Re: How can I pushdown of functions used in targetlist with FDW ?

Thank you for informations
I didn't know "CREATE ROUTINE MAPPING" thread.
In my development, it may be necessary to push down features, whether 
they are remote only or remote and local.
Now I understand community concerns about function pushdown.
I will investigate more and if needed I will create new design and post 
to hackers.

Regards,

Shigeo Hirose


On 2020/01/10 12:24, Kyotaro Horiguchi wrote:
> At Thu, 09 Jan 2020 20:25:20 -0500, Tom Lane <tgl@sss.pgh.pa.us> wrote in
>> Kyotaro Horiguchi <horikyota.ntt@gmail.com> writes:
>>> Isn't ROUTING MAPPING [1] that?  Definers should define one at their
>>> own risk as table constraints are.
>>
>> Hmm.  It looks like that patch is moribund, and I can't say that I'm
>> excited about reviving it.  Aside from the syntactical reserved-word
>> problems, it seems like a mighty heavyweight way of attacking the issue
> 
> I don't mean that the patch itself is usable right now, but am simply
> asking about the feature.
> 
>> --- that is, instead of a property directly attached to the function of
>> interest, you've now got this entire other system catalog structure that
>> has to be searched for relevant information.  And that needs all the usual
>> support for a new object type, eg ALTER infrastructure, pg_dump support,
>> etc etc etc.
> 
> Yeah, that sounds really combersome to me..
> 
>> Plus, once you've created a mapping, it's only good for one server so you
>> have to do it over again for each server.  I guess in the general case
>> you have to have that, but I'd sure want some kind of shortcut for the
>> common case where e.g. ABS() means the same thing everywhere.
> 
> As for the user side, I understand that they want to push down certain
> functions despite of the troubles, but it's better if simpler.
> 
> About the idea of function attrributes, I think push-down'ability is
> not one of function's properties. postgres_fdw can push down many
> intrinsic functions. oracle_fdw can push down less functions. file_fdw
> cannot at all. So it doesn't seem rather an ability of FDW drivers.
> Putting aside the maintenance trouble, it would work for intrinsic
> functions.
> 
> Even with the feature, we still cannot push down user-defined
> functions, or incompatible intrinsic functions, which the user wants
> to push down knowing about the incompatibility.  If we allow that, we
> need to have a means to tell foreign servers about such functions.  A
> means other than ROUTINE MAPPING I can come up with is server options
> and foreign table options.
> 
>> The SQL committee do love verbosity, don't they.
> 
> Sure..
> 
> regards.
>