Re: foreign table creation and NOT VALID check constraints

On Tue, Aug 1, 2017 at 2:41 PM, Amit Langote
<Langote_Amit_f8(at)lab(dot)ntt(dot)co(dot)jp> wrote:
> On 2017/08/01 17:54, Simon Riggs wrote:
>> On 1 August 2017 at 08:37, Amit Langote <Langote_Amit_f8(at)lab(dot)ntt(dot)co(dot)jp> wrote:
>>> On 2017/08/01 15:22, Simon Riggs wrote:
>>>> On 1 August 2017 at 07:16, Amit Langote <Langote_Amit_f8(at)lab(dot)ntt(dot)co(dot)jp> wrote:
>>>>> In f27a6b15e656 (9.6 & later), we decided to "Mark CHECK constraints
>>>>> declared NOT VALID valid if created with table." In retrospect,
>>>>> constraints on foreign tables should have been excluded from consideration
>>>>> in that commit, because the thinking behind the aforementioned commit
>>>>> (that the constraint is trivially validated because the newly created
>>>>> table contains no data) does not equally apply to the foreign tables case.
>>>>>
>>>>> Should we do something about that?
>>>>
>>>> In what way does it not apply? Do you have a failure case?
>>>
>>> Sorry for not mentioning the details.
>>>
>>> I was thinking that a foreign table starts containing the data of the
>>> remote object it points to the moment it's created (unlike local tables
>>> which contain no data to begin with). If a user is not sure whether a
>>> particular constraint being created in the same command holds for the
>>> remote data, they may mark it as NOT VALID and hope that the system treats
>>> the constraint as such until such a time that they mark it valid by
>>> running ALTER TABLE VALIDATE CONSTRAINT. Since the planner is the only
>>> consumer of pg_constraint.convalidated, that means the user expects the
>>> planner to ignore such a constraint. Since f27a6b15e656, users are no
>>> longer able to expect so.
>>
>> For Foreign Tables, it sounds like an issue. Don't think it exists for
>> normal tables.
>
> Yes. I was saying in my first email that we should not disregard user's
> request to mark a constraint NOT VALID if the table in question is a
> *foreign table*.
>
> So, it's OK that commit f27a6b15e656 changed things to ignore NOT VALID if
> it's in the following command:
>
> create table foo (a int, constraint check_a check (a > 0) not valid);
>
> But, not OK in the following command:
>
> create foreign table ffoo (
> a int,
> constraint check_a check (a > 0) not valid
> ) server loopback options (table_name 'foo');
>

If the user has specified "not valid" for a constraint on the foreign
table, there is high chance that s/he is aware of the fact that the
remote table that the foreign table points to has some rows which will
violet the constraint. So, +1.

--
Best Wishes,
Ashutosh Bapat
EnterpriseDB Corporation
The Postgres Database Company