Re: Do we want a hashset type?

[jian he]

On Tue, Jun 27, 2023 at 4:55 AM Joel Jacobson <j...@compiler.org> wrote:
>
> On Mon, Jun 26, 2023, at 13:06, jian he wrote:
> > Can you try to glue the attached to the hashset data type input
> > function.
> > the attached will parse cstring with double quote and not. so '{1,2,3}'
> > == '{"1","2","3"}'. obviously quote will preserve the inner string as
> > is.
> > currently int4hashset input is delimited by comma, if you want deal
> > with range then you need escape the comma.
>
> Not sure what you're trying to do here; what's the problem with
> the current int4hashset_in()?
>
> I think it might be best to focus on null semantics / three-valued logic
> before moving on and trying to implement support for more types,
> otherwise we would need to rewrite more code if we find general
> thinkos that are problems in all types.
>
> Help wanted to reason about what the following queries should return:
>
> SELECT hashset_union(NULL::int4hashset, '{}'::int4hashset);
>
> SELECT hashset_intersection(NULL::int4hashset, '{}'::int4hashset);
>
> SELECT hashset_difference(NULL::int4hashset, '{}'::int4hashset);
>
> SELECT hashset_symmetric_difference(NULL::int4hashset, '{}'::int4hashset);
>
> Should they return NULL, the empty set or something else?
>
> I've renamed hashset_merge() -> hashset_union() to better match
> SQL's MULTISET feature which has a MULTISET UNION.
>
> /Joel

in SQLMultiSets.pdf(previously thread) I found a related explanation
on page 45, 46.

(CASE WHEN OP1 IS NULL OR OP2 IS NULL THEN NULL ELSE MULTISET ( SELECT
T1.V FROM UNNEST (OP1) AS T1 (V) INTERSECT SQ SELECT T2.V FROM UNNEST
(OP2) AS T2 (V) ) END)

CASE WHEN OP1 IS NULL OR OP2 IS NULL THEN NULL ELSE MULTISET ( SELECT
T1.V FROM UNNEST (OP1) AS T1 (V) UNION SQ SELECT T2.V FROM UNNEST
(OP2) AS T2 (V) ) END

(CASE WHEN OP1 IS NULL OR OP2 IS NULL THEN NULL ELSE MULTISET ( SELECT
T1.V FROM UNNEST (OP1) AS T1 (V) EXCEPT SQ SELECT T2.V FROM UNNEST
(OP2) AS T2 (V) ) END)

In page11,
>
> Unlike the corresponding table operators UNION, INTERSECT and EXCEPT, we have 
> chosen ALL as the default, since this is the most natural interpretation of 
> MULTISET UNION, etc

also in page 11 aggregate name FUSION. (I like the name.................)