Re: [v9.5] Custom Plan API

On Mon, Sep 15, 2014 at 8:38 AM, Kouhei Kaigai <kaigai(at)ak(dot)jp(dot)nec(dot)com> wrote:
>> > The only reason why I put separate hooks here is, create_custom_scan()
>> > needs to know exact size of the CustomScan node (including private
>> > fields), however, it is helpful for extensions to kick its callback to
>> > initialize the node next to the common initialization stuff.
>>
>> Why does it need to know that? I don't see that it's doing anything that
>> requires knowing the size of that node, and if it is, I think it shouldn't
>> be. That should get delegated to the callback provided by the custom plan
>> provider.
>>
> Sorry, my explanation might be confusable. The create_custom_scan() does not
> need to know the exact size of the CustomScan (or its inheritance) because of
> the two separated hooks; one is node allocation time, the other is the tail
> of the series of initialization.
> If we have only one hook here, we need to have a mechanism to informs
> create_custom_scan() an exact size of the CustomScan node; including private
> fields managed by the provider, instead of the first hook on node allocation
> time. In this case, node allocation shall be processed by create_custom_scan()
> and it has to know exact size of the node to be allocated.
>
> How do I implement the feature here? Is the combination of static node size
> and callback on the tail more simple than the existing design that takes two
> individual hooks on create_custom_scan()?

I still don't get it. Right now, the logic in create_custom_scan(),
which I think should really be create_custom_plan() or
create_plan_from_custom_path(), basically looks like this:

1. call hook function CreateCustomPlan
2. compute values for tlist and clauses
3. pass those values to hook function InitCustomScan()
4. call copy_path_costsize

What I think we should do is:

1. compute values for tlist and clauses
2. pass those values to hook function PlanCustomPath(), which will return a Plan
3. call copy_path_costsize

create_custom_scan() does not need to allocate the node. You don't
need the node to be allocated before computing tlist and clauses.

--
Robert Haas
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