Re: pgsql: Allow insert and update tuple routing and COPY for foreign table

[Etsuro Fujita]

(2019/04/26 13:58), Etsuro Fujita wrote:

(2019/04/26 13:20), Amit Langote wrote:

+ Note that this function is also called when inserting routed tuples
into
+ a foreign-table partition or executingCOPY FROM on
+ a foreign table, in which case it is called in a different way than it
+ is in theINSERT case.

Maybe minor, but should the last part of this sentence read as:

...in which case it is called in a different way than it is in the case
INSERT is operating directly on the foreign table.

?


Yeah, but I think it would be OK to just say "the INSERT case" because
this note is added to the docs for ExecForeignInsert(), which allows the
FDW to directly insert into foreign tables as you know, so users will
read "the INSERT case" as "the case INSERT is
operating directly on the foreign table".


Pushed as-is.  I think we can change that later if necessary.

Best regards,
Etsuro Fujita

Re: pgsql: Allow insert and update tuple routing and COPY for foreign table

[Etsuro Fujita]

Amit-san,

(2019/04/26 13:20), Amit Langote wrote:

On 2019/04/25 22:17, Etsuro Fujita wrote:

(2019/04/24 22:04), Laurenz Albe wrote:

Before PostgreSQL v11, a foreign data wrapper could be certain that
BeginForeignModify is always called before ExecForeignInsert.
This is no longer true.


OK, how about something like the attached?  I reworded this a bit, though.


Thanks for the patch.

+ Note that this function is also called when inserting routed tuples into
+ a foreign-table partition or executingCOPY FROM  on
+ a foreign table, in which case it is called in a different way than it
+ is in theINSERT  case.

Maybe minor, but should the last part of this sentence read as:

...in which case it is called in a different way than it is in the case
INSERT  is operating directly on the foreign table.

?


Yeah, but I think it would be OK to just say "the INSERT case" because 
this note is added to the docs for ExecForeignInsert(), which allows the 
FDW to directly insert into foreign tables as you know, so users will 
read "the INSERT case" as "the case INSERT is 
operating directly on the foreign table".


Thanks for the comment!

Best regards,
Etsuro Fujita

Re: pgsql: Allow insert and update tuple routing and COPY for foreign table

[Amit Langote]

Fujita-san,

On 2019/04/25 22:17, Etsuro Fujita wrote:
> (2019/04/24 22:04), Laurenz Albe wrote:
>>    Before PostgreSQL v11, a foreign data wrapper could be certain that
>>    BeginForeignModify is always called before ExecForeignInsert.
>>    This is no longer true.
> 
> OK, how about something like the attached?  I reworded this a bit, though.

Thanks for the patch.

+ Note that this function is also called when inserting routed tuples into
+ a foreign-table partition or executing COPY FROM on
+ a foreign table, in which case it is called in a different way than it
+ is in the INSERT case.

Maybe minor, but should the last part of this sentence read as:

...in which case it is called in a different way than it is in the case
INSERT is operating directly on the foreign table.

?

Thanks,
Amit

Re: pgsql: Allow insert and update tuple routing and COPY for foreign table

[Laurenz Albe]

On Thu, 2019-04-25 at 22:17 +0900, Etsuro Fujita wrote:
> > The documentation of ExecForeignInsert should mention something like:
> >
> >ExecForeignInsert is called for INSERT statements as well
> >as for COPY FROM and tuples that are inserted into a foreign table
> >because it is a partition of a partitioned table.
> >
> >In the case of a normal INSERT, BeginForeignModify will be called
> >before ExecForeignInsert to perform any necessary setup.
> >In the other cases, this setup has to happen in BeginForeignInsert.
> 
> These seem a bit redundant to me [...]
> 
> OK, how about something like the attached?  I reworded this a bit, though.

I like your patch better than my wording.

Thanks for the effort!

Yours,
Laurenz Albe

Re: pgsql: Allow insert and update tuple routing and COPY for foreign table

[Etsuro Fujita]

(2019/04/24 22:04), Laurenz Albe wrote:

On Wed, 2019-04-24 at 20:54 +0900, Etsuro Fujita wrote:

How about adding to the documentation for BeginForeignInsert a mention
that if an FDW doesn't support COPY FROM and/or routable foreign tables,
it must throw an error in BeginForeignInsert accordingly.


Sure, some more documentation would be good.

The documentation of ExecForeignInsert should mention something like:

   ExecForeignInsert is called for INSERT statements as well
   as for COPY FROM and tuples that are inserted into a foreign table
   because it is a partition of a partitioned table.

   In the case of a normal INSERT, BeginForeignModify will be called
   before ExecForeignInsert to perform any necessary setup.
   In the other cases, this setup has to happen in BeginForeignInsert.


