Foreign tables don't enforce the partition constraint

On Fri, Mar 31, 2017 at 1:36 PM, Amit Langote
<Langote_Amit_f8@lab.ntt.co.jp> wrote:

We don't enforce the constraints defined on foreign tables in ExecInsert()
and ExecUpdate(). (COPY FROM does not support foreign tables at all.)
Since partition constraints are enforced using ExecConstraints() which is
not called for foreign tables, they will not be checked if one inserts
directly into foreign partitions. So:

create table p (a int) partition by list (a);
create table p1t (like p);
create table p2t (like p);
create foreign table p1 partition of p for values in (1)
server loopback options (table_name 'p1t');
create foreign table p2 partition of p for values in (2)
server loopback options (table_name 'p2t');
insert into p1 values (2); -- ungood
insert into p2 values (1); -- ungood

While we have the ability to mark check constraints as being NOT VALID so
that planner can ignore them, partition constraints are assumed to
*always* hold, giving possibly surprising results.

explain (costs off) select * from p where a = 1;
QUERY PLAN
--------------------------
Append
-> Foreign Scan on p1
(2 rows)

select * from p where a = 1;
a
---
(0 rows)

explain (costs off) select * from p where a = 2;
QUERY PLAN
--------------------------
Append
-> Foreign Scan on p2
(2 rows)

select * from p where a = 2;
a
---
(0 rows)

Should we do something about this (treat as an open item)?

Per https://www.postgresql.org/docs/devel/static/sql-createforeigntable.html,
constraints on the foreign table should represent a constraint that is
being enforced by the remote server. Similarly, a partition constraint
should also be enforced at the foreign server. Probably we should
update documentation of create foreign table to mention this. We have
updated ALTER TABLE ATTACH PARTITION documentation with a note on
foreign tables.
--
Best Wishes,
Ashutosh Bapat
EnterpriseDB Corporation
The Postgres Database Company

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On Fri, Mar 31, 2017 at 5:11 AM, Ashutosh Bapat
<ashutosh.bapat@enterprisedb.com> wrote:

Per https://www.postgresql.org/docs/devel/static/sql-createforeigntable.html,
constraints on the foreign table should represent a constraint that is
being enforced by the remote server.

Right. This is user error. Having the *local* server try to enforce
the constraint would slow down the system without guaranteeing
anything, because somebody could modify the table on the remote server
directly.

Similarly, a partition constraint
should also be enforced at the foreign server. Probably we should
update documentation of create foreign table to mention this.

That is a good idea.

--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company

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Similarly, a partition constraint
should also be enforced at the foreign server. Probably we should
update documentation of create foreign table to mention this.

That is a good idea.

Here's the patch. I am not able to build documents on my laptop because of

recent changes in d63762452434a3a046e8c7d130d5a77c594176e4. So, I was not
able to check whether the patch builds or not. But I am hoping it builds
well.

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

Hi Ashutosh,

On 2017/04/03 15:49, Ashutosh Bapat wrote:

Similarly, a partition constraint
should also be enforced at the foreign server. Probably we should
update documentation of create foreign table to mention this.

That is a good idea.

Here's the patch.

Thanks for creating the patch.

+ Constraints and partition bounds on foreign tables (such as

We use "partition constraint" instead of "partition bounds" to mean the
implicit constraint of a partition (there are a few instances of that in
the documentation). So, perhaps this could be written as: Constraints
(both the user-defined constraints such as <literal>CHECK</>
or <literal>NOT NULL</> clauses and the partition constraint) are not
enforced by the core <productname>PostgreSQL</> system, ...

And once we've mentioned that a constraint means one of these things, we
need not repeat "partition bounds/constraints" in the subsequent
paragraphs. If you agree, attached is the updated patch.

I am not able to build documents on my laptop because of
recent changes in d63762452434a3a046e8c7d130d5a77c594176e4. So, I was not
able to check whether the patch builds or not. But I am hoping it builds
well.

By the way, docs do build fine despite the error you see.

Thanks,
Amit

On 2017/04/03 16:44, Amit Langote wrote:

Hi Ashutosh,

On 2017/04/03 15:49, Ashutosh Bapat wrote:

Similarly, a partition constraint
should also be enforced at the foreign server. Probably we should
update documentation of create foreign table to mention this.

That is a good idea.

Here's the patch.

Thanks for creating the patch.

+ Constraints and partition bounds on foreign tables (such as

We use "partition constraint" instead of "partition bounds" to mean the
implicit constraint of a partition (there are a few instances of that in
the documentation). So, perhaps this could be written as: Constraints
(both the user-defined constraints such as <literal>CHECK</>
or <literal>NOT NULL</> clauses and the partition constraint) are not
enforced by the core <productname>PostgreSQL</> system, ...

And once we've mentioned that a constraint means one of these things, we
need not repeat "partition bounds/constraints" in the subsequent
paragraphs. If you agree, attached is the updated patch.

Since it seems that we agree that this documentation tweak is good idea, I
will add this to the open items list to avoid it being missed.

Thanks,
Amit

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