Обсуждение: Minor difference in behavior between +/-

SELECT NULL - date '2023-01-05';

-> result is null
SELECT NULL + date '2023-01-05';

-> result is [42725] ERROR: operator is not unique: unknown + date Hint: Could not choose a best candidate operator. You might need to add explicit type casts. Position: 13

I would expect both of those to be null.

At Wed, 25 Jan 2023 07:12:18 -0700, "David G. Johnston" <david.g.johnston@gmail.com> wrote in 
> On Tuesday, January 24, 2023, Ryan Murphy <ryanmurf@gmail.com> wrote:
> 
> > SELECT NULL - date '2023-01-05';
> > -> result is null
> > SELECT NULL + date '2023-01-05';
> > -> result is [42725] ERROR: operator is not unique: unknown + date Hint:
> > Could not choose a best candidate operator. You might need to add explicit
> > type casts. Position: 13
> >
> >
> There is no bug here.
> 
> 
> > I would expect both of those to be null.
> >
> 
> I get why you expect that but we expend little to no effort to handle null
> in a type agnostic way.  Type inference has to happen first and regardless
> of whether the literal is null.  Type inference failed in the + case but
> not the - case.  That is just happenstance of what operators happen to
> exist in the system.

In a bit more deatil, the type inference mechanism assumes the null as
a date since the other input is a date. In this case, the operator
"date - date" exists but "date + date" doesn't.

I'm not sure whether the SQL standard defines a binary operator should
be "strict/or return null on null input", but it seems to to me that
we assume a case of non-strict binary operators, or maybe we simply
dont' care about the case of null operands.

regards.

-- 
Kyotaro Horiguchi
NTT Open Source Software Center