These seem a bit redundant to me because the documentation already says:


void
BeginForeignModify(ModifyTableState *mtstate,
   ResultRelInfo *rinfo,
   List *fdw_private,
   int subplan_index,
   int eflags);


 Begin executing a foreign table modification operation.  This 
routine is

 called during executor startup.  It should perform any initialization
 needed prior to the actual table modifications.  Subsequently,
 *ExecForeignInsert*, 
ExecForeignUpdate
ion> or
 ExecForeignDelete will be called for each 
tuple to be

 inserted, updated, or deleted.

And


void
BeginForeignInsert(ModifyTableState *mtstate,
   ResultRelInfo *rinfo);


 Begin executing an insert operation on a foreign table.  This 
routine is

 called right before the first tuple is inserted into the foreign table
 in both cases when it is the partition chosen for tuple routing 
and the

 target specified in a COPY FROM command.  It should
 perform any initialization needed prior to the actual insertion.
 Subsequently, *ExecForeignInsert* will be 
called for

 each tuple to be inserted into the foreign table.


   Before PostgreSQL v11, a foreign data wrapper could be certain that
   BeginForeignModify is always called before ExecForeignInsert.
   This is no longer true.


OK, how about something like the attached?  I reworded this a bit, though.


On the other hand, if a FDW wants to support COPY in v11 and has no
need for BeginForeignInsert to support that, it is a simple exercise
for it to provide an empty BeginForeignInsert just to signal that it
wants to support COPY.


That seems to me inconsistent with the concept of the existing APIs for
updating foreign tables, because for an FDW that wants to support
INSERT/UPDATE/DELETE and has no need for
PlanForeignModify/BeginForeignModify, those APIs don't require the FDW
to provide empty PlanForeignModify/BeginForeignModify to tell the core
that it wants to support INSERT/UPDATE/DELETE.


That is true, but so far there hasn't been a change to the FDW API that
caused a callback to be invoked in a different fashion than it used to be.


I agree on that point.


Perhaps it would have been better to create a new callback like
ExecForeignCopy with the same signature as ExecForeignInsert so that
you can use the same callback function for both if you want.
That would also have avoided the breakage.


If so, we would actually need another new callback 
ExecForeignTupleRouting since ExecForeignInsert is also called for 
INSERT/UPDATE tuple routing (or another two new callbacks 
ExecForeignInsertTupleRouting and ExecForeignUpdateTupleRouting in case 
an FDW wants to support either of the tuple routing).  My concern about 
that is: introducing such a concept might lead to an increase in the 
number of callbacks as FDW evolves, increasing the maintenance cost of 
the core.  So I think it would be better to just have ExecForeignInsert 
as a foreign-table alternative for heap_insert, as that would keep the 
core much simple.



At the very least, this should have been mentioned in the list of
incompatible changes for v11.


Agreed.  In the attached, I added a mention to the release notes for PG11.

Best regards,
Etsuro Fujita
diff --git a/doc/src/sgml/fdwhandler.sgml b/doc/src/sgml/fdwhandler.sgml
index 7b758bdf09..c2d26ba3c1 100644
--- a/doc/src/sgml/fdwhandler.sgml
+++ b/doc/src/sgml/fdwhandler.sgml
@@ -587,6 +587,14 @@ ExecForeignInsert(EState *estate,
  with an error message.
 
 
+
+ Note that this function is also called when inserting routed tuples into
+ a foreign-table partition or executing COPY FROM on
+ a foreign table, in which case it is called in a different way than it
+ is in the INSERT case.  See the callback functions
+ described below to support that.
+
+
 
 
 TupleTableSlot *
@@ -743,6 +751,13 @@ BeginForeignInsert(ModifyTableState *mtstate,
  NULL, no action is taken for the initialization.
 
 
+
+ Note that if the FDW does not support routable foreign-table partitions
+ and/or executing COPY FROM 

Re: pgsql: Allow insert and update tuple routing and COPY for foreign table

[Laurenz Albe]

On Wed, 2019-04-24 at 14:25 +0100, Simon Riggs wrote:
> On Wed, 24 Apr 2019 at 12:55, Etsuro Fujita  
> wrote:
>  
> > > My point is that this should not be necessary.
> > 
> > In my opinion, I think this is necessary...
> 
> Could we decide by looking at what FDWs are known to exist?
> I hope we are trying to avoid breakage in the smallest number of FDWs.

A good idea.  I don't volunteer to go through the list, but I had a look
at Multicorn, which is a FDW framework used by many FDWs, and it seems
to rely on multicornBeginForeignModify being called before
multicornExecForeignInsert (the former sets up a MulticornModifyState
used by the latter).

https://github.com/Kozea/Multicorn/blob/master/src/multicorn.c

Multicorn obviously hasn't got the message yet that the API has
changed in an incompatible fashion, so I'd argue that every
Multicorn FDW with write support is currently broken.


As Andres has argued above, it is too late to do anything more about
it than to document this and warn FDW authors as good as we can.

