On Fri, Sep 2, 2016 at 5:00 PM, Etsuro Fujita <fujita.ets...@lab.ntt.co.jp>

> Hi Robert,
> Thanks for the comments!
> On 2016/09/02 11:55, Robert Haas wrote:
>> On Mon, Aug 1, 2016 at 5:44 PM, Etsuro Fujita
>> <fujita.ets...@lab.ntt.co.jp> wrote:
>>> I noticed that the following note about direct modification via
>>> GetForeignUpperPaths in fdwhandler.sgml is a bit confusing.  We have
>>> another
>>> approach using PlanDirectModify, so that should be reflected in the note
>>> as
>>> well.  Please find attached a patch.
>>>      <function>PlanForeignModify</> and the other callbacks described in
>>>      <xref linkend="fdw-callbacks-update"> are designed around the
>>> assumption
>>>      that the foreign relation will be scanned in the usual way and then
>>>      individual row updates will be driven by a local
>>> <literal>ModifyTable</>
>>>      plan node.  This approach is necessary for the general case where an
>>>      update requires reading local tables as well as foreign tables.
>>>      However, if the operation could be executed entirely by the foreign
>>>      server, the FDW could generate a path representing that and insert
>>> it
>>>      into the <literal>UPPERREL_FINAL</> upper relation, where it would
>>>      compete against the <literal>ModifyTable</> approach.
> I suppose this is factually correct, but I don't think it's very
>> illuminating.  I think that if we're going to document both
>> approaches, there should be some discussion of the pros and cons of
>> PlanDirectModify vs. PlanForeignModify.
> PlanDirectModify vs. GetForeignUpperPaths for an UPPERREL_FINAL upper
> relation?
> Of course either should be
>> better than an iterative ModifyTable, but how should the FDW author
>> decide between the two of them?
> That would apply to row locking.  We have two approaches for that too:
> GetForeignRowMarkType and GetForeignUpperPaths, which is documented in the
> same paragraph following the above documentation:
>      This approach
>      could also be used to implement remote <literal>SELECT FOR UPDATE</>,
>      rather than using the row locking callbacks described in
>      <xref linkend="fdw-callbacks-row-locking">.  Keep in mind that a path
> The point of the patch is just to let the FDW author know that there is
> another approach for implementing direct modification (ie,
> PlanDirectModify) just as for implementing row locking.
Considering the primary object of this patch is just to let the FDW author
that there is another approach for implementing direct modification, I like
idea of modifying the document.

I agree that the documentation about how the FDW author should decide
> between the two would be helpful, but I'd like to leave that for future
> work.

I performed basic test with patch,

a) patch get applied cleanly on latest source,
b) able to build documentation cleanly.

Marking this as ready for committer.

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Rushabh Lathia

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