Re: Do we want a hashset type?

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,
because then it can affect results of membership checks.

Consider these IN / ANY queries:

    test=# select 4 in (1,2,3);
     ?column?
    ----------
     f
    (1 row)

    test=# select 4 = ANY(ARRAY[1,2,3]);
     ?column?
    ----------
     f
    (1 row)

now add a NULL:

    test=# select 4 in (1,2,3,null);
     ?column?
    ----------

    (1 row)

    test=# select 4 = ANY(ARRAY[1,2,3,NULL]);
     ?column?
    ----------

    (1 row)

I don't see why a (hash)set should behave any differently. It's true
arrays don't behave like this:

    test=# select array[1,2,3,4,NULL] @> ARRAY[5];
     ?column?
    ----------
     f
    (1 row)

but I'd say that's more an anomaly than something we should replicate.

This is also what the SQL standard does for multisets - there's SQL:20nn
draft at http://www.wiscorp.com/SQLStandards.html, and the <member
predicate> section (p. 475) explains how this should work with NULL.

So if we see a set as a special case of multiset (with no duplicates),
then we have to handle NULLs this way too. It'd be weird to have this
behavior inconsistent.


regards

-- 
Tomas Vondra
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company