Yours,
Laurenz Albe

Re: pgsql: Allow insert and update tuple routing and COPY for foreign table

[Simon Riggs]

On Wed, 24 Apr 2019 at 12:55, Etsuro Fujita 
wrote:


> > My point is that this should not be necessary.
>
> In my opinion, I think this is necessary...
>

Could we decide by looking at what FDWs are known to exist?  I hope we are
trying to avoid breakage in the smallest number of FDWs.

-- 
Simon Riggshttp://www.2ndQuadrant.com/

PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services

Re: pgsql: Allow insert and update tuple routing and COPY for foreign table

[Laurenz Albe]

On Wed, 2019-04-24 at 20:54 +0900, Etsuro Fujita wrote:
> How about adding to the documentation for BeginForeignInsert a mention 
> that if an FDW doesn't support COPY FROM and/or routable foreign tables, 
> it must throw an error in BeginForeignInsert accordingly.

Sure, some more documentation would be good.

The documentation of ExecForeignInsert should mention something like:

  ExecForeignInsert is called for INSERT statements as well
  as for COPY FROM and tuples that are inserted into a foreign table
  because it is a partition of a partitioned table.

  In the case of a normal INSERT, BeginForeignModify will be called
  before ExecForeignInsert to perform any necessary setup.
  In the other cases, this setup has to happen in BeginForeignInsert.

  Before PostgreSQL v11, a foreign data wrapper could be certain that
  BeginForeignModify is always called before ExecForeignInsert.
  This is no longer true.

> > On the other hand, if a FDW wants to support COPY in v11 and has no
> > need for BeginForeignInsert to support that, it is a simple exercise
> > for it to provide an empty BeginForeignInsert just to signal that it
> > wants to support COPY.
> 
> That seems to me inconsistent with the concept of the existing APIs for 
> updating foreign tables, because for an FDW that wants to support 
> INSERT/UPDATE/DELETE and has no need for 
> PlanForeignModify/BeginForeignModify, those APIs don't require the FDW 
> to provide empty PlanForeignModify/BeginForeignModify to tell the core 
> that it wants to support INSERT/UPDATE/DELETE.

That is true, but so far there hasn't been a change to the FDW API that
caused a callback to be invoked in a different fashion than it used to be.

Perhaps it would have been better to create a new callback like
ExecForeignCopy with the same signature as ExecForeignInsert so that
you can use the same callback function for both if you want.
That would also have avoided the breakage.
But, of course it is too late for that now.

Note that postgres_fdw would have been broken by that API change as well
if it hasn't been patched.

At the very least, this should have been mentioned in the list of
incompatible changes for v11.

Yours,
Laurenz Albe

Re: pgsql: Allow insert and update tuple routing and COPY for foreign table

[Etsuro Fujita]

(2019/04/23 4:37), Laurenz Albe wrote:

On Mon, 2019-04-22 at 21:45 +0900, Etsuro Fujita wrote:

(2019/04/20 20:53), Laurenz Albe wrote:

On Fri, 2018-04-06 at 23:24 +, Robert Haas wrote:

Allow insert and update tuple routing and COPY for foreign tables.

Also enable this for postgres_fdw.

Etsuro Fujita, based on an earlier patch by Amit Langote. The larger
patch series of which this is a part has been reviewed by Amit
Langote, David Fetter, Maksim Milyutin, Álvaro Herrera, Stephen Frost,
and me.  Minor documentation changes to the final version by me.

Discussion: 
http://postgr.es/m/29906a26-da12-8c86-4fb9-d8f88442f...@lab.ntt.co.jp



If a FDW implements ExecForeignInsert, this commit automatically assumes
that it also supports COPY FROM.  It will call ExecForeignInsert without
calling PlanForeignModify and BeginForeignModify, and a FDW that does not
expect that will probably fail.


This is not 100% correct; the FDW documentation says:

  
   Tuples inserted into a partitioned table by
INSERT  or
   COPY FROM  are routed to partitions.  If an FDW
   supports routable foreign-table partitions, it should also provide the
   following callback functions.  These functions are also called when
   COPY FROM  is executed on a foreign table.
  


I don't see the difference between the documentation and what I wrote above.

Before v11, a FDW could expect that ExecForeignInsert is only called if
BeginForeignModify was called earlier.
That has silently changed with v11.


I have to admit that the documentation is not sufficient.


It's permissible to throw an error in BeginForeignInsert, so what I was
thinking for FDWs that don't want to support COPY FROM and
INSERT/UPDATE/COPY FROM tuple routing was to provide BeginForeignInsert
implementing something like this:

static void
fooBeginForeignInsert(ModifyTableState *mtstate,
ResultRelInfo *resultRelInfo)
{
  Relationrel = resultRelInfo->ri_RelationDesc;

  if (mtstate->ps.plan == NULL)
  ereport(ERROR,
  (errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
   errmsg("cannot copy to foreign table \"%s\"",
  RelationGetRelationName(rel;
  else
  ereport(ERROR,
  (errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
   errmsg("cannot route tuples into foreign table \"%s\"",
  RelationGetRelationName(rel;
}


Sure, it is not hard to modify a FDW to continue working with v11.


