Re: Custom Scan APIs (Re: Custom Plan node)

2014-03-04 23:09 GMT+09:00 Robert Haas <robertmhaas@gmail.com>:
> On Mon, Mar 3, 2014 at 5:15 PM, Stephen Frost <sfrost@snowman.net> wrote:
>>> As I mentioned
>>> up-thread, I'd really like to see FDW join push-down, FDW aggregate
>>> push-down, parallel query execution, and parallel remote-FDW execution
>>> and I don't see this CustomScan approach as the right answer to any of
>>> those.
>>
>> In accordance with the above, what I'd like to see with this patch is
>> removal of the postgres_fdw changes and any changes which were for that
>> support.  In addition, I'd like to understand why 'ctidscan' makes any
>> sense to have as an example of what to use this for- if that's valuable,
>> why wouldn't we simply implement that in core?  I do want an example in
>> contrib of how to properly use this capability, but I don't think that's
>> it.
>
> I suggested that example to KaiGai at last year's PGCon.  It may
> indeed be something we want to have in core, but right now we don't.
>
> More generally, I think this discussion is focusing on the wrong set
> of issues.  The threshold issue for this patch is whether there is a
> set of hook points that enable a workable custom-scan functionality,
> and whether KaiGai has correctly identified them.  In other words, I
> think we should be worrying about whether KaiGai's found all of the
> places that need to be modified to support a custom scan, and whether
> the modifications he's made to each of those places are correct and
> adequate.  Whether he's picked the best possible example does not
> strike me as a matter of principal concern, and it's far too late to
> tell him he's got to go pick a different one at this point anyway.
>
That is definitely the point to be discussed here. Even though I *believe*
I could put the callbacks needed to implement alternative join / scan,
it may lead different conclusion from other person's viewpoint.

At least, I could implement a custom-scan as an alternative of join
using postgres_fdw, however, it's uncertain whether I could cover
all the possible case we should care about.
So, I'd like to see comments from others.

Thanks,
-- 
KaiGai Kohei <kaigai@kaigai.gr.jp>