Message-ID: <52E5F3FD.6050000@lab.ntt.co.jp>
Date: Mon, 27 Jan 2014 14:51:57 +0900
From: Etsuro Fujita <fujita.etsuro@lab.ntt.co.jp>
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Subject: Re: inherit support for foreign tables
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(2014/01/22 4:09), Robert Haas wrote:
> On Mon, Jan 20, 2014 at 9:44 PM, Shigeru Hanada
> <shigeru.hanada@gmail.com> wrote:
>> Thanks for the comments.
>>
>> 2014/1/21 KaiGai Kohei <kaigai@ak.jp.nec.com>:
>>>> In addition, an idea which I can't throw away is to assume that all
>>>> constraints defined on foreign tables as ASSERTIVE.  Foreign tables
>>>> potentially have dangers to have "wrong" data by updating source data
>>>> not through foreign tables.  This is not specific to an FDW, so IMO
>>>> constraints defined on foreign tables are basically ASSERTIVE.  Of
>>>> course PG can try to maintain data correct, but always somebody might
>>>> break it.
>>>> qu
>>>>
>>> Does it make sense to apply "assertive" CHECK constraint on the qual
>>> of ForeignScan to filter out tuples with violated values at the local
>>> side, as if row-level security feature doing.
>>> It enables to handle a situation that planner expects only "clean"
>>> tuples are returned but FDW driver is unavailable to anomalies.
>>>
>>> Probably, this additional check can be turned on/off on the fly,
>>> if FDW driver has a way to inform the core system its capability,
>>> like FDW_CAN_ENFORCE_CHECK_CONSTRAINT that informs planner to skip
>>> local checks.
>>
>> Hmm, IIUC you mean that local users can't (or don't need to) know that
>> data which violates the local constraints exist on remote side.
>> Applying constraints to the data which is modified through FDW would
>> be necessary as well.  In that design, FDW is a bidirectional filter
>> which provides these features:
>>
>> 1) Don't push wrong data into remote data source, by applying local
>> constraints to the result of the modifying query executed on local PG.
>>   This is not perfect filter, because remote constraints don't mapped
>> automatically or perfectly (imagine constraints which is available on
>> remote but is not supported in PG).
>> 2) Don't retrieve wrong data from remote to local PG, by applying
>> local constraints
>>
>> I have a concern about consistency.  It has not been supported, but
>> let's think of Aggregate push-down invoked by a query below.
>>
>> SELECT count(*) FROM remote_table;
>>
>> If this query was fully pushed down, the result is the # of records
>> exist on remote side, but the result would be # of valid records when
>> we don't push down the aggregate.  This would confuse users.
>>
>>>> Besides CHECK constraints, currently NOT NULL constraints are
>>>> virtually ASSERTIVE (not enforcing).  Should it also be noted
>>>> explicitly?
>>>>
>>> Backward compatibility….
>>
>> Yep, backward compatibility (especially visible ones to users) should
>> be minimal, ideally zero.
>>
>>> NOT NULL [ASSERTIVE] might be an option.
>>
>> Treating [ASSERTIVE | NOT ASSERTIVE] like DEFERRABLE, and allow
>> ingASSERTIVE for only foreign tables?  It makes sense, though we need
>> consider exclusiveness .  But It needs to default to ASSERTIVE on
>> foreign tables, and NOT ASSERTIVE (means "forced") on others.  Isn't
>> is too complicated?
>>
>> CREATE FOREIGN TABLE foo (
>>      id int NOT NULL ASSERTIVE CHECK (id > 1) ASSERTIVE,
>>      …
>>      CONSTRAINT chk_foo_name_upper CHECK (upper(name) = name) ASSERTIVE
>> ) SERVER server;
>>
>> BTW, I noticed that this is like push-down-able expressions in
>> JOIN/WHERE.  We need to check a CHECK constraint defined on a foreign
>> tables contains only expressions which have same semantics as remote
>> side (in practice, built-in and immutable)?
>
> I don't think that that ASSERTIVE is going to fly, because "assertive"
> means (sayeth the Google) "having or showing a confident and forceful
> personality", which is not what we mean here.  It's tempting to do
> something like try to replace the keyword "check" with "assume" or
> "assert" or (stretching) "assertion", but that would require whichever
> one we picked to be a fully-reserved keyword, which I can't think is
> going to get much support here, for entirely understandable reasons.
> So I think we should look for another option.
>
> Currently, constraints can be marked NO INHERIT (though this seems to
> have not been fully documented, as the ALTER TABLE page doesn't
> mention it anywhere) or NOT VALID, so I'm thinking maybe we should go
> with something along those lines.  Some ideas:
>
> - NO CHECK.  The idea of writing CHECK (id > 1) NO CHECK is pretty
> hilarious, though.
> - NO VALIDATE.  But then people need to understand that NOT VALID
> means "we didn't validate it yet" while "no validate" means "we don't
> ever intend to validate it", which could be confusing.
> - NO ENFORCE.  Requires a new (probably unreserved) keyword.
> - NOT VALIDATED or NOT CHECKED.  Same problems as NO CHECK and NO
> VALIDATE, respectively, plus now we have to create a new keyword.
>
> Another idea is to apply an extensible-options syntax to constraints,
> like we do for EXPLAIN, VACUUM, etc.  Like maybe:
>
> CHECK (id > 1) OPTIONS (enforced false, valid true)
>
> Yet another idea is to consider validity a three-state property:
> either the constraint is valid (because we have checked it and are
> enforcing it), or it is not valid (because we are enforcing it but
> have not checked the pre-existing data), or it is assumed true
> (because we are not checking or enforcing it but are believing it
> anyway).  So then we could have a syntax like this:
>
> CHECK (id > 1) VALIDATE { ON | OFF | ASSERTION }
>
> Other ideas?
>
> One thing that's bugging me a bit about this whole line of attack is
> that, in the first instance, the whole goal here is to support
> inheritance hierarchies that mix ordinary tables with foreign tables.
> If you have a table with children some of which are inherited and
> others of which are not inherited, you're very likely going to want
> your constraints enforced for real on the children that are tables and
> assumed true on the children that are foreign tables, and none of what
> we're talking about here gets us to that, because we normally want the
> constraints to be identical throughout the inheritance hierarchy.
> Maybe there's some way around that, but I'm back to wondering if it
> wouldn't be better to simply silently force any constraints on a
> foreign-table into assertion mode.  That could be done without any new
> syntax at all, and frankly I think it's what people are going to want
> more often than not.

I'd like to vote for the idea of silently forcing any constraints on a 
foreign-table into assertion mode.  No new syntax and better documentation.

Thanks,

Best regards,
Etsuro Fujita


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