How about adding to the documentation for BeginForeignInsert a mention 
that if an FDW doesn't support COPY FROM and/or routable foreign tables, 
it must throw an error in BeginForeignInsert accordingly.



My point is that this should not be necessary.


In my opinion, I think this is necessary...


On the other hand, if a FDW wants to support COPY in v11 and has no
need for BeginForeignInsert to support that, it is a simple exercise
for it to provide an empty BeginForeignInsert just to signal that it
wants to support COPY.


That seems to me inconsistent with the concept of the existing APIs for 
updating foreign tables, because for an FDW that wants to support 
INSERT/UPDATE/DELETE and has no need for 
PlanForeignModify/BeginForeignModify, those APIs don't require the FDW 
to provide empty PlanForeignModify/BeginForeignModify to tell the core 
that it wants to support INSERT/UPDATE/DELETE.


Best regards,
Etsuro Fujita

Re: pgsql: Allow insert and update tuple routing and COPY for foreign table

[Amit Langote]

On 2019/04/22 21:45, Etsuro Fujita wrote:
> (2019/04/20 20:53), Laurenz Albe wrote:
>> I propose that PostgreSQL only allows COPY FROM on a foreign table if
>> the FDW
>> implements BeginForeignInsert.  The attached patch implements that.
> 
> I don't think that is a good idea, because there might be some FDWs that
> want to support COPY FROM on foreign tables without providing
> BeginForeignInsert.  (As for INSERT into foreign tables, we actually allow
> FDWs to support it without providing PlanForeignModify,
> BeginForeignModify, or EndForeignModify.)

I now understand why Laurenz's patch would in fact be a regression for
FDWs that do support COPY FROM and partition tuple routing without
providing BeginForeignInsert, although my first reaction was the opposite,
which was based on thinking (without confirming) that it's the core that
would crash due to initialization step being absent, but that's not the case.

The documentation [1] also says:

  If the BeginForeignInsert pointer is set to NULL, no action is taken for
  the initialization.

[1]
https://www.postgresql.org/docs/current/fdw-callbacks.html#FDW-CALLBACKS-UPDATE

> It's permissible to throw an error in BeginForeignInsert, so what I was
> thinking for FDWs that don't want to support COPY FROM and
> INSERT/UPDATE/COPY FROM tuple routing was to provide BeginForeignInsert
> implementing something like this:
> 
> static void
> fooBeginForeignInsert(ModifyTableState *mtstate,
>   ResultRelInfo *resultRelInfo)
> {
>     Relation    rel = resultRelInfo->ri_RelationDesc;
> 
>     if (mtstate->ps.plan == NULL)
>     ereport(ERROR,
>     (errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
>  errmsg("cannot copy to foreign table \"%s\"",
>     RelationGetRelationName(rel;
>     else
>     ereport(ERROR,
>     (errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
>  errmsg("cannot route tuples into foreign table \"%s\"",
>     RelationGetRelationName(rel;
> }

+1

Thanks,
Amit

Re: pgsql: Allow insert and update tuple routing and COPY for foreign table

[Laurenz Albe]

On Mon, 2019-04-22 at 14:07 -0700, Andres Freund wrote:
> How about just applying the patch from v12 on?
> > Then it is a behavior change in a major release, which is acceptable.
> > It requires the imaginary FDW above to add an empty BeginForeignInsert
> > callback function, but will unbreak FDW that slept through the change
> > that added COPY support.
> 
> I fail to see the advantage. It'll still require FDWs to be fixed to
> work correctly for v11, but additionally adds another set of API
> differences that needs to be fixed by another set of FDWs.  I think this
> ship simply has sailed.

I can accept that (having fixed my own FDW), but I am worried that it will
cause problems for FDW users.  Well, I guess they can always avoid COPY if
they don't want FDWs to crash.

Yours,
Laurenz Albe

Re: pgsql: Allow insert and update tuple routing and COPY for foreign table

[Andres Freund]

Hi,

On 2019-04-22 22:56:20 +0200, Laurenz Albe wrote:
> On Mon, 2019-04-22 at 13:27 -0700, Andres Freund wrote:
> > On 2019-04-22 21:37:25 +0200, Laurenz Albe wrote:
> > > Commit 3d956d956a introduced support for foreign tables as partitions
> > > and COPY FROM on foreign tables.
> > > 
> > > If a foreign data wrapper supports data modifications, but either has
> > > not adapted to this change yet or doesn't want to support it
> > > for other reasons, it probably got broken by the above commit,
> > > because COPY will just call ExecForeignInsert anyway, which might not
> > > work because neither PlanForeignModify nor BeginForeignModify have
> > > been called.
> > > 
> > > To avoid breaking third-party foreign data wrappers in that way, allow
> > > COPY FROM and tuple routing for foreign tables only if the foreign data
> > > wrapper implements BeginForeignInsert.
> > 
> > Isn't this worse though? Before this it's an API change between major
> > versions. With this it's an API change in a *minor* version. Sure, it's
> > one that doesn't crash, but it's still a pretty substantial function
> > regression, no?
> 
> Hm, that's a good point.
> You could say that this patch is too late, because a FDW might already be
> relying on COPY FROM to work without BeginForeignInsert in v11.

