Minor difference in behavior between +/-

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.

David J.

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