Re: [HACKERS] Foreign tables don't enforce the partition constraint

[Amit Langote]

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 CHECK
> or NOT NULL clauses and the partition constraint) are not
> enforced by the core 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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Re: [HACKERS] Foreign tables don't enforce the partition constraint

[Amit Langote]

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 CHECK
or NOT NULL clauses and the partition constraint) are not
enforced by the core 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
diff --git a/doc/src/sgml/ref/create_foreign_table.sgml b/doc/src/sgml/ref/create_foreign_table.sgml
index 5d0dcf567b..57b3156b21 100644
--- a/doc/src/sgml/ref/create_foreign_table.sgml
+++ b/doc/src/sgml/ref/create_foreign_table.sgml
@@ -279,10 +279,10 @@ CHECK ( expression ) [ NO INHERIT ]
   Notes
 

-Constraints on foreign tables (such as CHECK
-or NOT NULL clauses) are not enforced by the
-core PostgreSQL system, and most foreign data wrappers
-do not attempt to enforce them either; that is, the constraint is
+Constraints (both the user-defined constraints such as CHECK
+or NOT NULL clauses and the partition constraint) are not
+enforced by the core PostgreSQL system, and most foreign
+data wrappers do not attempt to enforce them either; that is, the they is
 simply assumed to hold true.  There would be little point in such
 enforcement since it would only apply to rows inserted or updated via
 the foreign table, and not to rows modified by other means, such as

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Re: [HACKERS] Foreign tables don't enforce the partition constraint

[Ashutosh Bapat]

> > 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


cft_doc_change.patch
Description: Binary data

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Re: [HACKERS] Foreign tables don't enforce the partition constraint

[Robert Haas]

On Fri, Mar 31, 2017 at 5:11 AM, Ashutosh Bapat
 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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Re: [HACKERS] Foreign tables don't enforce the partition constraint

[Ashutosh Bapat]

On Fri, Mar 31, 2017 at 1:36 PM, Amit Langote
 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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[HACKERS] Foreign tables don't enforce the partition constraint

[Amit Langote]

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)?

Thanks,
Amit




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