I think that's the case.


> How about just applying the patch from v12 on?
> Then it is a behavior change in a major release, which is acceptable.
> It requires the imaginary FDW above to add an empty BeginForeignInsert
> callback function, but will unbreak FDW that slept through the change
> that added COPY support.

I fail to see the advantage. It'll still require FDWs to be fixed to
work correctly for v11, but additionally adds another set of API
differences that needs to be fixed by another set of FDWs.  I think this
ship simply has sailed.

Greetings,

Andres Freund

Re: pgsql: Allow insert and update tuple routing and COPY for foreign table

[Laurenz Albe]

On Mon, 2019-04-22 at 13:27 -0700, Andres Freund wrote:
> On 2019-04-22 21:37:25 +0200, Laurenz Albe wrote:
> > Commit 3d956d956a introduced support for foreign tables as partitions
> > and COPY FROM on foreign tables.
> > 
> > If a foreign data wrapper supports data modifications, but either has
> > not adapted to this change yet or doesn't want to support it
> > for other reasons, it probably got broken by the above commit,
> > because COPY will just call ExecForeignInsert anyway, which might not
> > work because neither PlanForeignModify nor BeginForeignModify have
> > been called.
> > 
> > To avoid breaking third-party foreign data wrappers in that way, allow
> > COPY FROM and tuple routing for foreign tables only if the foreign data
> > wrapper implements BeginForeignInsert.
> 
> Isn't this worse though? Before this it's an API change between major
> versions. With this it's an API change in a *minor* version. Sure, it's
> one that doesn't crash, but it's still a pretty substantial function
> regression, no?

Hm, that's a good point.
You could say that this patch is too late, because a FDW might already be
relying on COPY FROM to work without BeginForeignInsert in v11.

How about just applying the patch from v12 on?
Then it is a behavior change in a major release, which is acceptable.
It requires the imaginary FDW above to add an empty BeginForeignInsert
callback function, but will unbreak FDW that slept through the change
that added COPY support.

Yours,
Laurenz Albe

Re: pgsql: Allow insert and update tuple routing and COPY for foreign table

[Laurenz Albe]

On Mon, 2019-04-22 at 16:24 -0400, Robert Haas wrote:
> On Mon, Apr 22, 2019 at 3:37 PM Laurenz Albe  wrote:
> > Sure, it is not hard to modify a FDW to continue working with v11.
> > 
> > My point is that this should not be necessary.
> 
> I'm not sure whether this proposal is a good idea or a bad idea, but I
> think that it's inevitable that FDWs are going to need some updating
> for new releases as the API evolves.  That has happened before and
> will continue to happen.

Absolutely.
I am just unhappy that this change caused unnecessary breakage.

If you developed a read-only FDW for 9.2, it didn't break with the
write support introduced in 9.3, because that used different API
functions.  That's how it should be IMHO.

If you developed a FDW for 9.1, it got broken in 9.2, because the
API had to change to allow returning multiple paths.
That was unfortunate but necessary, so it is ok.

Nothing in this patch required an incompatible change.

Yours,
Laurenz Albe

Re: pgsql: Allow insert and update tuple routing and COPY for foreign table

[Andres Freund]

Hi,

On 2019-04-22 21:37:25 +0200, Laurenz Albe wrote:
> Subject: [PATCH] Foreign table COPY FROM and tuple routing requires
>  BeginForeignInsert
> 
> Commit 3d956d956a introduced support for foreign tables as partitions
> and COPY FROM on foreign tables.
> 
> If a foreign data wrapper supports data modifications, but either has
> not adapted to this change yet or doesn't want to support it
> for other reasons, it probably got broken by the above commit,
> because COPY will just call ExecForeignInsert anyway, which might not
> work because neither PlanForeignModify nor BeginForeignModify have
> been called.
> 
> To avoid breaking third-party foreign data wrappers in that way, allow
> COPY FROM and tuple routing for foreign tables only if the foreign data
> wrapper implements BeginForeignInsert.

Isn't this worse though? Before this it's an API change between major
versions. With this it's an API change in a *minor* version. Sure, it's
one that doesn't crash, but it's still a pretty substantial function
regression, no?

Greetings,

Andres Freund

Re: pgsql: Allow insert and update tuple routing and COPY for foreign table

[Robert Haas]

On Mon, Apr 22, 2019 at 3:37 PM Laurenz Albe  wrote:
> Sure, it is not hard to modify a FDW to continue working with v11.
>
> My point is that this should not be necessary.

