Re: inherit support for foreign tables

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
constraintsdon'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)?
--
Shigeru HANADA