Re: Do we want a hashset type?

On Thu, Jun 22, 2023, at 07:51, Joel Jacobson wrote:
> For instance, how should hashset_count() work?
>
> Given the query,
>
> SELECT hashset_count('{1,2,3,null}'::int4hashset);
>
> Should we,
>
> a) threat NULL as a distinct value and return 4?
>
> b) ignore NULL and return 3?
>
> c) return NULL? (since the presence of NULL can be thought to render
> the entire count indeterminate)
>
> I think my personal preference is (b) since it is then consistent with
> how COUNT() works.

Having thought a bit more on this matter,
I think it's better to remove hashset_count() since the semantics are not obvious,
and instead provide a hashset_cardinality() function, that would obviously
include a possible null value in the number of elements:

SELECT hashset_cardinality('{1,2,3,null}'::int4hashset);
4

SELECT hashset_cardinality('{null}'::int4hashset);
1

SELECT hashset_cardinality('{null,null}'::int4hashset);
1

SELECT hashset_cardinality('{}'::int4hashset);
0

SELECT hashset_cardinality(NULL::int4hashset);
NULL

Sounds good?

/Joel