I'm not sure whether this proposal is a good idea or a bad idea, but I
think that it's inevitable that FDWs are going to need some updating
for new releases as the API evolves.  That has happened before and
will continue to happen.

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

Re: pgsql: Allow insert and update tuple routing and COPY for foreign table

[Laurenz Albe]

On Mon, 2019-04-22 at 21:45 +0900, Etsuro Fujita wrote:

Thanks for looking into this!

> (2019/04/20 20:53), Laurenz Albe wrote:
> > On Fri, 2018-04-06 at 23:24 +, Robert Haas wrote:
> > > Allow insert and update tuple routing and COPY for foreign tables.
> > > 
> > > Also enable this for postgres_fdw.
> > > 
> > > Etsuro Fujita, based on an earlier patch by Amit Langote. The larger
> > > patch series of which this is a part has been reviewed by Amit
> > > Langote, David Fetter, Maksim Milyutin, Álvaro Herrera, Stephen Frost,
> > > and me.  Minor documentation changes to the final version by me.
> > > 
> > > Discussion: 
> > > http://postgr.es/m/29906a26-da12-8c86-4fb9-d8f88442f...@lab.ntt.co.jp
> > 
> > This commit makes life hard for foreign data wrappers that support
> > data modifications.
> > 
> > If a FDW implements ExecForeignInsert, this commit automatically assumes
> > that it also supports COPY FROM.  It will call ExecForeignInsert without
> > calling PlanForeignModify and BeginForeignModify, and a FDW that does not
> > expect that will probably fail.
> 
> This is not 100% correct; the FDW documentation says:
> 
>  
>   Tuples inserted into a partitioned table by 
> INSERT or
>   COPY FROM are routed to partitions.  If an FDW
>   supports routable foreign-table partitions, it should also provide the
>   following callback functions.  These functions are also called when
>   COPY FROM is executed on a foreign table.
>  

I don't see the difference between the documentation and what I wrote above.

Before v11, a FDW could expect that ExecForeignInsert is only called if
BeginForeignModify was called earlier.
That has silently changed with v11.

> > maybe there are FDWs that support INSERT but don't want to support COPY
> > for some reason.
> 
> I agree on that point.
> 
> > I propose that PostgreSQL only allows COPY FROM on a foreign table if the 
> > FDW
> > implements BeginForeignInsert.  The attached patch implements that.
> 
> I don't think that is a good idea, because there might be some FDWs that 
> want to support COPY FROM on foreign tables without providing 
> BeginForeignInsert.  (As for INSERT into foreign tables, we actually 
> allow FDWs to support it without providing PlanForeignModify, 
> BeginForeignModify, or EndForeignModify.)
> 
> It's permissible to throw an error in BeginForeignInsert, so what I was 
> thinking for FDWs that don't want to support COPY FROM and 
> INSERT/UPDATE/COPY FROM tuple routing was to provide BeginForeignInsert 
> implementing something like this:
> 
> static void
> fooBeginForeignInsert(ModifyTableState *mtstate,
>ResultRelInfo *resultRelInfo)
> {
>  Relationrel = resultRelInfo->ri_RelationDesc;
> 
>  if (mtstate->ps.plan == NULL)
>  ereport(ERROR,
>  (errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
>   errmsg("cannot copy to foreign table \"%s\"",
>  RelationGetRelationName(rel;
>  else
>  ereport(ERROR,
>  (errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
>   errmsg("cannot route tuples into foreign table \"%s\"",
>  RelationGetRelationName(rel;
> }

Sure, it is not hard to modify a FDW to continue working with v11.

My point is that this should not be necessary.

If a FDW worked well with v10, it should continue to work with v11
unless there is a necessity for a compatibility-breaking change.

On the other hand, if a FDW wants to support COPY in v11 and has no
need for BeginForeignInsert to support that, it is a simple exercise
for it to provide an empty BeginForeignInsert just to signal that it
wants to support COPY.

I realized that my previous patch forgot to check for tuple routing,
updated patch attached.

Yours,
Laurenz Albe
From 545ac9d567ea6ca610ce08355451923cc787e13d Mon Sep 17 00:00:00 2001
From: Laurenz Albe 
Date: Mon, 22 Apr 2019 21:35:06 +0200
Subject: [PATCH] Foreign table COPY FROM and tuple routing requires
 BeginForeignInsert

Commit 3d956d956a introduced support for foreign tables as partitions
and COPY FROM on foreign tables.

If a foreign data wrapper supports data modifications, but either has
not adapted to this change yet or doesn't want to support it
for other reasons, it probably got broken by the above commit,
because COPY will just call ExecForeignInsert anyway, which might not
work because neither PlanForeignModify nor BeginForeignModify have
been called.

