.

I think we have an opportunity here to innovate and potentially influence a
future set concept in the SQL standard.

However, I see how one could argue against this reasoning, on the basis that
PostgreSQL users might be more familiar with and expect NULLs can exist
everywhere in all data structures.

A different perspective is to look at what use-cases we can foresee.

I've been trying hard, but I can't find compelling use-cases where a NULL element
in a set would offer a more natural SQL query than handling NULLs within SQL and
keeping the set NULL-free.

Does anyone else have a strong realistic example where including NULLs in the
set would simplify the SQL query?

/Joel