Re: Foreign join pushdown vs EvalPlanQual

On 2015/12/09 13:26, Kouhei Kaigai wrote:
>> On Tue, Dec 8, 2015 at 10:00 PM, Etsuro Fujita
>> <fujita(dot)etsuro(at)lab(dot)ntt(dot)co(dot)jp> wrote:
>>>> I think the actual regression test outputs are fine, and that your
>>>> desire to suppress part of the plan tree from showing up in the
>>>> EXPLAIN output is misguided. I like it just the way it is. To
>>>> prevent user confusion, I think that when we add support to
>>>> postgres_fdw for this we might also want to add some documentation
>>>> explaining how to interpret this EXPLAIN output, but I don't think
>>>> there's any problem with the output itself.

>>> I'm not sure that that's a good idea. one reason for that is I think that
>>> that would be more confusing to users when more than two foreign tables are
>>> involved in a foreign join as shown in the following example. Note that the
>>> outer plans will be shown recursively. Another reason is there is no
>>> consistency between the costs of the outer plans and that of the main plan.

>> I still don't really see a problem here, but, regardless, the solution
>> can't be to hide nodes that are in fact present from the user. We can
>> talk about making further changes here, but hiding the nodes
>> altogether is categorically out in my mind.

> If you really want to hide the alternative sub-plan, you can move the
> outer planstate onto somewhere private field on BeginForeignScan,
> then kick ExecProcNode() at the ForeignRecheck callback by itself.
> Explain walks down the sub-plan if outerPlanState(planstate) is
> valid. So, as long as your extension keeps the planstate privately,
> it is not visible from the EXPLAIN.
>
> Of course, I don't recommend it.

Sorry, my explanation might be not enough, but I'm not saying to hide
the subplan. I think it would be better to show the subplan somewhere
in the EXPLAIN outout, but I'm not sure that it's a good idea to show
that in the current form. We have two plan trees; one for normal query
execution and another for EvalPlanQual testing. I think it'd be better
to show the EXPLAIN output the way that allows users to easily identify
each of the plan trees.

Best regards,
Etsuro Fujita