To avoid breaking third-party foreign data wrappers in that way, allow
COPY FROM and tuple routing for foreign tables only if the foreign data
wrapper implements BeginForeignInsert.
---
 doc/src/sgml/fdwhandler.sgml |  2 ++
 src/backend/commands/copy.c  |  5 +
 src/backend/executor/execPartition.c | 17 -
 3 files changed, 19 insertions(+), 5 deletions(-)

diff --git a/doc/src/sgml/fdwhandler.sgml b/doc/src/sgml/fdwhandler.sgml
index 

Re: pgsql: Allow insert and update tuple routing and COPY for foreign table

[Etsuro Fujita]

(2019/04/20 20:53), Laurenz Albe wrote:

On Fri, 2018-04-06 at 23:24 +, Robert Haas wrote:

Allow insert and update tuple routing and COPY for foreign tables.

Also enable this for postgres_fdw.

Etsuro Fujita, based on an earlier patch by Amit Langote. The larger
patch series of which this is a part has been reviewed by Amit
Langote, David Fetter, Maksim Milyutin, Álvaro Herrera, Stephen Frost,
and me.  Minor documentation changes to the final version by me.

Discussion: 
http://postgr.es/m/29906a26-da12-8c86-4fb9-d8f88442f...@lab.ntt.co.jp


This commit makes life hard for foreign data wrappers that support
data modifications.

If a FDW implements ExecForeignInsert, this commit automatically assumes
that it also supports COPY FROM.  It will call ExecForeignInsert without
calling PlanForeignModify and BeginForeignModify, and a FDW that does not
expect that will probably fail.


This is not 100% correct; the FDW documentation says:


 Tuples inserted into a partitioned table by 
INSERT or

 COPY FROM are routed to partitions.  If an FDW
 supports routable foreign-table partitions, it should also provide the
 following callback functions.  These functions are also called when
 COPY FROM is executed on a foreign table.



maybe there are FDWs that support INSERT but don't want to support COPY
for some reason.


I agree on that point.


I propose that PostgreSQL only allows COPY FROM on a foreign table if the FDW
implements BeginForeignInsert.  The attached patch implements that.


I don't think that is a good idea, because there might be some FDWs that 
want to support COPY FROM on foreign tables without providing 
BeginForeignInsert.  (As for INSERT into foreign tables, we actually 
allow FDWs to support it without providing PlanForeignModify, 
BeginForeignModify, or EndForeignModify.)


It's permissible to throw an error in BeginForeignInsert, so what I was 
thinking for FDWs that don't want to support COPY FROM and 
INSERT/UPDATE/COPY FROM tuple routing was to provide BeginForeignInsert 
implementing something like this:


static void
fooBeginForeignInsert(ModifyTableState *mtstate,
  ResultRelInfo *resultRelInfo)
{
Relationrel = resultRelInfo->ri_RelationDesc;

if (mtstate->ps.plan == NULL)
ereport(ERROR,
(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
 errmsg("cannot copy to foreign table \"%s\"",
RelationGetRelationName(rel;
else
ereport(ERROR,
(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
 errmsg("cannot route tuples into foreign table \"%s\"",
RelationGetRelationName(rel;
}

Best regards,
Etsuro Fujita

Re: pgsql: Allow insert and update tuple routing and COPY for foreign table

[Amit Langote]

Hi,

On 2019/04/20 20:53, Laurenz Albe wrote:
> On Fri, 2018-04-06 at 23:24 +, Robert Haas wrote:
>> Allow insert and update tuple routing and COPY for foreign tables.
>>
>> Also enable this for postgres_fdw.
>>
>> Etsuro Fujita, based on an earlier patch by Amit Langote. The larger
>> patch series of which this is a part has been reviewed by Amit
>> Langote, David Fetter, Maksim Milyutin, Álvaro Herrera, Stephen Frost,
>> and me.  Minor documentation changes to the final version by me.
>>
>> Discussion: 
>> http://postgr.es/m/29906a26-da12-8c86-4fb9-d8f88442f...@lab.ntt.co.jp
> 
> This commit makes life hard for foreign data wrappers that support
> data modifications.
> 
> If a FDW implements ExecForeignInsert, this commit automatically assumes
> that it also supports COPY FROM.  It will call ExecForeignInsert without
> calling PlanForeignModify and BeginForeignModify, and a FDW that does not
> expect that will probably fail.
> 
> So this commit silently turns a functioning FDW into a broken FDW.
> That is not nice.  Probably not every FDW is aware of this change, and
> maybe there are FDWs that support INSERT but don't want to support COPY
> for some reason.

That seems like an oversight to me.  I agree that we had better checked
that a table's FDW provides BeginForeignInsert() before proceeding with
copying into the table, as your patch teaches CopyFrom() to do.

> I propose that PostgreSQL only allows COPY FROM on a foreign table if the FDW
> implements BeginForeignInsert.  The attached patch implements that.

Looks good to me, including the documentation change.

