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 +0000, 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-d8f88442f2b9@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:
> 
>      <para>
>       Tuples inserted into a partitioned table by 
> <command>INSERT</command> or
>       <command>COPY FROM</command> 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
>       <command>COPY FROM</command> is executed on a foreign table.
>      </para>

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)
> {
>      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))));
> }

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

On Wed, 24 Apr 2019 at 12:55, Etsuro Fujita <fujita.etsuro@lab.ntt.co.jp> 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 Riggs http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services

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

From

Simon Riggs

Date:

24 April 2019, 13:25:04

On Wed, 24 Apr 2019 at 12:55, Etsuro Fujita <fujita.etsuro@lab.ntt.co.jp> 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 Riggs http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services

Re: pgsql: Allow insert and update tuple routing and COPY forforeign table

From

Laurenz Albe

Date:

24 April 2019, 14:03:12

On Wed, 2019-04-24 at 14:25 +0100, Simon Riggs wrote:
> On Wed, 24 Apr 2019 at 12:55, Etsuro Fujita <fujita.etsuro@lab.ntt.co.jp> 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 forforeign table

From

Laurenz Albe

Date:

24 April 2019, 14:03:12

On Wed, 2019-04-24 at 14:25 +0100, Simon Riggs wrote:
> On Wed, 24 Apr 2019 at 12:55, Etsuro Fujita <fujita.etsuro@lab.ntt.co.jp> 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 foreigntable

From

Etsuro Fujita

Date:

25 April 2019, 13:17:12

(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:

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

      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,
      <function>*ExecForeignInsert*</function>, 
<function>ExecForeignUpdate</funct\
ion> or
      <function>ExecForeignDelete</function> will be called for each 
tuple to be
      inserted, updated, or deleted.

And

<programlisting>
void
BeginForeignInsert(ModifyTableState *mtstate,
                    ResultRelInfo *rinfo);
</programlisting>

      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 <command>COPY FROM</command> command.  It should
      perform any initialization needed prior to the actual insertion.
      Subsequently, <function>*ExecForeignInsert*</function> 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

Re: pgsql: Allow insert and update tuple routing and COPY for foreigntable

From

Etsuro Fujita

Date:

25 April 2019, 13:17:12

(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:

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

      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,
      <function>*ExecForeignInsert*</function>, 
<function>ExecForeignUpdate</funct\
ion> or
      <function>ExecForeignDelete</function> will be called for each 
tuple to be
      inserted, updated, or deleted.

And

<programlisting>
void
BeginForeignInsert(ModifyTableState *mtstate,
                    ResultRelInfo *rinfo);
</programlisting>

      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 <command>COPY FROM</command> command.  It should
      perform any initialization needed prior to the actual insertion.
      Subsequently, <function>*ExecForeignInsert*</function> 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

(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 executing<command>COPY FROM</command> on
>> + a foreign table, in which case it is called in a different way than it
>> + is in the<command>INSERT</command> 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
>> <command>INSERT</command> 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 <command>INSERT</command> is
> operating directly on the foreign table".

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

Best regards,
Etsuro Fujita