Re: Custom Plan node

2013/9/7 Kohei KaiGai <kaigai(at)kaigai(dot)gr(dot)jp>:
> 2013/9/7 Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>:
>> Robert Haas <robertmhaas(at)gmail(dot)com> writes:
>>> I find this a somewhat depressing response. Didn't we discuss this
>>> exact design at the developer meeting in Ottawa? I thought it sounded
>>> reasonable to you then, or at least I don't remember you panning it.
>>
>> What I recall saying is that I didn't see how the planner side of it would
>> work ... and I still don't see that. I'd be okay with committing
>> executor-side fixes only if we had a vision of where we'd go on the
>> planner side; but this patch doesn't offer any path forward there.
>>
> The reason why this patch stick on executor-side is we concluded
> not to patch the planner code from the beginning in Ottawa because
> of its complexity.
> I'd also like to agree that planner support for custom plan is helpful
> to construct better execution plan, however, it also make sense even
> if this feature begins a functionality that offers a way to arrange a plan
> tree being already constructed.
>
> Anyway, let me investigate what's kind of APIs to be added for planner
> stage also.
>
It is a brief idea to add planner support on custom node, if we need it
from the beginning. Of course, it is not still detailed considered and
needs much brushing up, however, it may be a draft to implement this
feature.

We may be able to categorize plan node types into three; scan, join
and others.

Even though planner tries to test various combinations of join and scan
to minimize its estimated cost, we have less options on other types
like T_Agg and so on. It seems to me the other types are almost put
according to the query's form, so it does not make a big problem even
if all we can do is manipulation of plan-tree at planner_hook.
That is similar to what proposed patch is doing.

So, let's focus on join and scan. It needs to give extensions a chance
to override built-in path if they can offer more cheap path.
It leads an API that allows to add alternative paths when built-in feature
is constructing candidate paths. Once path was added, we can compare
them according to the estimated cost.
For example, let's assume a query tries to join foreign table A and B
managed by same postgres_fdw server, remote join likely has cheaper
cost than local join. If extension has a capability to handle the case
correctly, it may be able to add an alternative "custom-join" path with
cheaper-cost.
Then, this path shall be transformed to "CustomJoin" node that launches
a query to get a result of A join B being remotely joined.
In this case, here is no matter even if "CustomJoin" has underlying
ForeignScan nodes on the left-/right-tree, because extension can handle
the things to do with its arbitrary.

So, the following APIs may be needed for planner support, at least.

* API to add an alternative join path, in addition to built-in join logic.
* API to add an alternative scan path, in addition to built-in scan logic.
* API to construct "CustomJoin" according to the related path.
* API to construct "CustomScan" according to the related path.

Any comment please.

Thanks,
--
KaiGai Kohei <kaigai(at)kaigai(dot)gr(dot)jp>