> I think this should be backpatched to v11.

Agreed.

Thanks,
Amit

Re: pgsql: Allow insert and update tuple routing and COPY for foreign table

[Etsuro Fujita]

(2019/04/20 20:53), Laurenz Albe wrote:

On Fri, 2018-04-06 at 23:24 +, Robert Haas wrote:

Allow insert and update tuple routing and COPY for foreign tables.

Also enable this for postgres_fdw.

Etsuro Fujita, based on an earlier patch by Amit Langote. The larger
patch series of which this is a part has been reviewed by Amit
Langote, David Fetter, Maksim Milyutin, Álvaro Herrera, Stephen Frost,
and me.  Minor documentation changes to the final version by me.

Discussion: 
http://postgr.es/m/29906a26-da12-8c86-4fb9-d8f88442f...@lab.ntt.co.jp


This commit makes life hard for foreign data wrappers that support
data modifications.


Will look into this.

Best regards,
Etsuro Fujita

Re: pgsql: Allow insert and update tuple routing and COPY for foreign table

[Laurenz Albe]

On Fri, 2018-04-06 at 23:24 +, Robert Haas wrote:
> Allow insert and update tuple routing and COPY for foreign tables.
> 
> Also enable this for postgres_fdw.
> 
> Etsuro Fujita, based on an earlier patch by Amit Langote. The larger
> patch series of which this is a part has been reviewed by Amit
> Langote, David Fetter, Maksim Milyutin, Álvaro Herrera, Stephen Frost,
> and me.  Minor documentation changes to the final version by me.
> 
> Discussion: 
> http://postgr.es/m/29906a26-da12-8c86-4fb9-d8f88442f...@lab.ntt.co.jp

This commit makes life hard for foreign data wrappers that support
data modifications.

If a FDW implements ExecForeignInsert, this commit automatically assumes
that it also supports COPY FROM.  It will call ExecForeignInsert without
calling PlanForeignModify and BeginForeignModify, and a FDW that does not
expect that will probably fail.

So this commit silently turns a functioning FDW into a broken FDW.
That is not nice.  Probably not every FDW is aware of this change, and
maybe there are FDWs that support INSERT but don't want to support COPY
for some reason.

I propose that PostgreSQL only allows COPY FROM on a foreign table if the FDW
implements BeginForeignInsert.  The attached patch implements that.

I think this should be backpatched to v11.

Yours,
Laurenz Albe
From c4b0e871658c757124dad992578da0b60fccf962 Mon Sep 17 00:00:00 2001
From: Laurenz Albe 
Date: Sat, 20 Apr 2019 13:36:56 +0200
Subject: [PATCH] COPY FROM on foreign tables requires BeginForeignInsert

Commit 3d956d956a introduced COPY FROM support for foreign tables.

If a foreign data wrapper supports data modifications, but either has
not adapted to this change yet or doesn't want to support COPY FROM
for other reasons, it probably got broken by the above commit,
because COPY will just call ExecForeignInsert anyway, which might not
work because neither PlanForeignModify nor BeginForeignModify have
been called.

To avoid breaking third-party foreign data wrappers in that way, allow
COPY FROM for foreign tables only if the foreign data wrapper implements
BeginForeignInsert.
---
 doc/src/sgml/fdwhandler.sgml | 2 ++
 src/backend/commands/copy.c  | 5 +
 2 files changed, 7 insertions(+)

diff --git a/doc/src/sgml/fdwhandler.sgml b/doc/src/sgml/fdwhandler.sgml
index 2c07a86637..8cd174a204 100644
--- a/doc/src/sgml/fdwhandler.sgml
+++ b/doc/src/sgml/fdwhandler.sgml
@@ -724,6 +724,8 @@ BeginForeignInsert(ModifyTableState *mtstate,
  perform any initialization needed prior to the actual insertion.
  Subsequently, ExecForeignInsert will be called for
  each tuple to be inserted into the foreign table.
+ All foreign data wrappers that support COPY FROM have
+ to provide this callback function.
 
 
 
diff --git a/src/backend/commands/copy.c b/src/backend/commands/copy.c
index c39218f8db..944d558cd4 100644
--- a/src/backend/commands/copy.c
+++ b/src/backend/commands/copy.c
@@ -2857,6 +2857,11 @@ CopyFrom(CopyState cstate)
 		resultRelInfo->ri_FdwRoutine->BeginForeignInsert != NULL)
 		resultRelInfo->ri_FdwRoutine->BeginForeignInsert(mtstate,
 		 resultRelInfo);
+	else
+		ereport(ERROR,
+(errcode(ERRCODE_FEATURE NOT SUPPORTED),
+ errmsg("cannot copy to foreign table \"%s\"",
+		RelationGetRelationName(cstate->rel;
 
 	/* Prepare to catch AFTER triggers. */
 	AfterTriggerBeginQuery();
-- 
2.20.1